SH(1)SH(1)NAMEsh - command interpreter (shell)
SYNOPSISsh [-/+aCefnuvxIimsVEb] [-/+o longname] [arg ...]
DESCRIPTION
Sh is the standard command interpreter for the system. The current
version of sh is in the process of being changed to conform with the
POSIX 1003.2 and 1003.2a specifications for the shell. This version
has many features which make it appear similar in some respects to the
Korn shell, but it is not a Korn shell clone (run GNU's bash if you
want that). Only features designated by POSIX, plus a few Berkeley
extensions, are being incorporated into this shell. We expect POSIX
conformance by the time 4.4 BSD is released. This man page is not
intended to be a tutorial or a complete specification of the shell.
Overview
The shell is a command that reads lines from either a file or the ter‐
minal, interprets them, and generally executes other commands. It is
the program that is running when a user logs into the system (although
a user can select a different shell with the chsh(1) command). The
shell implements a language that has flow control constructs, a macro
facility that provides a variety of features in addition to data stor‐
age, along with built in history and line editing capabilities. It
incorporates many features to aid interactive use and has the advantage
that the interpretative language is common to both interactive and non-
interactive use (shell scripts). That is, commands can be typed
directly to the running shell or can be put into a file and the file
can be executed directly by the shell.
Invocation
If no args are present and if the standard input of the shell is con‐
nected to a terminal (or if the -i flag is set), the shell is consid‐
ered an interactive shell. An interactive shell generally prompts
before each command and handles programming and command errors differ‐
ently (as described below). When first starting, the shell inspects
argument 0, and if it begins with a dash '-', the shell is also consid‐
ered a login shell. This is normally done automatically by the system
when the user first logs in. A login shell first reads commands from
the files /etc/profile and .profile if they exist. If the environment
variable ENV is set on entry to a shell, or is set in the .profile of a
login shell, the shell next reads commands from the file named in ENV.
Therefore, a user should place commands that are to be executed only at
login time in the .profile file, and commands that are executed for
every shell inside the ENV file. To set the ENV variable to some file,
place the following line in your .profile of your home directory
ENV=$HOME/.shinit; export ENV
substituting for ``.shinit'' any filename you wish. Since the ENV file
is read for every invocation of the shell, including shell scripts and
non-interactive shells, the following paradigm is useful for restrict‐
ing commands in the ENV file to interactive invocations. Place com‐
mands within the ``case'' and ``esac'' below (these commands are
described later):
case $- in *i*)
# commands for interactive use only
...
esac
If command line arguments besides the options have been specified, then
the shell treats the first argument as the name of a file from which to
read commands (a shell script), and the remaining arguments are set as
the positional parameters of the shell ($1, $2, etc). Otherwise, the
shell reads commands from its standard input.
Argument List Processing
All of the single letter options have a corresponding name that can be
used as an argument to the '-o' option. The set -o name is provided
next to the single letter option in the description below. Specifying
a dash ``-'' turns the option on, while using a plus ``+'' disables the
option. The following options can be set from the command line or with
the set(1) builtin (described later).
-a allexport
Export all variables assigned to. (UNIMPLEMENTED for 4.4alpha)
-C noclobber
Don't overwrite existing files with ``>''. (UNIMPLEMENTED for
4.4alpha)
-e errexit
If not interactive, exit immediately if any untested command
fails. The exit status of a command is considered to be explic‐
itly tested if the command is used to control an if, elif,
while, or until; or if the command is the left hand operand of
an ``&&'' or ``||'' operator.
-f noglob
Disable pathname expansion.
-n noexec
If not interactive, read commands but do not execute them. This
is useful for checking the syntax of shell scripts.
-u nounset
Write a message to standard error when attempting to expand a
variable that is not set, and if the shell is not interactive,
exit immediately. (UNIMPLEMENTED for 4.4alpha)
-v verbose
The shell writes its input to standard error as it is read.
Useful for debugging.
-x xtrace
Write each command to standard error (preceded by a '+ ') before
it is executed. Useful for debugging.
-I ignoreeof
Ignore EOF's from input when interactive.
-i interactive
Force the shell to behave interactively.
-m monitor
Turn on job control (set automatically when interactive).
-s stdin
Read commands from standard input (set automatically if no file
arguments are present). This option has no effect when set
after the shell has already started running (i.e. with set(1)).
-V vi
Enable the builtin vi(1) command line editor (disables -E if it
has been set).
-E emacs
Enable the builtin emacs(1) command line editor (disables -V if
it has been set).
-b notify
Enable asynchronous notification of background job completion.
(UNIMPLEMENTED for 4.4alpha)
Lexical Structure
The shell reads input in terms of lines from a file and breaks it up
into words at whitespace (blanks and tabs), and at certain sequences of
characters that are special to the shell called ``operators''. There
are two types of operators: control operators and redirection operators
(their meaning is discussed later). Following is a list of operators:
Control operators: & && ( ) ; ;; | || <newline>
Redirection operator: < > >| << >> <& >& <<- <>
Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords. There
are three types of quoting: matched single quotes, matched double
quotes, and backslash.
Backslash
A backslash preserves the literal meaning of the following character,
with the exception of <newline>. A backslash preceding a <newline> is
treated as a line continuation.
Single Quotes
Enclosing characters in single quotes preserves the literal meaning of
all the characters.
Double Quotes
Enclosing characters within double quotes preserves the literal meaning
of all characters except dollarsign ($), backquote (`), and backslash
(\). The backslash inside double quotes is historically weird, and
serves to quote only the following characters: $ ` " \ <newline>.
Otherwise it remains literal.
Reserved Words
Reserved words are words that have special meaning to the shell and are
recognized at the beginning of a line and after a control operator.
The following are reserved words:
! elif fi while case
else for then { }
do done until if esac
Their meaning is discussed later.
Aliases
An alias is a name and corresponding value set using the alias(1)
builtin command. Whenever a reserved word may occur (see above), and
after checking for reserved words, the shell checks the word to see if
it matches an alias. If it does, it replaces it in the input stream
with its value. For example, if there is an alias called ``lf'' with
the value ``ls -F'', then the input
lf foobar <return>
would become
ls -F foobar <return>
Aliases provide a convenient way for naive users to create shorthands
for commands without having to learn how to create functions with argu‐
ments. They can also be used to create lexically obscure code. This
use is discouraged.
Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to
the BNF in the POSIX 1003.2 document). Essentially though, a line is
read and if the first word of the line (or after a control operator) is
not a reserved word, then the shell has recognized a simple command.
Otherwise, a complex command or some other special construct may have
been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the follow‐
ing actions:
1) Leading words of the form ``name=value'' are stripped off and
assigned to the environment of the simple command. Redirection opera‐
tors and their arguments (as described below) are stripped off and
saved for processing.
2) The remaining words are expanded as described in the section called
``Expansions'', and the first remaining word is considered the command
name and the command is located. The remaining words are considered
the arguments of the command. If no command name resulted, then the
``name=value'' variable assignments recognized in 1) affect the current
shell.
3) Redirections are performed as described in the next section.
Redirections
Redirections are used to change where a command reads its input or
sends its output. In general, redirections open, close, or duplicate
an existing reference to a file. The overall format used for redirect‐
ion is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previ‐
ously. Following is a list of the possible redirections. The [n] is
an optional number, as in '3' (not '[3]'), that refers to a file
descriptor.
[n]> file
Redirect standard output (or n) to file.
[n]>| file
Same, but override the -C option.
[n]>> file
Append standard output (or n) to file.
[n]< file
Redirect standard input (or n) from file.
[n1]<&n2
Duplicate standard input (or n1) from file descriptor n2.
[n]<&-
Close standard input (or n).
[n1]>&n2
Duplicate standard output (or n) from n2.
[n]>&-
Close standard output (or n).
[n]<> file
Open file for reading and writing on standard input (or n).
The following redirection is often called a ``here-document''.
[n]<< delimiter
here-doc-text...
delimiter
All the text on successive lines up to the delimiter is saved away and
made available to the command on standard input, or file descriptor n
if it is specified. If the delimiter as specified on the initial line
is quoted, then the here-doc-text is treated literally, otherwise the
text is subjected to parameter expansion, command substitution, and
arithmetic expansion (as described in the section on ``Expansions'').
If the operator is ``<<-'' instead of ``<<'', then leading tabs in the
here-doc-text are stripped.
Search and Execution
There are three types of commands: shell functions, builtin commands,
and normal programs -- and the command is searched for (by name) in
that order. They each are executed in a different way.
When a shell function is executed, all of the shell positional parame‐
ters (except $0, which remains unchanged) are set to the arguments of
the shell function. The variables which are explicitly placed in the
environment of the command (by placing assignments to them before the
function name) are made local to the function and are set to the values
given. Then the command given in the function definition is executed.
The positional parameters are restored to their original values when
the command completes.
Shell builtins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or builtin, the
command is searched for as a normal program in the filesystem (as
described in the next section). When a normal program is executed, the
shell runs the program, passing the arguments and the environment to
the program. If the program is a shell procedure, the shell will inter‐
pret the program in a subshell. The shell will reinitialize itself in
this case, so that the effect will be as if a new shell had been
invoked to handle the shell procedure, except that the location of com‐
mands located in the parent shell will be remembered by the child.
Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name. Then it looks for a builtin command by that
name. Finally, it searches each entry in PATH in turn for the command.
The value of the PATH variable should be a series of entries separated
by colons. Each entry consists of a directory name. The current
directory may be indicated by an empty directory name.
Command names containing a slash are simply executed without performing
any of the above searches.
Command Exit Status
Each command has an exit status that can influence the behavior of
other shell commands. The paradigm is that a command exits with zero
for normal or success, and non-zero for failure, error, or a false
indication. The man page for each command should indicate the various
exit codes and what they mean. Additionally, the builtin commands
return exit codes, as does an executed function.
Complex Commands
Complex commands are combinations of simple commands with control oper‐
ators or reserved words, together creating a larger complex command.
More generally, a command is one of the following:
- simple command
- pipeline
- list or compound-list
- compound command
- function definition
Unless otherwise stated, the exit status of a command is that of the
last simple command executed by the command.
Pipeline
A pipeline is a sequence of one or more commands separated by the con‐
trol operator |. The standard output of all but the last command is
connected to the standard input of the next command.
The format for a pipeline is:
[!] command1 [ | command2 ...]
The standard output of command1 is connected to the standard input of
command2. The standard input, standard output, or both of a command is
considered to be assigned by the pipeline before any redirection speci‐
fied by redirection operators that are part of the command.
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
If the reserved word ! does not precede the pipeline, the exit status
is the exit status of the last command specified in the pipeline. Oth‐
erwise, the exit status is the logical NOT of the exit status of the
last command. That is, if the last command returns zero, the exit sta‐
tus is 1; if the last command returns greater than zero, the exit sta‐
tus is zero.
Because pipeline assignment of standard input or standard output or
both takes place before redirection, it can be modified by redirection.
For example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the
standard input of command2.
A ; or <newline> terminator causes the preceding AND-OR-list (described
next) to be executed sequentially; a & causes asynchronous execution of
the preceding AND-OR-list.
Background Commands -- &
If a command is terminated by the control operator ampersand (&), the
shell executes the command asynchronously -- that is, the shell does
not wait for the command to finish before executing the next command.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an asynchronous
command is set to /dev/null.
Lists -- Generally Speaking
A list is a sequence of zero or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these
three characters. The commands in a list are executed in the order
they are written. If command is followed by an ampersand, the shell
starts the command and immediately proceed onto the next command; oth‐
erwise it waits for the command to terminate before proceeding to the
next one.
``&&'' and ``||'' are AND-OR list operators. ``&&'' executes the first
command, and then executes the second command iff the exit status of
the first command is zero. ``||'' is similar, but executes the second
command iff the exit status of the first command is nonzero. ``&&''
and ``||'' both have the same priority.
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of the
first list is zero. The until command is similar, but has the word
until in place of while repeat until the exit status of the first list
is zero.
The syntax of the for command is
for variable in word...
do list
done
The words are expanded, and then the list is executed repeatedly with
the variable set to each word in turn. do and done may be replaced
with ``{'' and ``}''.
The syntax of the break and continue command is
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue con‐
tinues with the next iteration of the innermost loop. These are imple‐
mented as builtin commands.
The syntax of the case command is
case word in
pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see Shell Patterns
described later), separated by ``|'' characters.
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell.
Functions
The syntax of a function definition is
name ( ) command
A function definition is an executable statement; when executed it
installs a function named name and returns an exit status of zero.
The command is normally a list enclosed between ``{'' and ``}''.
Variables may be declared to be local to a function by using a local
command. This should appear as the first statement of a function, and
the syntax is
local [ variable | - ] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and
exported and readonly flags from the variable with the same name in the
surrounding scope, if there is one. Otherwise, the variable is ini‐
tially unset. The shell uses dynamic scoping, so that if you make the
variable x local to function f, which then calls function g, references
to the variable x made inside g will refer to the variable x declared
inside f, not to the global variable named x.
The only special parameter than can be made local is ``-''. Making
``-'' local any shell options that are changed via the set command
inside the function to be restored to their original values when the
function returns.
The syntax of the return command is
return [ exitstatus ]
It terminates the currently executing function. Return is implemented
as a builtin command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name
is called a variable. When starting up, the shell turns all the envi‐
ronment variables into shell variables. New variables can be set using
the form
name=value
Variables set by the user must have a name consisting solely of alpha‐
betics, numerics, and underscores - the first of which must not be
numeric. A parameter can also be denoted by a number or a special
character as explained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The
shell sets these initially to the values of its command line arguments
that follow the name of the shell script. The set(1) builtin can also
be used to set or reset them.
Special Parameters
A special parameter is a parameter denoted by one of the following spe‐
cial characters. The value of the parameter is listed next to its
character.
* Expands to the positional parameters, starting from one. When
the expansion occurs within a double-quoted string it expands to
a single field with the value of each parameter separated by the
first character of the IFS variable, or by a <space> if IFS is
unset.
@ Expands to the positional parameters, starting from one. When
the expansion occurs within double-quotes, each positional
parameter expands as a separate argument. If there are no posi‐
tional parameters, the expansion of @ generates zero arguments,
even when @ is double-quoted. What this basically means, for
example, is if $1 is ``abc'' and $2 is ``def ghi'', then "$@"
expands to the two arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent pipeline.
- (Hyphen)
Expands to the current option flags (the single-letter option
names concatenated into a string) as specified on invocation, by
the set builtin command, or implicitly by the shell.
$ Expands to the process ID of the invoked shell. A subshell
retains the same value of $ as its parent.
! Expands to the process ID of the most recent background command
executed from the current shell. For a pipeline, the process ID
is that of the last command in the pipeline.
0 (Zero.)
Expands to the name of the shell or shell script.
Word Expansions
This clause describes the various expansions that are performed on
words. Not all expansions are performed on every word, as explained
later.
Tilde expansions, parameter expansions, command substitutions, arith‐
metic expansions, and quote removals that occur within a single word
expand to a single field. It is only field splitting or pathname
expansion that can create multiple fields from a single word. The sin‐
gle exception to this rule is the expansion of the special parameter @
within double-quotes, as was described above.
The order of word expansion is:
(1) Tilde Expansion, Parameter Expansion, Command Substitution, Arith‐
metic Expansion (these all occur at the same time).
(2) Field Splitting is performed on fields generated by step (1)
unless the IFS variable is null.
(3) Pathname Expansion (unless set -f is in effect).
(4) Quote Removal.
The $ character is used to introduce parameter expansion, command sub‐
stitution, or arithmetic evaluation.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to
tilde expansion. All the characters up to a slash (/) or the end of
the word are treated as a username and are replaced with the user's
home directory. If the username is missing (as in ~/foobar), the tilde
is replaced with the value of the HOME variable (the current user's
home directory).
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching }. Any
} escaped by a backslash or within a quoted string, and characters in
embedded arithmetic expansions, command substitutions, and variable
expansions, are not examined in determining the matching }.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are
optional except for positional parameters with more than one digit or
when parameter is followed by a character that could be interpreted as
part of the name. If a parameter expansion occurs inside double-
quotes:
1) Pathname expansion is not performed on the results of the expansion.
2) Field splitting is not performed on the results of the expansion,
with the exception of @.
In addition, a parameter expansion can be modified by using one of the
following formats.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expan‐
sion of word is substituted; otherwise, the value of parameter
is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter. In all cases, the
final value of parameter is substituted. Only variables, not
positional parameters or special parameters, can be assigned in
this way.
${parameter:?[word]}
Indicate Error if Null or Unset. If parameter is unset or null,
the expansion of word (or a message indicating it is unset if
word is omitted) is written to standard error and the shell
exits with a nonzero exit status. Otherwise, the value of
parameter is substituted. An interactive shell need not exit.
${parameter:+word}
Use Alternate Value. If parameter is unset or null, null is
substituted; otherwise, the expansion of word is substituted.
In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omis‐
sion of the colon results in a test for a parameter that is only unset.
${#parameter}
String Length. The length in characters of the value of parame‐
ter.
The following four varieties of parameter expansion provide for sub‐
string processing. In each case, pattern matching notation (see Shell
Patterns), rather than regular expression notation, is used to evaluate
the patterns. If parameter is * or @, the result of the expansion is
unspecified. Enclosing the full parameter expansion string in double-
quotes does not cause the following four varieties of pattern charac‐
ters to be quoted, whereas quoting characters within the braces has
this effect.
${parameter%word}
Remove Smallest Suffix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the smallest portion of the suffix matched by the pattern
deleted.
${parameter%%word}
Remove Largest Suffix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the largest portion of the suffix matched by the pattern
deleted.
${parameter#word}
Remove Smallest Prefix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the smallest portion of the prefix matched by the pattern
deleted.
${parameter##word}
Remove Largest Prefix Pattern. The word is expanded to produce
a pattern. The parameter expansion then results in parameter,
with the largest portion of the prefix matched by the pattern
deleted.
Command Substitution
Command substitution allows the output of a command to be substituted
in place of the command name itself. Command substitution occurs when
the command is enclosed as follows:
$(command)
or (``backquoted'' version):
`command`
The shell expands the command substitution by executing command in a
subshell environment and replacing the command substitution with the
standard output of the command, removing sequences of one or more <new‐
line>s at the end of the substitution. (Embedded <newline>s before the
end of the output are not removed; however, during field splitting,
they may be translated into <space>s, depending on the value of IFS and
quoting that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expan‐
sion is as follows:
$((expression))
The expression is treated as if it were in double-quotes, except that a
double-quote inside the expression is not treated specially. The shell
expands all tokens in the expression for parameter expansion, command
substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and substitutes
the value of the expression.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expan‐
sion the shell scans the results of expansions and substitutions that
did not occur in double-quotes for field splitting and multiple fields
can result.
The shell treats each character of the IFS as a delimiter and use the
delimiters to split the results of parameter expansion and command sub‐
stitution into fields.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed after word
splitting is complete. Each word is viewed as a series of patterns,
separated by slashes. The process of expansion replaces the word with
the names of all existing files whose names can be formed by replacing
each pattern with a string that matches the specified pattern. There
are two restrictions on this: first, a pattern cannot match a string
containing a slash, and second, a pattern cannot match a string start‐
ing with a period unless the first character of the pattern is a
period. The next section describes the patterns used for both Pathname
Expansion and the case(1) command.
Shell Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are ``!'', ``*'', ``?'', and
``[''. These characters lose there special meanings if they are
quoted. When command or variable substitution is performed and the
dollar sign or back quotes are not double quoted, the value of the
variable or the output of the command is scanned for these characters
and they are turned into meta-characters.
An asterisk (``*'') matches any string of characters. A question mark
matches any single character. A left bracket (``['') introduces a char‐
acter class. The end of the character class is indicated by a ``]'';
if the ``]'' is missing then the ``['' matches a ``['' rather than
introducing a character class. A character class matches any of the
characters between the square brackets. A range of characters may be
specified using a minus sign. The character class may be complemented
by making an exclamation point the first character of the character
class.
To include a ``]'' in a character class, make it the first character
listed (after the ``!'', if any). To include a minus sign, make it the
first or last character listed
Builtins
This section lists the builtin commands which are builtin because they
need to perform some operation that can't be performed by a separate
process. In addition to these, there are several other commands that
may be builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).
: A null command that returns a 0 (true) exit value.
. file The commands in the specified file are read and executed by the
shell.
alias [ name[=string] ... ]
If name=string is specified, the shell defines the alias
``name'' with value ``string''. If just ``name'' is specified,
the value of the alias ``name'' is printed. With no arguments,
the alias builtin prints the names and values of all defined
aliases (see unalias).
bg [ job ] ...
Continue the specified jobs (or the current job if no jobs are
given) in the background.
command command arg...
Execute the specified builtin command. (This is useful when you
have a shell function with the same name as a builtin command.)
cd [ directory ]
Switch to the specified directory (default $HOME). If the an
entry for CDPATH appears in the environment of the cd command or
the shell variable CDPATH is set and the directory name does not
begin with a slash, then the directories listed in CDPATH will
be searched for the specified directory. The format of CDPATH
is the same as that of PATH. In an interactive shell, the cd
command will print out the name of the directory that it actu‐
ally switched to if this is different from the name that the
user gave. These may be different either because the CDPATH
mechanism was used or because a symbolic link was crossed.
eval string...
Concatenate all the arguments with spaces. Then re-parse and
execute the command.
exec [ command arg... ]
Unless command is omitted, the shell process is replaced with
the specified program (which must be a real program, not a shell
builtin or function). Any redirections on the exec command are
marked as permanent, so that they are not undone when the exec
command finishes.
exit [ exitstatus ]
Terminate the shell process. If exitstatus is given it is used
as the exit status of the shell; otherwise the exit status of
the preceding command is used.
export name...
The specified names are exported so that they will appear in the
environment of subsequent commands. The only way to un-export a
variable is to unset it. The shell allows the value of a vari‐
able to be set at the same time it is exported by writing
export name=value
With no arguments the export command lists the names of all
exported variables.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands previ‐
ously entered to an interactive shell.
-e editor
Use the editor named by editor to edit the commands. The editor
string is a command name, subject to search via the PATH vari‐
able. The value in the FCEDIT variable is used as a default
when -e is not specified. If FCEDIT is null or unset, the value
of the EDITOR variable is used. If EDITOR is null or unset,
ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them. The
commands are written in the sequence indicated by the first and
last operands, as affected by -r, with each command preceded by
the command number.
-n
Suppress command numbers when listing with -l.
-r
Reverse the order of the commands listed (with -l) or edited
(with neither -l nor -s).
-s
Re-execute the command without invoking an editor.
first
last
Select the commands to list or edit. The number of previous
commands that can be accessed are determined by the value of the
HISTSIZE variable. The value of first or last or both are one
of the following:
[+]number
A positive number representing a command number; command numbers
can be displayed with the -l option.
-number
A negative decimal number representing the command that was exe‐
cuted number of commands previously. For example, -1 is the
immediately previous command.
string
A string indicating the most recently entered command that
begins with that string. If the old=new operand is not also
specified with -s, the string form of the first operand cannot
contain an embedded equal sign.
The following environment variables affect the execution of fc:
FCEDIT
Name of the editor to use.
HISTSIZE
The number of previous commands that are accessable.
fg [ job ]
Move the specified job or the current job to the foreground.
getopts optstring var
The POSIX getopts command.
hash -rv command...
The shell maintains a hash table which remembers the locations
of commands. With no arguments whatsoever, the hash command
prints out the contents of this table. Entries which have not
been looked at since the last cd command are marked with an
asterisk; it is possible for these entries to be invalid.
With arguments, the hash command removes the specified commands
from the hash table (unless they are functions) and then locates
them. With the -v option, hash prints the locations of the
commands as it finds them. The -r option causes the hash com‐
mand to delete all the entries in the hash table except for
functions.
jobid [ job ]
Print the process id's of the processes in the job. If the job
argument is omitted, use the current job.
jobs This command lists out all the background processes which are
children of the current shell process.
pwd Print the current directory. The builtin command may differ
from the program of the same name because the builtin command
remembers what the current directory is rather than recomputing
it each time. This makes it faster. However, if the current
directory is renamed, the builtin version of pwd will continue
to print the old name for the directory.
read [ -p prompt ] [ -e ] variable...
The prompt is printed if the -p option is specified and the
standard input is a terminal. Then a line is read from the
standard input. The trailing newline is deleted from the line
and the line is split as described in the section on word split‐
ting above, and the pieces are assigned to the variables in
order. If there are more pieces than variables, the remaining
pieces (along with the characters in IFS that separated them)
are assigned to the last variable. If there are more variables
than pieces, the remaining variables are assigned the null
string.
The -e option causes any backslashes in the input to be treated
specially. If a backslash is followed by a newline, the back‐
slash and the newline will be deleted. If a backslash is fol‐
lowed by any other character, the backslash will be deleted and
the following character will be treated as though it were not in
IFS, even if it is.
readonly name...
The specified names are marked as read only, so that they cannot
be subsequently modified or unset. The shell allows the value
of a variable to be set at the same time it is marked read only
by writing
readonly name=value
With no arguments the readonly command lists the names of all
read only variables.
set [ { -options | +options | -- } ] arg...
The set command performs three different functions.
With no arguments, it lists the values of all shell variables.
If options are given, it sets the specified option flags, or
clears them as described in the section called ``Argument List
Processing''.
The third use of the set command is to set the values of the
shell's positional parameters to the specified args. To change
the positional parameters without changing any options, use
``--'' as the first argument to set. If no args are present,
the set command will clear all the positional parameters (equiv‐
alent to executing ``shift $#''.
setvar variable value
Assigns value to variable. (In general it is better to write
variable=value rather than using setvar. Setvar is intended to
be used in functions that assign values to variables whose names
are passed as parameters.)
shift [ n ]
Shift the positional parameters n times. A shift sets the value
of $1 to the value of $2, the value of $2 to the value of $3,
and so on, decreasing the value of $# by one. If there are zero
positional parameters, shifting doesn't do anything.
trap [ action ] signal...
Cause the shell to parse and execute action when any of the
specified signals are received. The signals are specified by
signal number. Action may be null or omitted; the former causes
the specified signal to be ignored and the latter causes the
default action to be taken. When the shell forks off a subshell,
it resets trapped (but not ignored) signals to the default
action. The trap command has no effect on signals that were
ignored on entry to the shell.
umask [ mask ]
Set the value of umask (see umask(2)) to the specified octal
value. If the argument is omitted, the umask value is printed.
unalias [-a] [name]
If ``name'' is specified, the shell removes that alias. If
``-a'' is specified, all aliases are removed.
unset name...
The specified variables and functions are unset and unexported.
If a given name corresponds to both a variable and a function,
both the variable and the function are unset.
wait [ job ]
Wait for the specified job to complete and return the exit sta‐
tus of the last process in the job. If the argument is omitted,
wait for all jobs to complete and the return an exit status of
zero.
Command Line Editing
When sh is being used interactively from a terminal, the current com‐
mand and the command history (see fc in Builtins) can be edited using
vi-mode command-line editing. This mode uses commands, described below,
similar to a subset of those described in the vi man page. The command
set -o vi enables vi-mode editing and place sh into vi insert mode.
With vi-mode enabled, sh can be switched between insert mode and com‐
mand mode. The editor is not described in full here, but will be in a
later document. It's similar to vi: typing <ESC> will throw you into
command VI command mode. Hitting <return> while in command mode will
pass the line to the shell.
SH(1)