DIFF(1P) POSIX Programmer's Manual DIFF(1P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
diff - compare two files
diff [-c| -e| -f| -C n][-br] file1 file2
The diff utility shall compare the contents of file1 and file2 and
write to standard output a list of changes necessary to convert file1
into file2. This list should be minimal. No output shall be produced if
the files are identical.
The diff utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-b Cause any amount of white space at the end of a line to be
treated as a single <newline> (that is, the white-space charac‐
ters preceding the <newline> are ignored) and other strings of
white-space characters, not including <newline>s, to compare
-c Produce output in a form that provides three lines of context.
-C n Produce output in a form that provides n lines of context (where
n shall be interpreted as a positive decimal integer).
-e Produce output in a form suitable as input for the ed utility,
which can then be used to convert file1 into file2.
-f Produce output in an alternative form, similar in format to -e,
but not intended to be suitable as input for the ed utility, and
in the opposite order.
-r Apply diff recursively to files and directories of the same name
when file1 and file2 are both directories.
The following operands shall be supported:
A pathname of a file to be compared. If either the file1 or
file2 operand is '-', the standard input shall be used in its
If both file1 and file2 are directories, diff shall not compare block
special files, character special files, or FIFO special files to any
files and shall not compare regular files to directories. Further
details are as specified in Diff Directory Comparison Format . The
behavior of diff on other file types is implementation-defined when
found in directories.
If only one of file1 and file2 is a directory, diff shall be applied to
the non-directory file and the file contained in the directory file
with a filename that is the same as the last component of the non-
The standard input shall be used only if one of the file1 or file2 op‐
erands references standard input. See the INPUT FILES section.
The input files may be of any type.
The following environment variables shall affect the execution of diff:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables used
to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all
the other internationalization variables.
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input files).
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error
and informative messages written to standard output.
Determine the locale for affecting the format of file timestamps
written with the -C and -c options.
Determine the location of message catalogs for the processing of
TZ Determine the timezone used for calculating file timestamps
written with the -C and -c options. If TZ is unset or null, an
unspecified default timezone shall be used.
Diff Directory Comparison Format
If both file1 and file2 are directories, the following output formats
shall be used.
In the POSIX locale, each file that is present in only one directory
shall be reported using the following format:
"Only in %s: %s\n", <directory pathname>, <filename>
In the POSIX locale, subdirectories that are common to the two directo‐
ries may be reported with the following format:
"Common subdirectories: %s and %s\n", <directory1 pathname>,
For each file common to the two directories if the two files are not to
be compared, the following format shall be used in the POSIX locale:
"File %s is a %s while file %s is a %s\n", <directory1 pathname>,
<file type of directory1 pathname>, <directory2 pathname>,
<file type of directory2 pathname>
For each file common to the two directories, if the files are compared
and are identical, no output shall be written. If the two files differ,
the following format is written:
"diff %s %s %s\n", <diff_options>, <filename1>, <filename2>
where <diff_options> are the options as specified on the command line.
All directory pathnames listed in this section shall be relative to the
original command line arguments. All other names of files listed in
this section shall be filenames (pathname components).
Diff Binary Output Format
In the POSIX locale, if one or both of the files being compared are not
text files, an unspecified format shall be used that contains the path‐
names of two files being compared and the string "differ" .
If both files being compared are text files, depending on the options
specified, one of the following formats shall be used to write the dif‐
Diff Default Output Format
The default (without -e, -f, -c, or -C options) diff utility output
shall contain lines of these forms:
"%da%d\n", <num1>, <num2>
"%da%d,%d\n", <num1>, <num2>, <num3>
"%dd%d\n", <num1>, <num2>
"%d,%dd%d\n", <num1>, <num2>, <num3>
"%dc%d\n", <num1>, <num2>
"%d,%dc%d\n", <num1>, <num2>, <num3>
"%dc%d,%d\n", <num1>, <num2>, <num3>
"%d,%dc%d,%d\n", <num1>, <num2>, <num3>, <num4>
These lines resemble ed subcommands to convert file1 into file2. The
line numbers before the action letters shall pertain to file1; those
after shall pertain to file2. Thus, by exchanging a for d and reading
the line in reverse order, one can also determine how to convert file2
into file1. As in ed, identical pairs (where num1= num2) are abbrevi‐
ated as a single number.
Following each of these lines, diff shall write to standard output all
lines affected in the first file using the format:
"< %s", <line>
and all lines affected in the second file using the format:
"> %s", <line>
If there are lines affected in both file1 and file2 (as with the c sub‐
command), the changes are separated with a line consisting of three
Diff -e Output Format
With the -e option, a script shall be produced that shall, when pro‐
vided as input to ed, along with an appended w (write) command, convert
file1 into file2. Only the a (append), c (change), d (delete), i
(insert), and s (substitute) commands of ed shall be used in this
script. Text lines, except those consisting of the single character
period ( '.' ), shall be output as they appear in the file.
Diff -f Output Format
With the -f option, an alternative format of script shall be produced.
It is similar to that produced by -e, with the following differences:
1. It is expressed in reverse sequence; the output of -e orders
changes from the end of the file to the beginning; the -f from
beginning to end.
2. The command form <lines> <command-letter> used by -e is reversed.
For example, 10c with -e would be c10 with -f.
3. The form used for ranges of line numbers is <space>-separated,
rather than comma-separated.
Diff -c or -C Output Format
With the -c or -C option, the output format shall consist of affected
lines along with surrounding lines of context. The affected lines shall
show which ones need to be deleted or changed in file1, and those added
from file2. With the -c option, three lines of context, if available,
shall be written before and after the affected lines. With the -C
option, the user can specify how many lines of context are written. The
exact format follows.
The name and last modification time of each file shall be output in the
"*** %s %s\n", file1, <file1 timestamp>
"--- %s %s\n", file2, <file2 timestamp>
Each <file> field shall be the pathname of the corresponding file being
compared. The pathname written for standard input is unspecified.
In the POSIX locale, each <timestamp> field shall be equivalent to the
output from the following command:
date "+%a %b %e %T %Y"
without the trailing <newline>, executed at the time of last modifica‐
tion of the corresponding file (or the current time, if the file is
Then, the following output formats shall be applied for every set of
First, a line shall be written in the following format:
Next, the range of lines in file1 shall be written in the following
format if the range contains two or more lines:
"*** %d,%d ****\n", <beginning line number>, <ending line number>
and the following format otherwise:
"*** %d ****\n", <ending line number>
The ending line number of an empty range shall be the number of the
preceding line, or 0 if the range is at the start of the file.
Next, the affected lines along with lines of context (unaffected lines)
shall be written. Unaffected lines shall be written in the following
" %s", <unaffected_line>
Deleted lines shall be written as:
"- %s", <deleted_line>
Changed lines shall be written as:
"! %s", <changed_line>
Next, the range of lines in file2 shall be written in the following
format if the range contains two or more lines:
"--- %d,%d ----\n", <beginning line number>, <ending line number>
and the following format otherwise:
"--- %d ----\n", <ending line number>
Then, lines of context and changed lines shall be written as described
in the previous formats. Lines added from file2 shall be written in the
"+ %s", <added_line>
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
0 No differences were found.
1 Differences were found.
>1 An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
If lines at the end of a file are changed and other lines are added,
diff output may show this as a delete and add, as a change, or as a
change and add; diff is not expected to know which happened and users
should not care about the difference in output as long as it clearly
shows the differences between the files.
If dir1 is a directory containing a directory named x, dir2 is a direc‐
tory containing a directory named x, dir1/x and dir2/x both contain
files named date.out, and dir2/x contains a file named y, the command:
diff -r dir1 dir2
could produce output similar to:
Common subdirectories: dir1/x and dir2/x
Only in dir2/x: y
diff -r dir1/x/date.out dir2/x/date.out
< Mon Jul 2 13:12:16 PDT 1990
> Tue Jun 19 21:41:39 PDT 1990
The -h option was omitted because it was insufficiently specified and
does not add to applications portability.
Historical implementations employ algorithms that do not always produce
a minimum list of differences; the current language about making every
effort is the best this volume of IEEE Std 1003.1-2001 can do, as there
is no metric that could be employed to judge the quality of implementa‐
tions against any and all file contents. The statement "This list
should be minimal'' clearly implies that implementations are not
expected to provide the following output when comparing two 100-line
files that differ in only one character on a single line:
all 100 lines from file1 preceded with "< "
all 100 lines from file2 preceded with "> "
The "Only in" messages required when the -r option is specified are not
used by most historical implementations if the -e option is also speci‐
fied. It is required here because it provides useful information that
must be provided to update a target directory hierarchy to match a
source hierarchy. The "Common subdirectories" messages are written by
System V and 4.3 BSD when the -r option is specified. They are allowed
here but are not required because they are reporting on something that
is the same, not reporting a difference, and are not needed to update a
The -c option, which writes output in a format using lines of context,
has been included. The format is useful for a variety of reasons, among
them being much improved readability and the ability to understand dif‐
ference changes when the target file has line numbers that differ from
another similar, but slightly different, copy. The patch utility is
most valuable when working with difference listings using the context
format. The BSD version of -c takes an optional argument specifying
the amount of context. Rather than overloading -c and breaking the
Utility Syntax Guidelines for diff, the standard developers decided to
add a separate option for specifying a context diff with a specified
amount of context ( -C). Also, the format for context diffs was
extended slightly in 4.3 BSD to allow multiple changes that are within
context lines from each other to be merged together. The output format
contains an additional four asterisks after the range of affected lines
in the first filename. This was to provide a flag for old programs
(like old versions of patch) that only understand the old context for‐
mat. The version of context described here does not require that multi‐
ple changes within context lines be merged, but it does not prohibit it
either. The extension is upwards-compatible, so any vendors that wish
to retain the old version of diff can do so by adding the extra four
asterisks (that is, utilities that currently use diff and understand
the new merged format will also understand the old unmerged format, but
not vice versa).
The substitute command was added as an additional format for the -e
option. This was added to provide implementations with a way to fix the
classic "dot alone on a line" bug present in many versions of diff.
Since many implementations have fixed this bug, the standard developers
decided not to standardize broken behavior, but rather to provide the
necessary tool for fixing the bug. One way to fix this bug is to output
two periods whenever a lone period is needed, then terminate the append
command with a period, and then use the substitute command to convert
the two periods into one period.
The BSD-derived -r option was added to provide a mechanism for using
diff to compare two file system trees. This behavior is useful, is
standard practice on all BSD-derived systems, and is not easily repro‐
ducible with the find utility.
The requirement that diff not compare files in some circumstances, even
though they have the same name, is based on the actual output of his‐
torical implementations. The message specified here is already in use
when a directory is being compared to a non-directory. It is extended
here to preclude the problems arising from running into FIFOs and other
files that would cause diff to hang waiting for input with no indica‐
tion to the user that diff was hung. In most common usage, diff -r
should indicate differences in the file hierarchies, not the difference
of contents of devices pointed to by the hierarchies.
Many early implementations of diff require seekable files. Since the
System Interfaces volume of IEEE Std 1003.1-2001 supports named pipes,
the standard developers decided that such a restriction was unreason‐
able. Note also that the allowed filename - almost always refers to a
No directory search order is specified for diff. The historical order‐
ing is, in fact, not optimal, in that it prints out all of the differ‐
ences at the current level, including the statements about all common
subdirectories before recursing into those subdirectories.
"diff %s %s %s\n", <diff_options>, <filename1>, <filename2>
does not vary by locale because it is the representation of a command,
not an English sentence.
cmp, comm, ed, find
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 DIFF(1P)