sh(1p)sh(1p)NAMEsh - Shell, the standard command language interpreter (POSIX Shell)
SYNOPSISsh [-ir] [-c command_string | -s] [+ | -abCefhkmnopstuvx] [+|-o]
[option...]|[+|-A name] [argument...]|[file] [argument...]
The POSIX shell is an interactive command interpreter and a command
programming language.
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
sh: XCU5.0 and POSIX.2
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
OPTIONS
Causes sh to read commands from command_string. Causes sh to run as an
interactive shell. The SIGTERM signal is thus ignored, and the SIGINT
signal is caught, causing the current command to be terminated and a
new prompt to be output. [Tru64 UNIX] Causes sh to run as a
restricted shell. Causes sh to read commands from standard input. If
you do not specify the -c option or do not specify any arguments to sh
other than options, sh automatically invokes the -s option. The -c
option overrides the -s option.
[Tru64 UNIX] The rest of the options that can be used with sh are
described under the set subcommand in the subsection Special sh Com‐
mands.
DESCRIPTION
The POSIX shell carries out commands either interactively from a termi‐
nal keyboard or from a file.
Some important features of the shell are as follows: Command aliasing
File name substitution Tilde substitution Command substitution Parame‐
ter substitution Job control Inline editing
A file from which the shell carries out commands is usually called a
shell script, a shell procedure, or a command file.
A simple command is a sequence of words separated by spaces or tabs. A
word is a sequence of characters that contains no unquoted spaces or
tabs. The first word in the sequence (numbered as 0), usually speci‐
fies the name of a command. Any remaining words, with a few excep‐
tions, are passed to that command. A space refers to both spaces and
tabs.
[Tru64 UNIX] The value of a simple command is its exit value if it
ends normally, or (octal) 200 added to the signal number if it termi‐
nates due to a signal. For a list of status values, see the signal(2)
reference page.
A pipeline is a sequence of one or more commands separated by a | (ver‐
tical bar) or, for historical compatibility, by a ^ (circumflex). In a
pipeline, the standard output of each command becomes the standard
input of the next command. Each command runs as a separate process, and
the shell waits for the last command to end. A filter is a command
that reads its standard input, transforms it in some way, then writes
it to its standard output. A pipeline normally consists of a series of
filters. Although the processes in a pipeline (except the first
process) can execute in parallel, they are synchronized to the extent
that each program needs to read the output of its predecessor.
The exit value of a pipeline is the exit value of the last command.
A list is a sequence of one or more pipelines separated by ; (semi‐
colon), & (ampersand), && (two ampersands), or || (two vertical bars)
and optionally ended by a ; (semicolon), an & (ampersand), a |& (copro‐
cess), or a newline. These separators and terminators have the follow‐
ing effects: Causes sequential execution of the preceding pipeline; the
shell waits for the pipeline to finish. Causes asynchronous execution
of the preceding pipeline; the shell does not wait for the pipeline to
finish. Causes the list following it to be executed only if the pre‐
ceding pipeline returns a 0 (zero) exit value. Causes the list follow‐
ing it to be executed only if the preceding pipeline returns a nonzero
exit value.
[Tru64 UNIX] The cd command is an exception; if it returns a
nonzero exit value, no subsequent commands in a list are exe‐
cuted, regardless of the separator characters.
The ; and & separators have equal precedence, as do && and ||. The
single-character separators have lower precedence than the double-char‐
acter separators. An unquoted newline character following a pipeline
functions the same as a ; (semicolon).
Comments
The shell treats as a comment any word that begins with a # character
and ignores that word and all characters following up to the next new‐
line character.
Shell Flow Control Statements
Unless otherwise stated, the value returned by a command is that of the
last simple command executed in the command. Each time a for command
is executed, identifier is set to the next word taken from the in word
list. If in word ... is omitted, the for command executes the do list
once for each positional parameter that is set. (See Parameter Substi‐
tution.) Execution ends when there are no more words in the list.
Prints on standard error (file descriptor 2), the set of words, each
preceded by a number. If in word... is omitted, then the positional
parameters are used instead. (See Parameter Substitution.) The PS3
prompt is printed and a line is read from the standard input. If this
line consists of the number of one of the listed words, then the value
of the parameter identifier is set to the word corresponding to this
number. If this line is empty, the selection list is printed again.
Otherwise, the value of the parameter identifier is set to null. The
contents of the line read from standard input is saved in the REPLY
parameter. The list is executed for each selection until a break or
End-of-File is encountered. Executes the list associated with the
first pattern that matches word. The form of the patterns is the same
as that used for file name generation. (See File name Generation.)
Executes the list following if and, if it returns a 0 (zero) exit sta‐
tus, executes the list following the first then. Otherwise, the list
following elif is executed and, if its value is 0 (zero), the list fol‐
lowing the next then is executed. Failing that, the else list is exe‐
cuted. If no else list or then list is executed, then the if command
returns a 0 (zero) exit status. Executes the while list repeatedly,
and if the exit status of the last command in the list is 0 (zero),
executes the do list; otherwise the loop terminates. If no commands in
the do list are executed, then the while command returns a 0 (zero)
exit status; until can be used in place of while to negate the loop
termination test. Executes list in a separate environment. If two
adjacent open parentheses are needed for nesting, a space must be
inserted to avoid arithmetic evaluation as described later. Executes
list. Unlike the metacharacters ( and ), { and } are reserved words
and must be at the beginning of a line or after a ; (semicolon) in
order to be recognized. Evaluates expression and returns a 0 (zero)
exit status when expression is TRUE. See Conditional Expressions for a
description of expression. Defines a function that is referenced by
identifier. The body of the function is the list of commands between {
and }. (See Functions.) Executes pipeline and prints the elapsed time
as well as the user and system time on standard error.
The following reserved words are recognized only when they appear,
without single or double quotes, as the first word of a command:
if for case then while esac else until function elif do
select fi done time { } [[ ]]
Command Aliasing
The first word of each command is replaced by the text of an alias (if
an alias for this word was defined). The first character of an alias
name can be any nonspecial printable character, but the rest of the
characters must be the same as for a valid identifier. The replacement
string can contain any valid shell script, including the metacharacters
previously listed. The first word of each command in the replaced
text, other than any that are in the process of being replaced, is
tested for aliases. If the last character of the alias value is a
space, the word following the alias will also be checked for alias sub‐
stitution.
Aliases can be used to redefine special built-in commands but cannot be
used to redefine the reserved words previously listed. Aliases can be
created, listed, and exported with the alias command and can be removed
with the unalias command. Exported aliases remain in effect for
scripts invoked by name, but must be reinitialized for separate invoca‐
tions of the shell. (See Invocation.)
Aliasing is performed when scripts are read, not while they are exe‐
cuted. Therefore, for an alias to take effect, the alias definition
command has to be executed before the command that references the alias
is read.
Aliases are frequently used as shorthand for full pathnames. An option
to the aliasing facility allows the value of the alias to be automati‐
cally set to the full pathname of the corresponding command. These
aliases are called tracked aliases.
[Tru64 UNIX] The value of a tracked alias is defined the first time
the corresponding command is looked up and becomes undefined each time
the PATH environment variable is reset. These aliases remain tracked
so that the next subsequent reference will redefine the value. Several
tracked aliases are compiled into the shell. The -h option of the set
command makes each referenced command name into a tracked alias.
[Tru64 UNIX] The following exported aliases are compiled into the
shell, but can be unset or redefined:
autoload='typeset -fu' false='let 0' functions='typeset -f' hash='alias
-t' history='fc -l' integer='typeset -i' nohup='nohup ' r='fc -e -'
true=':' type='whence -v'
Tilde Substitution
After alias substitution is performed, each word is checked to see if
it begins with an unquoted ~ (tilde). If it does, then the word up to
a / (slash) is checked to see if it matches a username in the
/etc/passwd file. If a match is found, the tilde and the matched login
name are replaced by the login directory of the matched user. This is
called a tilde substitution. If no match is found, the original text
is left unchanged. A tilde by itself, or in front of a /, is replaced
by the value of the HOME parameter. A tilde followed by a + (plus
sign) or - (dash) is replaced by $PWD and $OLDPWD, respectively.
In addition, tilde substitution is attempted when the value of a vari‐
able assignment parameter begins with a tilde.
Command Substitution
The standard output from a command enclosed in parentheses preceded by
a dollar sign $( ) or a pair of `` (grave accents) can be used as part
or all of a word; trailing newlines are removed. In the second
(archaic) form, the string between the grave accents is processed for
special quoting characters before the command is executed. (See Quot‐
ing.) The command substitution $(cat file) can be replaced by the
equivalent but faster $(<file). Command substitution of most special
commands that do not perform input/output redirection are carried out
without creating a separate process. An arithmetic expression enclosed
in double parentheses preceded by a dollar sign ( $(( )) ) is replaced
by the value of the arithmetic expression within the double parenthe‐
ses.
Parameter Substitution
A parameter is an identifier, one or more digits, or any of the charac‐
ters *, @, #, ?, -, $, and !. A named parameter (a parameter denoted
by an identifier) has a value and 0 (zero) or more attributes. Named
parameters can be assigned values and attributes by using the typeset
special command. The attributes supported by the shell are described
later with the typeset special command. Exported parameters pass val‐
ues and attributes to the environment.
The shell supports a 1-dimensional array facility. An element of an
array parameter is referenced by a subscript. A subscript is denoted
by an arithmetic expression enclosed with [ ] (brackets). To assign
values to an array, use set -A name value ... The value of all sub‐
scripts must be in the range of 0 to 1023. Arrays need not be
declared. Any reference to a named parameter with a valid subscript is
legal and an array is created if necessary. Referencing an array with‐
out a subscript is equivalent to referencing the element 0 (zero).
The value of a named parameter can be assigned by the following:
name=value [ name=value ]
If the integer attribute, -i, is set for name, the value is subject to
arithmetic evaluation, as described later. Positional parameters,
which are denoted by a number, can be assigned values with the set spe‐
cial command. Parameter $0 is set from argument 0 (zero) when the
shell is invoked. The $ (dollar sign) character is used to introduce
substitutable parameters. Reads all the characters from the ${ (dollar
sign left brace) to the matching } (right brace) as part of the same
word even if it contains braces or metacharacters. The value, if any,
of the parameter is substituted. The braces are required when parame‐
ter is followed by a letter, digit, or underscore that is not to be
interpreted as part of its name or when a named parameter is sub‐
scripted. If parameter is one or more digits, it is a positional
parameter. A positional parameter of more than one digit must be
enclosed in braces. If parameter is * (asterisk) or @ (at sign), all
the positional parameters, starting with $1, are substituted (separated
by a field separator character). If an array identifier with subscript
* or @ is used, the value for each of the elements is substituted (sep‐
arated by a field separator character). Substitutes the number of
positional parameters if parameter is * or @; otherwise, the length of
the value of the parameter is substituted. Substitutes the number of
elements in the array identifier. Substitutes the value of parameter
if it is set and non-null; otherwise, substitute word. Sets parameter
to word if it is not set or is null; the value of the parameter is then
substituted. Positional parameters cannot be assigned values in this
way. Substitutes the value of parameter if it is set and is non-null;
otherwise, print word and exit from the shell. If word is omitted, a
standard message is printed. Substitute word if parameter is set and
is non-null; otherwise, substitute nothing. Causes the value of this
substitution to be the value of parameter with the matched portion
deleted if the shell pattern matches the beginning of the value of
parameter; otherwise the value of parameter is substituted. In the
first form, the smallest matching pattern is deleted and in the second
form, the largest matching pattern is deleted. Causes the value of
this substitution to be the value of parameter with the matched part
deleted if the shell pattern matches the end of the value of parameter;
otherwise, substitute the value of parameter. In the first form, the
smallest matching pattern is deleted and in the second form, the
largest matching pattern is deleted.
If the : (colon) is omitted from the previous expressions, then the
shell checks only whether parameter is set or not.
In the previous expressions, word is not evaluated unless it is to be
used as the substituted string, so that, in the following example, pwd
is executed only if d is not set or is null: echo ${d:-$(pwd)}
The following parameters are automatically set by the shell: The number
of positional parameters in decimal. Flags supplied to the shell on
invocation or by the set command. The decimal value returned by the
last executed command. The process number of this shell. [Tru64
UNIX] Initially, the value of _ (underscore) is an absolute pathname
of the shell or script being executed as passed in the environment.
Subsequently, it is assigned the last argument of the previous command.
This parameter is not set for commands that are asynchronous. This
parameter is also used to hold the name of the matching MAIL file when
checking for mail. The process number of the last background command
invoked. [Tru64 UNIX] The value of errno as set by the most recently
failed system call. This value is system dependent and is intended for
debugging purposes. The line number of the current line within the
script or function being executed. The previous working directory set
by the cd command. The value of the last option argument processed by
the getopts special command. The index of the last option argument
processed by the getopts special command. The process number of the
parent of the shell. The present working directory set by the cd com‐
mand. [Tru64 UNIX] Each time this parameter is referenced, a random
integer, uniformly distributed between 0 and 32767, is generated. The
sequence of random numbers can be initialized by assigning a numeric
value to RANDOM. [Tru64 UNIX] This parameter is set by the select
statement and by the read special command when no arguments are sup‐
plied. [Tru64 UNIX] Each time this parameter is referenced, the num‐
ber of seconds since shell invocation is returned. If this parameter
is assigned a value, then the value returned upon reference is the
value that was assigned plus the number of seconds since the assign‐
ment.
The following parameters are used by the shell: The search path for the
cd command. If this variable is set, the value is used to define the
width of the edit window for the shell edit modes and for printing
select lists. If the value of this variable ends in emacs, gmacs, or
vi and the VISUAL variable is not set, then the corresponding option
(see set under Special sh Commands) is turned on. If this parameter is
set, then parameter substitution is performed on the value to generate
the pathname of the script that is executed when the shell is invoked.
(See Invocation.) This file is typically used for alias and function
definitions. The default editor name for the fc command. [Tru64
UNIX] The search path for function definitions. This path is searched
when a function with the -u attribute is referenced and when a command
is not found. If an executable file is found, then it is read and exe‐
cuted in the current environment. Internal field separators, normally
spaces, tabs, and newlines that are used to separate command words
which result from command or parameter substitution and for separating
words with the read special command. The first character of the IFS
parameter is used to separate arguments for the $* substitution. (See
Quoting.) If this parameter is set when the shell is invoked, then the
value is the pathname of the file that is used to store the command
history. (See Command Reentry.) If this parameter is set when the
shell is invoked, the number of previously entered commands that are
accessible by this shell is greater than or equal to this number. The
default is 128. The default argument (home directory) for the cd com‐
mand. Specifies the locale of your system, which is comprised of three
parts: language, territory, and codeset. The default locale is the C
locale, which specifies the value English for language, U.S. for ter‐
ritory, and ASCII for codeset. The locale specified for the LANG vari‐
able controls the language applied to messages. Unless set explicitly,
the LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and
LC_TIME variables also derive their settings from the locale set for
LANG. Specifies the collating sequence to use when sorting names and
when character ranges occur in patterns. The default value is the col‐
lating sequence for American English. If absent, the collating
sequence can be taken from the LANG parameter. If both LC_COLLATE and
LANG are absent, the ANSI C collating sequence is used. Specifies the
character classification information to use on your system. The
default value is American English. Specifies the language that the
system expects for user input of yes and no strings. The default value
is American English. Specifies the monetary format for your system.
The default value is the monetary format for American English. Speci‐
fies the numeric format for your system. The default value is the
numeric format for American English. Specifies the date and time for‐
mat for your system. The default value is the date and time format for
American English. [Tru64 UNIX] If this variable is set, the value is
used to determine the column length for printing select lists. Select
lists will print vertically until about two-thirds of LINES lines are
filled. [Tru64 UNIX] The name of the user's login account correspond‐
ing to the login name in the user database. If this parameter is set
to the name of a mail file and the MAILPATH parameter is not set, the
shell informs you of the arrival of mail in the specified file. This
variable specifies how often (in seconds) the shell checks for changes
in the modification time of any of the files specified by the MAILPATH
or MAIL parameters. The default value is 600 seconds. When the time
has elapsed, the shell checks before issuing the next prompt. A list
of file names separated by : (colons). If this parameter is set, the
shell informs you of any modifications to the specified files that have
occurred within the last MAILCHECK seconds. Each file name can be fol‐
lowed by a ? (question mark) and a message that is printed. The mes‐
sage will undergo parameter substitution with the parameter, $_ defined
as the name of the file that has changed. The default message is you
have mail in $_. Specifies a list of directories to search to find
message catalogs. The search path for commands. (See Execution.) The
value of this parameter is expanded for parameter substitution to
define the primary prompt string which by default is the $ (dollar
sign). The ! (exclamation point) in the primary prompt string is
replaced by the command number. (See Command Reentry.) Secondary
prompt string, by default > (right angle bracket). Selection prompt
string used within a select loop, by default #? (number sign, question
mark). The value of this parameter is expanded for parameter substitu‐
tion and precedes each line of an execution trace. If omitted, the
execution trace prompt is + (plus sign). [Tru64 UNIX] The pathname of
the shell is kept in the environment. [Tru64 UNIX] If set to a value
greater than 0 (zero), the shell terminates if a command is not entered
within the prescribed number of seconds after issuing the PS1 prompt.
(Note that the shell can be compiled with a maximum bound for this
value that cannot be exceeded.) If the value of this variable ends in
emacs, gmacs, or vi, the corresponding option (see the set command in
Special sh Commands) is turned on.
[Tru64 UNIX] The shell gives default values to PATH, PS1, PS2,
MAILCHECK, TMOUT, and IFS, while HOME, SHELL, ENV, and MAIL are not set
by the shell (although HOME is set by the login command). On some sys‐
tems, MAIL and SHELL are also set by the login command.
Interpretation of Spaces
After parameter and command substitution, the results of substitutions
are scanned for the field separator characters (those found in IFS),
and split into distinct arguments where such characters are found.
Explicit null arguments (`` or '') are retained. Implicit null argu‐
ments (those resulting from parameters that have no values) are
removed.
File name Generation
Following substitution, each command word is scanned for the characters
* (asterisk), ? (question mark), and [ ] (brackets), unless the -f
option was set. If one of these characters appears, the word is
regarded as a pattern. The word is replaced with lexicographically
sorted file names that match the pattern. If no file name is found
that matches the pattern, the word is left unchanged. When a pattern
is used for file name generation, the (dot) character at the start of a
file name or immediately following a / (slash), as well as the / char‐
acter itself, must be matched explicitly. In other instances of pat‐
tern matching, the / and are not treated specially. Matches any
string, including the null string. Matches any single character.
Matches any one of the enclosed characters. In an expression such as
[a-z], the - (dash) means “through” according to the current collating
sequence. The collating sequence is determined by the value of the
LC_COLLATE environment variable. If the first character following the [
(left bracket) is a ! (exclamation point), then any character not
enclosed is matched. A - can be included in the character set by
putting it as the first or last character.
A pattern_list is a list of one or more patterns separated from each
other with a | (vertical bar). Composite patterns can be formed with
one or more of the following: Optionally matches any one of the given
patterns. Matches zero or more occurrences of the given patterns.
Matches one or more occurrences of the given patterns. Matches exactly
one of the given patterns. Matches anything, except one of the given
patterns.
Character Classes
You can use the following notation to match file names within a range
indication:
[:charclass:]
This format instructs the system to match any single character belong‐
ing to charclass; the defined classes correspond to ctype() subroutines
as follows:
alnum alpha cntrl digit graph lower print punct space upper xdigit
Your locale might define additional character properties, such as the
following:
[:vowel:]
The preceding character class could be TRUE for a, e, i, o, u, or y.
You could then use [:vowel] inside a set construction to match any
vowel. Refer to The LC_CTYPE Category section of the locale file for‐
mat reference page for more information.
Quoting
The following characters have a special meaning to the shell and cause
termination of a word unless quoted:
; & ( ) | ^ < > <newline> <space> <tab>
Each of the metacharacters previously listed has a special meaning to
the shell and causes termination of a word unless quoted. A character
can be quoted (that is, made to stand for itself) by preceding it with
a \ (backslash). The pair \newline is ignored. All characters enclosed
between a pair of '' (single quotes) are quoted. A single quote cannot
appear within single quotes.
Inside "" (double quotes) parameter and command substitution occurs and
\ quotes the characters \, `, ', and $. The meaning of $* and $@ is
identical when not quoted or when used as a parameter assignment value
or as a file name. However, when used as a command argument, '$*' is
equivalent to '$1d$2d. . .', where d is the first character of the IFS
parameter, whereas '$@' is equivalent to '$1' '$2' . . . Inside ``
(grave accents) \ (backslash) quotes the characters \, `, and $. If
the grave accents occur within double quotes, then \ also quotes the '
(single quote) character.
The special meaning of reserved words or aliases can be removed by
quoting any character of the reserved word. The recognition of func‐
tion names or special command names listed later cannot be altered by
quoting them.
Arithmetic Evaluation
[Tru64 UNIX] An ability to perform integer arithmetic is provided with
the let special command. Evaluations are performed using long arith‐
metic. Constants are of the form [base#]n, where base is a decimal
number between 2 and 36 representing the arithmetic base and n is a
number in that base. If base is omitted, then base 10 is used.
[Tru64 UNIX] An arithmetic expression uses the same syntax, prece‐
dence, and associativity of expression as the C language. All the
integral operators, other than ++, --, ?:, and , are supported. Named
parameters can be referenced by name within an arithmetic expression
without using the parameter substitution syntax. When a named parameter
is referenced, its value is evaluated as an arithmetic expression.
An internal integer representation of a named parameter can be speci‐
fied with the -i option of the typeset special command. Arithmetic
evaluation is performed on the value of each assignment to a named
parameter with the -i attribute. If you do not specify an arithmetic
base, the first assignment to the parameter determines the arithmetic
base. This base is used when parameter substitution occurs.
Because many of the arithmetic operators require quoting, an alterna‐
tive form of the let command is provided. For any command that begins
with a ((, all the characters until a matching )) are treated as a
quoted expression. More precisely, ((...)) is equivalent to let
"...".
Note that ((...)) is a command with a return value, whereas $((...))
is the way to put the string representation of the value of an arith‐
metic expression into the command line (that is, it is like a $ vari‐
able).
Prompting
When used interactively, the shell prompts with the value of PS1 before
reading a command. If at any time a newline is typed and further input
is needed to complete a command, then the secondary prompt (that is,
the value of PS2) is issued.
Conditional Expressions
A conditional expression is used with the [[ compound command to test
attributes of files and to compare strings. Word splitting and file
name generation are not performed on the words between [[ and ]]. Each
expression can be constructed from one or more of the following unary
or binary expressions: [Tru64 UNIX] TRUE, if file exists. [Tru64
UNIX] TRUE, if file exists and is a block-special file. [Tru64
UNIX] TRUE, if file exists and is a character-special file. [Tru64
UNIX] TRUE, if file exists and is a directory. [Tru64 UNIX] TRUE, if
file exists. [Tru64 UNIX] TRUE, if file exists and is an ordinary
file. [Tru64 UNIX] TRUE, if file exists and has its setgid bit set.
[Tru64 UNIX] TRUE, if file exists and its group matches the effective
group ID of this process. [Tru64 UNIX] TRUE, if file exists and has
its sticky bit set. [Tru64 UNIX] TRUE, if file exists and is a sym‐
bolic link. [Tru64 UNIX] TRUE, if length of string is nonzero.
[Tru64 UNIX] TRUE, if option named option is on. [Tru64 UNIX] TRUE,
if file exists and is owned by the effective user ID of this process.
[Tru64 UNIX] TRUE, if file exists and is a FIFO special file or a
pipe. [Tru64 UNIX] TRUE, if file exists and is readable by current
process. [Tru64 UNIX] TRUE, if file exists and has size greater than
0 (zero). [Tru64 UNIX] TRUE, if file exists and is a socket. [Tru64
UNIX] TRUE, if file descriptor number file_des is open and associated
with a terminal device. [Tru64 UNIX] TRUE, if file exists and has its
setuid bit set. [Tru64 UNIX] TRUE, if file exists and is writable by
current process. [Tru64 UNIX] TRUE, if file exists and is executable
by current process. If file exists and is a directory, then the cur‐
rent process has permission to search in the directory. [Tru64
UNIX] TRUE, if length of string is 0 (zero). [Tru64 UNIX] TRUE, if
file1 exists and is newer than file2. [Tru64 UNIX] TRUE, if file1
exists and is older than file2. [Tru64 UNIX] TRUE, if file1 and file2
exist and refer to the same file. [Tru64 UNIX] TRUE, if string
matches pattern. [Tru64 UNIX] TRUE, if string does not match pattern.
[Tru64 UNIX] TRUE, if string1 collates before string2. [Tru64
UNIX] TRUE, if string1 collates after string2. [Tru64 UNIX] TRUE, if
expression1 is equal to expression2. [Tru64 UNIX] TRUE, if expres‐
sion1 is not equal to expression2. [Tru64 UNIX] TRUE, if expression1
is less than expression2. [Tru64 UNIX] TRUE, if expression1 is
greater than expression2. [Tru64 UNIX] TRUE, if expression1 is less
than or equal to expression2. [Tru64 UNIX] TRUE, if expression1 is
greater than or equal to expression2.
[Tru64 UNIX] A compound expression can be constructed from these prim‐
itives by using any of the following, listed in decreasing order of
precedence. [Tru64 UNIX] TRUE, if expression is TRUE. Used to group
expressions. [Tru64 UNIX] TRUE if expression is FALSE. [Tru64
UNIX] TRUE, if expression1 and expression2 are both TRUE. [Tru64
UNIX] TRUE, if either expression1 or expression2 is TRUE.
Input/Output
Before a command is executed, you can redirect its input and output by
using a special notation interpreted by the shell. The following can
appear anywhere in a simple command or can precede or follow a command
and are not passed on to the invoked command. Command and parameter
substitution occurs before word or digit is used, except as noted in
the following text. File name generation occurs only if the pattern
matches a single file and interpretation of spaces is not performed.
Use file word as standard input (file descriptor 0). Use file word as
standard output (file descriptor 1). If the file does not exist, it is
created. If the file exists, and the noclobber option is on, this
causes an error; otherwise, it is truncated to 0 (zero) length. Same
as >, except that it overrides the noclobber option. Use file word as
standard output. If the file exists, output is appended to it (by
first seeking to the End-of-File); otherwise, the file is created.
Open file word for reading and writing as standard input. The shell
input is read up to a line that is the same as word, or to an End-of-
File. No parameter substitution, command substitution, or file name
generation is performed on word. The resulting document, called a here
document, becomes the standard input. If any character of word is
quoted, then no interpretation is placed upon the characters of the
document; otherwise, parameter and command substitution occurs, \new‐
line is ignored, and \ must be used to quote the characters \, $, `,
and the first character of word. If - is appended to <<, then all lead‐
ing tabs are stripped from word and from the document. The standard
input is duplicated from file descriptor digit (see the dup(2) refer‐
ence page). The standard output is duplicated using >& digit. The
standard input is closed. The standard output is closed using >&-.
The input from the coprocess (or background process) is moved to stan‐
dard input. The output to the coprocess is moved to standard output.
If one of the preceding redirections is preceded by a digit, then the
file descriptor number referred to is that specified by the digit
(instead of the default 0 or 1). For example:
... 2>&1
means file descriptor 2 is to be opened for writing as a duplicate of
file descriptor 1.
The order in which redirections are specified is significant. The shell
evaluates each redirection in terms of the (file descriptor, file)
association at the time of evaluation. For example:
... 1>fname 2>&1
first associates file descriptor 1 with file fname. It then associates
file descriptor 2 with the file associated with file descriptor 1 (that
is, fname). If the order of redirections were reversed, file descrip‐
tor 2 is associated with the terminal (assuming file descriptor 1 is)
and then file descriptor 1 is associated with file fname.
If a command is followed by & and job control is not active, the
default standard input for the command is the empty /dev/null file.
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by input/output
specifications.
Environment
The environment is a list of name-value pairs that is passed to an exe‐
cuted program in the same way as a normal argument list. The names
must be identifiers and the values are character strings. The shell
interacts with the environment in several ways. On invocation, the
shell scans the environment and creates a parameter for each name
found, giving it the corresponding value and marking it export. Exe‐
cuted commands inherit the environment. If you modify the values of
these parameters or create new ones, using the export or typeset -x
commands, they become part of the environment. The environment seen by
any executed command is thus composed of any name-value pairs origi‐
nally inherited by the shell, whose values can be modified by the cur‐
rent shell, plus any additions that must be noted in the export or
typeset -x commands.
[Tru64 UNIX] When the value of an exported parameter is changed, the
shell automatically exports the new value to all child processes. This
behavior is different from that of the Bourne shell, sh(1b), which does
not automatically reexport a changed parameter.
You can augment the environment for any simple command or function by
prefixing it with one or more parameter assignments. A parameter
assignment argument is a word of the form identifier=value.
Thus, the following two expressions are equivalent (as far as the exe‐
cution of command is concerned):
TERM=450 command argument ...
(export TERM; TERM=450; command argument ...)
[Tru64 UNIX] If the -k option is set, all parameter assignment argu‐
ments are placed in the environment, even if they occur after the com‐
mand name. The following first prints a=b c and then c: echo a=b c set
-k echo a=b c
[Tru64 UNIX] This feature is intended for use with scripts written for
early versions of the shell; its use in new scripts is strongly dis‐
couraged. It is likely to disappear someday.
Functions
The function reserved word is used to define shell functions. Shell
functions are read in and stored internally. Alias names are resolved
when the function is read. Functions are executed like commands with
the arguments passed as positional parameters. (See Execution.)
Functions execute in the same process as the caller and share all files
and the present working directory with the caller. Traps caught by the
caller are reset to their default action inside the function. A trap
condition that is not caught or ignored by the function causes the
function to terminate and the condition to be passed on to the caller.
A trap on EXIT set inside a function is executed after the function
completes in the environment of the caller. Ordinarily, variables are
shared between the calling program and the function. However, the spe‐
cial command typeset used within a function defines local variables
whose scope includes the current function and all functions it calls.
The special command return is used to return from function calls.
Errors within functions return control to the caller.
Function identifiers can be listed with the -f or +f option of the
typeset special command. The text of functions is also listed with -f.
Function can be undefined with the -f option of the unset special com‐
mand.
Ordinarily, functions are unset when the shell executes a shell script.
The -xf option of the typeset command allows a function to be exported
to scripts that are executed without a separate invocation of the
shell. Functions that need to be defined across separate invocations
of the shell should be specified in the ENV file with the -xf option of
typeset.
Jobs
If the monitor option of the set command is turned on, an interactive
shell associates a job with each pipeline. It keeps a table of current
jobs, printed by the jobs command, and assigns them small integer num‐
bers. When a job is started asynchronously with &, the shell prints a
line that looks like: [1] 1234
This line indicates that the job, which was started asynchronously, was
job number 1 and had one (top-level) process, whose process ID was
1234.
If you are running a job and want to do something else, you can enter
the Suspend key sequence (normally <Ctrl-z>, which sends a SIGINT sig‐
nal to the current job. The shell then normally indicates that the job
has been stopped, and it prints another prompt. You can then manipu‐
late the state of this job, putting it in the background with the bg
command, or run some other commands and then eventually bring the job
back into the foreground with the foreground command fg. The job sus‐
pension takes effect immediately, and corresponds to the Interrupt key
sequence in that pending output and unread input are discarded. A spe‐
cial key sequence, <Ctrl-y>, does not generate a SIGINT signal until a
program attempts to read it. (See the read(2) reference page for more
information.) This key sequence can be typed ahead when you have pre‐
pared some commands for a job that you wish to stop after it has read
them.
A job being run in the background will stop if it tries to read from
the terminal. Background jobs are normally allowed to produce output,
but this can be disabled by issuing the stty tostop command. If you
set this terminal option, then background jobs will stop when they try
to produce output like they do when they try to read input.
There are several ways to refer to jobs in the shell. A job can be
referred to by the process ID of any process of the job, or by one of
the following: The job with the given number. Any job whose command
line begins with string. Any job whose command line contains string.
Current job. Equivalent to %%. Previous job.
This shell learns immediately whenever a process changes state. It nor‐
mally informs you whenever a job becomes blocked so that no further
progress is possible, but only just before it prints a prompt. This is
done so that it does not otherwise disturb your work.
When the monitor mode is on, each background job that is completed
triggers any trap set for CHLD.
When you try to leave the shell while jobs are stopped or running, you
are warned that You have stopped(running) jobs. You can use the jobs
command to see what they are. If you do this or immediately try to
exit again, the shell does not warn you a second time, and the stopped
jobs are terminated.
Signals
The SIGINT and SIGQUIT signals for an invoked command are ignored if
the command is followed by & and job monitor option is not active.
Otherwise, signals have the values inherited by the shell from its par‐
ent (but see also the trap command).
Execution
Each time a command is executed, the previous substitutions are carried
out. If the command name matches one of the special commands listed
later, it is executed within the current shell process. Next, the com‐
mand name is checked to see if it matches one of the user-defined func‐
tions. If it does, the positional parameters are saved and then reset
to the arguments of the function call. When the function is completed
or issues a return, the positional parameter list is restored and any
trap set on EXIT within the function is executed. The value of a func‐
tion is the value of the last command executed. A function is also
executed in the current shell process. If a command name is not a spe‐
cial command or a user-defined function, a process is created and an
attempt is made to execute the command via exec.
The PATH shell parameter defines the search path for the directory con‐
taining the command. Alternative directory names are separated by a :
(colon). The default path is :/usr/bin: (specifying /usr/bin, and the
current directory in that order). The current directory can be speci‐
fied by two or more adjacent colons, or by a colon at the beginning or
end of the path list. If the command name contains a / (slash), then
the search path is not used. Otherwise, each directory in the path is
searched for an executable file.
If the file has execute permission but is not a directory or an a.out
file, it is assumed to be a file containing shell commands. A subshell
is spawned to read it. All nonexported aliases, functions, and named
parameters are removed in this case. If the shell command file does
not have read permission, or if the setuid and/or setgid bits are set
on the file, the shell executes an agent whose job it is to set up the
permissions and execute the shell with the shell command file passed
down as an open file. A command in parentheses is executed in a sub‐
shell without removing nonexported quantities.
Command Reentry
The text of the last HISTSIZE (default 128) commands entered from a
terminal device is saved in a history file. The $HOME/.sh_history file
is used if the HISTFILE variable is not set or is not writable. A
shell can access the commands of all interactive shells that use the
same named HISTFILE. The fc special command is used to list or edit a
portion of this file. The portion of the file to be edited or listed
can be selected by number or by giving the first character or charac‐
ters of the command. A single command or range of commands can be
specified. If you do not specify an editor program as an argument to
fc, then the value of the FCEDIT parameter is used. If FCEDIT is not
defined, then /usr/bin/ed is used. The edited commands are printed and
reexecuted upon leaving the editor. The editor name - (dash) is used
to skip the editing phase and to reexecute the command. In this case,
a substitution parameter of the form old=new can be used to modify the
command before execution. For example, if r is aliased to 'fc -e -',
then typing `r bad=good c' reexecutes the most recent command, which
starts with the letter c, replacing the first occurrence of the string
bad with the string good.
Inline Editing Options
Normally, each command line entered from a terminal device is simply
typed followed by a newline (<Return> or linefeed). If the emacs,
gmacs, or vi option is active, you can edit the command line. To be in
any of these edit modes, set the corresponding option. An editing
option is automatically selected each time the VISUAL or EDITOR vari‐
able is assigned a value ending in either of these option names.
[Tru64 UNIX] The editing features require that the terminal accept
<Return> as carriage-return without linefeed and that a space must
overwrite the current character on the screen. ADM terminal users
should set the space-advance switch to Space. Hewlett-Packard series
2621 terminal users should set the straps to bcGHxZ etX.
[Tru64 UNIX] The editing modes create the impression that the user is
looking through a window at the current line. The window width is the
value of COLUMNS if it is defined, otherwise it is 80 bytes. If the
line is longer than the window width minus 2, a mark is displayed at
the end of the window to notify the user. As the cursor moves and
reaches the window boundaries, the window is centered about the cursor.
The mark is a > (right angle bracket) if the line extends on the right
side of the window, a < (left angle bracket) if the line extends on the
left side of the window, and an * (asterisk) if the line extends on
both sides of the window.
[Tru64 UNIX] The search commands in each edit mode provide access to
the history file. Only strings are matched, not patterns, although if
the leading character in the string is a ^ (circumflex), the match is
restricted to begin at the first character in the line.
The emacs Editing Mode
This mode is entered by enabling either the emacs or gmacs option. The
only difference between these two modes is the way they handle <Ctrl-
t>. To edit, the user moves the cursor to the point needing correction
and then inserts or deletes characters or words as needed. All the
editing commands are control characters or escape sequences. The nota‐
tion for control characters is ^ (circumflex) followed by the charac‐
ter. For example, ^F is the notation for <Ctrl-f>. This is entered by
pressing f while holding down <Ctrl>. <Shift> is not depressed. (The
notation ^? indicates <Delete>.)
The notation for escape sequences is M- followed by a character. For
example, M-f (pronounced Meta f) is entered by pressing <Esc> (ASCII
033) followed by f. (M-F would be the notation for <Esc> followed by
<Shift> (capital) F.)
All edit commands operate from any place on the line (not just at the
beginning). Do not press <Return> or linefeed after edit commands
except when noted. Moves the cursor forward (right) one character.
Moves the cursor forward one word. (The emacs editor's definition of a
word is a string of characters, consisting of only letters, digits, and
underscores, and delimited with spaces or tabs.) Moves the cursor
backward (left) one character. Moves the cursor backward one word.
Moves the cursor to the start of the line. Moves the cursor to the end
of the line. Moves the cursor forward on the current line to the char‐
acter indicated by the character argument. Moves the cursor backward
on the current line to the character indicated by the character argu‐
ment. Interchanges the cursor and mark. Deletes the previous charac‐
ter. (User-defined Erase character as defined by the stty command,
often <Ctrl-h> or #.) Deletes the current character. Deletes the cur‐
rent word. Deletes the previous word. Deletes the previous word.
Deletes the previous word (if your Interrupt character is <Delete>,
this command does not work). Transposes the current character with
next character in emacs mode. Transposes two previous characters in
gmacs mode. Capitalizes the current character. Capitalizes the cur‐
rent word. Changes the current word to lowercase. Deletes from the
cursor to the end of the line. If preceded by a numerical parameter
whose value is less than the current cursor position, deletes from
given position up to the cursor. If preceded by a numerical parameter
whose value is greater than the current cursor position, deletes from
the cursor up to given cursor position. Deletes from the cursor to the
mark. Pushes the region from the cursor to the mark on the stack.
Kills the entire current line. If two Kill characters are entered in
succession, all Kill characters from then on cause a linefeed (useful
when using paper terminals). (User-defined Kill character as defined
by the stty command, often <Ctrl-g> or @.) Restores the last item
removed from the line. (Yanks the item back to the line.) Performs a
linefeed and prints the current line. (Null character.) Sets the
mark. Sets the mark. Executes the current line (newline). Executes
the current line (enter). The End-of-File character is processed as an
End-of-File only if the current line is null. Fetches the previous
command. Each time <Ctrl-p> is entered, the previous command back in
time is accessed. Moves back one line when not on the first line of a
multiline command. Fetches the least recent (oldest) history line.
Fetches the most recent (youngest) history line. Fetches the next com‐
mand line. Each time <Ctrl-n> is entered, the next command line for‐
ward in time is accessed. Reverses the search history for a previous
command line containing string. If an argument of 0 (zero) is given,
the search is forward. The string variable is terminated by a <Return>
or newline character. If string is preceded by a ^ (circumflex), the
matched line must begin with string. If string is omitted, then the
next command line containing the most recent string is accessed. In
this case, an argument of 0 (zero) reverses the direction of the
search. Executes the current line and fetches the next line relative
to current line from the history file. (Operate) Defines the numeric
parameter (escape). The digits are taken as an argument to the next
command. The commands that accept a parameter are <Ctrl-f>, <Ctrl-b>,
<Erase>, <Ctrl-c>, <Ctrl-d>, <Ctrl-k>, <Ctrl-r>, <Ctrl-p>, <Ctrl-n>,
<Ctrl-]>, <Esc-.>, <Esc-Ctrl-]>, <Esc-_>, <Esc-b>, <Esc-c>, <Esc-d>,
<Esc-f>, <Esc-h>, <Esc-l> and <Esc-Ctrl-h>. Your alias list is
searched for an alias by the name _letter and if an alias of this name
is defined, its value is inserted on the input queue. The letter must
not be one of the preceding metafunctions. (Soft-key) [Tru64
UNIX] Your alias list is searched for an alias by the name __letter
and if an alias of this name is defined, its value is inserted on the
input queue (Soft-key). This can be used to program functions keys on
many systems. [Tru64 UNIX] Same as <Esc-]> letter. The last word of
the previous command is inserted on the line. If preceded by a numeric
parameter, the value of this parameter determines which word to insert,
rather than the last word. Same as the <Esc-.> combination. Attempts
file name generation on the current word. An * (asterisk) is appended
if the word does not match any file or contain any special pattern
characters. File name completion. Replaces the current word with the
longest common prefix of all file names matching the current word with
an asterisk appended. If the match is unique, a / (slash) is appended
if the file is a directory, and a space is appended if the file is not
a directory. Lists the files matching current word pattern if an *
(asterisk) were appended. Multiplies the argument of the next command
by four. Escapes the next character. Editing characters, the user's
Erase, Kill, and Interrupt (normally by using <Delete>) characters can
be entered in a command line or in a search string if preceded by a \
(backslash). The backslash removes the next character's editing fea‐
tures (if any). Displays the version of the shell. Inserts a # (num‐
ber sign) at the beginning of the line and executes it. This causes a
comment to be inserted in the history file.
The vi Editing Mode
There are two typing modes. Initially, when you enter a command you
are in the input mode. To edit, the user enters control mode by typing
<Esc> (ASCII 033) and moves the cursor to the place needing correction
and then inserts or deletes characters or words as needed. Most con‐
trol commands accept an optional repeat count prior to the command.
When in vi mode on most systems, canonical processing is initially
enabled and the command is echoed again if the speed is 1200 baud or
greater, if it contains any control characters, or if less than 1 sec‐
ond has elapsed since the prompt was printed. The Escape character
terminates canonical processing for the remainder of the command and
the user can then modify the command line.
[Tru64 UNIX] This scheme has the advantages of canonical processing
with the type-ahead echoing of raw mode. If the option viraw is also
set, the terminal always has canonical processing disabled. This mode
is implicit for systems that do not support two alternate End-of-Line
delimiters, and can be helpful for certain terminals.
Input Edit Commands
By default the editor is in input mode. (User-defined Erase character
as defined by the stty command, often <Ctrl-h> or #.) Deletes the pre‐
vious character. Deletes the previous space-separated word. Termi‐
nates the shell. Escapes the next character. Editing characters and
the user's Erase or Kill characters can be entered in a command line or
in a search string if preceded by a <Ctrl-v>. <Ctrl-v> removes the
next character's editing features (if any). Escapes the next Erase or
Kill character.
Motion Edit Commands
These commands move the cursor: Cursor forward (right) one character.
Cursor forward one word. A word is a string of characters delimited by
spaces or tabs. Cursor to the beginning of the next word that follows
a space. Cursor to the end of the word. Cursor to end of the current
space-delimited word. Cursor backward (left) one character. Cursor
backward one word. Cursor to the preceding space-delimited word. Cur‐
sor to the column count. Finds the next character c in the current
line. Finds the previous character c in the current line. Equivalent
to f followed by h. Equivalent to F followed by l. Repeats count
times, the last single character find command: f, F, t, or T. Reverses
the last single character find command count times. Cursor to the
start of the line. Cursor to the first nonspace character in the line.
Cursor to the end of the line.
Search Edit Commands
These commands access your command history. Fetches the previous com‐
mand. Each time k is entered, the previous command back in time is
accessed. Equivalent to k. Fetches the next command. Each time j is
entered, the next command forward in time is accessed. Equivalent to
j. Fetches the command number count. The default is the least recent
history command. Searches backward through history for a previous com‐
mand containing the specified string. The string variable is termi‐
nated by <Return> or a newline character. If the specified string is
preceded by a ^ (circumflex), the matched line must begin with string.
If string is null, the previous string is used. Same as / (slash)
except that the search is in the forward direction. Searches for next
match of the last pattern to the / or ? commands. Searches for next
match of the last pattern to the / or ? commands, but in reverse
direction. Searches the command history for the string entered by the
previous / command.
Text Modification Edit Commands
These commands modify the line. Enters input mode and enters text
after the current character. Appends text to the end of the line.
Equivalent to $a. Deletes the current character through the character
to which motion would move the cursor, and enters input mode. If
motion is c, the entire line is deleted and input mode is entered.
Deletes the current character through the end of line, and enters input
mode. Equivalent to c$. Equivalent to cc. Deletes the current char‐
acter through the end of line. Equivalent to d$. Deletes the current
character through the character to which motion would move. If motion
is d, the entire line is deleted. Enters input mode and inserts text
before the current character. Inserts text before the beginning of the
line. Equivalent to 0i. Places the previous text modification before
the cursor. Places the previous text modification after the cursor.
Enters input mode and replaces characters on the screen with the char‐
acters you type, overlay fashion. Replaces the count characters,
starting at the current cursor position with c and advancing the cur‐
sor. Deletes the current character. Deletes the preceding character.
Repeats the previous text modification command. Inverts the case of
the count characters, starting at the current cursor position and
advancing the cursor. Causes the count word of the previous command to
be appended and input mode entered. The last word is used if count is
omitted. Causes an * (asterisk) to be appended to the current word and
file name generation to be attempted. If no match is found, it rings
the bell. Otherwise, the word is replaced by the matching pattern and
input mode is entered. File name completion. Replaces the current
word with the longest common prefix of all file names matching the cur‐
rent word with an * (asterisk) appended. If the match is unique, a /
(slash) is appended if the file is a directory; a space is appended if
the file is not a directory.
Miscellaneous vi Commands
Yanks the current character through the character to which motion would
move the cursor and puts the characters into the delete buffer. The
text and cursor are unchanged. Yanks from current position to the end
of line. Equivalent to y$. Undoes the last text-modifying command.
Undoes all the text-modifying commands performed on the line. Returns
the command fc -e ${VISUAL:-${EDITOR:-vi}} count in the input buffer.
If count is omitted, the current line is used. Performs a linefeed and
prints the current line. Effective only in control mode. Executes the
current line, regardless of mode (newline). Executes the current line,
regardless of mode (enter). Sends the line after inserting a # (number
sign) in front of the line. Useful for causing the current line to be
inserted in the history without being executed. Lists the file names
that match the current word if an * (asterisk) is appended to it.
Searches the alias list for an alias by the name _letter . If an alias
of this name is defined, its value is inserted in the input queue for
processing.
Special sh Commands
The following simple commands are executed in the shell process.
Input/output redirection is permitted. Unless otherwise indicated, the
output is written on file descriptor 1 and the exit status, when there
is no syntax error, is 0 (zero).
Commands that are indicated as command1 or command2 are treated spe‐
cially in the following ways: Parameter assignment lists that precede
the command remain in effect when the command completes. I/O redirec‐
tions are processed after parameter assignments. Errors cause a script
that contains the commands so marked to abort. Words, following a com‐
mand specified as command2 that are in the format of a parameter
assignment, are expanded with the same rules as a parameter assignment.
This means that ~ (tilde) substitution is performed after the = (equal
sign). Word splitting and file name generation are not performed. The
command only expands arguments. Reads the complete file and executes
the commands. The commands are executed in the current shell environ‐
ment. The search path specified by PATH is used to find the directory
containing file. If any arguments are specified, they become the posi‐
tional parameters. Otherwise, the positional parameters are unchanged.
The exit status is the exit status of the last command executed. See
the alias(1) reference page. See the bg(1) reference page. Exits from
the enclosing for, while, until, or select loop, if any. If n is spec‐
ified, breaks n levels. Resumes the next iteration of the enclosing
for, while, until, or select loop. If n is specified, resumes at the
nth enclosing loop. See the cd(1) reference page. See the echo(1)
reference page. The arguments are read as input to the shell and the
resulting commands are executed. If argument is given, the command
specified by the arguments is executed in place of this shell without
creating a new process. Input/output arguments can appear and affect
the current process. If no arguments are given, the effect of this
command is to modify file descriptors as prescribed by the input/output
redirection list. In this case, any file descriptor numbers greater
than 2 that are opened with this mechanism are closed when invoking
another program. Causes the shell to exit with the exit status speci‐
fied by n. If n is omitted, the exit status is that of the last com‐
mand executed. An End-of-File also causes the shell to exit, except
for a shell which has the ignoreeof option (see set) turned on. The
given names are marked for automatic export to the environment of sub‐
sequently executed commands. The export -p command outputs the names
and values of all exported variables, one per line, in the format:
export variable=value See the fc(1) reference page. See the fg(1) ref‐
erence page. See the getopts(1) reference page. See the hash(1) ref‐
erence page. [Tru64 UNIX] This command is no longer supported. See
the loader(5) reference page for information on using shared libraries.
See the jobs(1) reference page. Set the kill(1) reference page. Each
argument is a separate arithmetic expression to be evaluated. (See
Arithmetic Evaluation for a description of arithmetic expression evalu‐
ation.) The exit status is 0 (zero) if the value of the last expres‐
sion is nonzero, and 1 otherwise. See the newgrp(1) reference page.
[Tru64 UNIX] The shell output mechanism. With no options or with
option - or --, the arguments are printed on standard output as
described by echo. In raw mode, -R or -r, the escape conventions of
echo are ignored. The -R option prints all subsequent arguments and
options other than -n.
The -p option causes the arguments to be written to the pipe of
the process spawned with |& instead of standard output. The -s
option causes the arguments to be written onto the history file
instead of standard output. The -u option can be used to spec‐
ify a 1-digit file descriptor unit number n on which the output
is placed. The default is 1. If the -n option is used, no new‐
line is added to the output. [Tru64 UNIX] Equivalent to print
-r - $PWD. [Tru64 UNIX] The shell input mechanism. One line
is read and is broken up into fields using the characters in IFS
as separators. In raw mode, a \ (backslash) at the end of a
line does not signify line continuation. The first field is
assigned to the first name, the second field to the second name,
and so on, with leftover fields assigned to the last name. The
-p option causes the input line to be taken from the input pipe
of a process spawned by the shell using |&. If the -s option is
present, the input is saved as a command in the history file.
The -u option can be used to specify a 1-digit file descriptor
unit to read from. The file descriptor can be opened with the
exec special command. The default value of n is 0 (zero). If
name is omitted, REPLY is used as the default name. The exit
status is 0 (zero) unless an End-of-File is encountered. An End-
of-File with the -p option causes cleanup for this process so
that another can be spawned. If the first argument contains a ?
(question mark), the remainder of this word is used as a prompt
on standard error when the shell is interactive. The exit sta‐
tus is 0 (zero) unless an End-of-File is encountered. The vari‐
ables whose names are given are marked read-only. These vari‐
ables can not be unset or changed by subsequent assignment. The
-p option outputs the names and values of all readonly vari‐
ables, one per line, in the format: readonly variable=value
Causes a shell function to return to the invoking script with
the return status specified by n. If n is omitted, the return
status is that of the last command executed. If return is
invoked while not in a function or a (dot) script, it is the
same as an exit. [Tru64 UNIX] This command is no longer sup‐
ported. See the loader(5) reference page for information on
using shared libraries. Using + rather than - causes these
options to be turned off. These options can also be used upon
invocation of the shell. The options for the set command have
the following meanings: [Tru64 UNIX] Array assignment. Unsets
the variable name and assign values sequentially from the list
argument. If +A is used, the variable name is not unset first.
Automatically exports subsequent parameters that are defined.
Causes the shell to notify the user asynchronously of background
job completions. Prevent existing files from being overwritten
by the shell's > redirection operator; the >| redirection opera‐
tor overrides this noclobber option for an individual file. If
a command has a nonzero exit status, executes the ERR trap, if
set, and exits. This mode is disabled while reading profiles.
Disables file name generation. Each command becomes a tracked
alias when first encountered. [Tru64 UNIX] All parameter
assignment arguments are placed in the environment for a com‐
mand, not just those that precede the command name. Background
jobs will run in a separate process group and a line will print
upon completion. The exit status of background jobs is reported
in a completion message. On systems with job control, this
option is turned on automatically for interactive shells. Reads
commands and checks them for syntax errors, but does not execute
them. Ignored for interactive shells. The argument can be one
of the following option names: Same as a. Same as e. [Tru64
UNIX] Runs all background jobs at a lower priority. This is
the default mode. [Tru64 UNIX] Invokes an emacs-style inline
editor for command entry. [Tru64 UNIX] Invokes a gmacs-style
inline editor for command entry. The shell does not exit on
End-of-File. The exit command must be used. [Tru64 UNIX] Same
as k. [Tru64 UNIX] All directory names resulting from file
name generation have a trailing / (slash) appended. Same as m.
Prevents redirection > from truncating existing files. Requires
>| to truncate a file when turned on. Same as n. Same as f.
Does not save function definitions in history file. Same as u.
[Tru64 UNIX] Same as p. Same as v. [Tru64 UNIX] Same as h.
Invokes, in insert mode, a vi-style inline editor until you
press Escape (ASCII 033). This changes to move mode. A return
sends the line. [Tru64 UNIX] Each character is processed as it
is entered in vi mode. Same as x.
[Tru64 UNIX] If no option name is supplied, then the current
option settings are printed. [Tru64 UNIX] Disables processing
of the $HOME/.profile file and uses the /etc/suid_profile file
instead of the ENV file. This mode is on whenever the effective
user ID or group ID is not equal to the real user ID or group
ID. Turning this off causes the effective user ID and group ID
to be set to the real user ID and group ID. [Tru64 UNIX] Sorts
the positional parameters. [Tru64 UNIX] Exits after reading
and executing one command. Treats unset parameters as an error
when substituting. Prints shell input lines as they are read.
Prints commands and their arguments as they are executed.
Unsets x and v options and stops examining arguments for
options. Does not change any of the options; useful in setting
$1 to a value beginning with -. If no arguments follow this
option, the positional parameters are unset.
These options can also be used upon invocation of the shell.
The current set of options can be found in $-. Unless -A is
specified, the remaining arguments are positional parameters and
are assigned, in order, to $1 $2 .... If no arguments are
given, the names and values of all named parameters are printed
on the standard output. If the only argument is +, the names of
all named parameters are printed. The positional parameters
from $n+1 ... are renamed $1 ...; the default n is 1. The argu‐
ment n can be any arithmetic expression that evaluates to a non‐
negative number less than or equal to $#. Prints the accumu‐
lated user and system times for the shell and for processes run
from the shell. The argument variable specifies a command to be
read and executed when the shell receives the specified signals.
(Note that argument is scanned once when the trap is set and
once when the trap is taken.) Each signal can be given as a num‐
ber or as the name of the signal. Trap commands are executed in
order of signal number. Any attempt to set a trap on a signal
that was ignored on entry to the current shell is ineffective.
If argument is omitted or is -, all traps associated with signal
are reset to their original values. If argument is the null
string, signal is ignored by the shell and by the commands it
invokes. If signal is ERR, the command argument is executed
whenever a command has a nonzero exit status. If signal is
DEBUG, argument is executed after each command. If signal is 0
(zero) or EXIT and the trap statement is executed inside the
body of a function, the command argument is executed after the
function completes. If signal is 0 (zero) or EXIT for a trap
set outside any function, the command argument is executed on
exit from the shell. The trap command with no variables prints
a list of commands associated with each signal number.
Note
Although signal is an optional parameter, using argument without
specifying a value for signal will have no effect. This is not
considered an error. [Tru64 UNIX] Sets attributes and values
for shell parameters. When invoked inside a function, a new
instance of the parameter name is created. The parameter value
and type are restored when the function completes. The follow‐
ing list of attributes can be specified: [Tru64 UNIX] The names
refer to function names rather than parameter names. No assign‐
ments can be made and the only other valid options are -t, -u,
and -x. The -t option turns on execution tracing for this func‐
tion. The -u option causes this function to be marked unde‐
fined. The FPATH variable is searched to find the function def‐
inition when the function is referenced. The -x option allows
the function definition to remain in effect across shell proce‐
dures invoked by name. [Tru64 UNIX] Provides system-to-host‐
name file mapping on machines that restrict the set of charac‐
ters in file names. [Tru64 UNIX] Parameter is an integer.
This makes arithmetic faster. If n is nonzero, it defines the
output arithmetic base; otherwise, the first assignment deter‐
mines the output base. [Tru64 UNIX] All uppercase characters
are converted to lowercase. The uppercase -u option is turned
off. [Tru64 UNIX] Left justifies and removes leading spaces
from value. If n is nonzero, it defines the width of the field;
otherwise, it is determined by the width of the value of first
assignment. When the parameter is assigned, it is filled on the
right with spaces or truncated, if necessary, to fit into the
field. Leading zeros are removed if the -Z option is also set.
The -R option is turned off. [Tru64 UNIX] The given names are
marked read-only and these names cannot be changed by subsequent
assignment. [Tru64 UNIX] Right justifies and fills with lead‐
ing spaces. If n is nonzero, it defines the width of the field;
otherwise, it is determined by the width of the value of first
assignment. The field is left-filled with spaces or truncated
from the end if the parameter is reassigned. The L option is
turned off. [Tru64 UNIX] Tags the named parameters. Tags are
user definable and have no special meaning to the shell. [Tru64
UNIX] All lowercase characters are converted to uppercase char‐
acters. The lowercase -l option is turned off. [Tru64
UNIX] The given names are marked for export. [Tru64
UNIX] Right justifies and fills with leading zeros if the first
nonspace character is a digit and the -L option was not set. If
n is nonzero, it defines the width of the field; otherwise, it
is determined by the width of the value of first assignment.
Using + (plus sign) rather than - (dash) causes these options to
be turned off. If no name arguments are given but options are
specified, a list of names (and optionally the values) of the
parameters that have these options set is printed. (Using +
rather than - keeps the values from being printed.) If no names
and options are given, the names and attributes of all parame‐
ters are printed. See the ulimit(1) reference page. See the
umask(1) reference page. See the unalias(1) reference page.
The variables or functions given by the list of names are unas‐
signed, that is, their values and attributes are erased. Read-
only variables cannot be unset. If the -f option is specified,
the names refer to function names. If no options or the -v
option is specified, the names refer to variables. Unsetting
ERRNO, LINENO, MAILCHECK, OPTARG, OPTIND, RANDOM, SECONDS,
TMOUT, and _ removes their special meaning even if they are sub‐
sequently assigned. See the wait(1) reference page. [Tru64
UNIX] For each name, indicates how it would be interpreted if
used as a command name. The -v option produces a more verbose
report. The -p option does a path search for name even if name
is an alias, a function, or a reserved word.
Invocation
If the shell is invoked by exec, and the first character of argument
zero ($0) is - (dash), the shell is assumed to be a login shell and
commands are read from /etc/profile and then from either in the current
directory or $HOME/.profile, if either file exists. Next, commands are
read from the file named by performing parameter substitution on the
value of the ENV environment variable, if the file exists. If the -s
option is not present and argument is present, a path search is per‐
formed on the first argument to determine the name of the script to
execute. The script argument must have read permission and any setuid
and getgid settings are ignored. Commands are then read, as described
in the following text.
See the OPTIONS section for a complete description of options that can
be interpreted by the shell when it is invoked.
NOTES
If a command is executed, and a command with the same name is installed
in a directory in the search path before the directory where the origi‐
nal command was found, the shell will execute the original command.
Use the hash command to correct this situation. When the shell encoun‐
ters the >> characters, it does not open the file in append mode;
instead, the shell opens the file for writing and seeks to the end.
Failure (nonzero exit status) of a special command preceding a || sym‐
bol prevents the list following || from executing. If a command that
is a tracked alias is executed, and then a command with the same name
is installed in a directory in the search path before the directory
where the original command was found, the shell continues to exec the
original command. Use the -t option of the alias command to correct
this situation. Using the fc built-in command within a compound com‐
mand causes the whole command to disappear from the history file. The
built-in command reads the whole file before any commands are executed.
Therefore, the alias and unalias commands in the file do not apply to
any functions defined in the file. Traps are not processed while a job
is waiting for a foreground process. Thus, a trap on CHLD is not exe‐
cuted until the foreground job terminates.
RETURN VALUES
Errors detected by the shell, such as syntax errors, cause the shell to
return a nonzero exit status. Otherwise, the shell returns the exit
status of the last command executed. (See also the exit command,
described previously.) If the shell is being used noninteractively,
execution of the shell file is abandoned. Run-time errors detected by
the shell are reported by printing the command or function name and the
error condition. If the line number that the error occurred on is
greater than 1, the line number is also printed in [ ] (brackets) after
the command or function name.
FILES
System profile User profile Contains user information Contains the
names of available and permitted shells
SEE ALSO
Commands: alias(1), bg(1), cat(1), cd(1), chmod(1), csh(1), echo(1),
emacs(1), env(1), fc(1), fg(1), hash(1), jobs(1), kill(1), ksh(1), new‐
grp(1), nice(1), nohup(1), sh(1), sh(1b), stty(1), test(1), time(1),
ulimit(1), umask(1)umask(1), unalias(1), vi(1), wait(1)
Functions: exec(2), fcntl(2), fork(2), ioctl(2), lseek(2), pipe(2),
read(2), sigaction(2), signal(2), umask(2), wait(2)
Routines: ctype(3), rand(3), ulimit(3)
Files: null(7), passwd(4), shells(4)
Miscellaneous: loader(5)
Standards: standards(5)sh(1p)
