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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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DIFF(1P) POSIX Programmer's Manual DIFF(1P)
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|-u|-C n|-U 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
POSIX.1‐2017, 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 characters preceding the <newline> are ignored)
and other strings of white-space characters, not
including <newline> characters, to compare equal.
-c Produce output in a form that provides three lines of
copied context.
-C n Produce output in a form that provides n lines of copied
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 diff utility shall detect infinite loops; that is,
entering a previously visited directory that is an
ancestor of the last file encountered. When it detects
an infinite loop, diff shall write a diagnostic message
to standard error and shall either recover its position
in the hierarchy or terminate.
-u Produce output in a form that provides three lines of
unified context.
-U n Produce output in a form that provides n lines of
unified context (where n shall be interpreted as a non-
negative decimal integer).
The following operands shall be supported:
file1, file2
A pathname of a file to be compared. If either the file1
or file2 operand is '-', the standard input shall be
used in its place.
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-directory file.
The standard input shall be used only if one of the file1 or file2
operands 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 POSIX.1‐2017, Section 8.2,
Internationalization Variables 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.
LC_CTYPE 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).
LC_MESSAGES
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.
LC_TIME Determine the locale for affecting the format of file
timestamps written with the -C and -c options.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
TZ Determine the timezone used for calculating file
timestamps written with a context format. If TZ is unset
or null, an unspecified default timezone shall be used.
Default.
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
directories may be reported with the following format:
"Common subdirectories: %s and %s\n", <directory1 pathname>,
<directory2 pathname>
For each file common to the two directories, if the two files are
not to be compared: if the two files have the same device ID and
file serial number, or are both block special files that refer to
the same device, or are both character special files that refer to
the same device, in the POSIX locale the output format is
unspecified. Otherwise, in the POSIX locale an unspecified format
shall be used that contains the pathnames of the two files.
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, it is implementation-defined whether diff uses
the binary file output format or the other formats as specified
below. The binary file output format shall contain the pathnames
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 differences.
Diff Default Output Format
The default (without -e, -f, -c, -C, -u, or -U 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 abbreviated 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
subcommand), the changes are separated with a line consisting of
three <hyphen-minus> characters:
"---\n"
Diff -e Output Format
With the -e option, a script shall be produced that shall, when
provided 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 following format:
"*** %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
modification of the corresponding file (or the current time, if
the file is standard input).
Then, the following output formats shall be applied for every set
of changes.
First, a line shall be written in the following format:
"***************\n"
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 format:
" %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 following format:
"+ %s", <added_line>
Diff -u or -U Output Format
The -u or -U options behave like the -c or -C options, except that
the context lines are not repeated; instead, the context, deleted,
and added lines are shown together, interleaved. The exact format
follows.
The name and last modification time of each file shall be output
in the following format:
"--- %s\t%s%s %s\n", file1, <file1 timestamp>, <file1 frac>, <file1 zone>
"+++ %s\t%s%s %s\n", file2, <file2 timestamp>, <file2 frac>, <file2 zone>
Each <file> field shall be the pathname of the corresponding file
being compared, or the single character '-' if standard input is
being compared. However, if the pathname contains a <tab> or a
<newline>, or if it does not consist entirely of characters taken
from the portable character set, the behavior is implementation-
defined.
Each <timestamp> field shall be equivalent to the output from the
following command:
date '+%Y-%m-%d %H:%M:%S'
without the trailing <newline>, executed at the time of last
modification of the corresponding file (or the current time, if
the file is standard input).
Each <frac> field shall be either empty, or a decimal point
followed by at least one decimal digit, indicating the fractional-
seconds part (if any) of the file timestamp. The number of
fractional digits shall be at least the number needed to represent
the file's timestamp without loss of information.
Each <zone> field shall be of the form "shhmm", where "shh" is a
signed two-digit decimal number in the range -24 through +25, and
"mm" is an unsigned two-digit decimal number in the range 00
through 59. It represents the timezone of the timestamp as the
number of hours (hh) and minutes (mm) east (+) or west (-) of UTC
for the timestamp. If the hours and minutes are both zero, the
sign shall be '+'. However, if the timezone is not an integral
number of minutes away from UTC, the <zone> field is
implementation-defined.
Then, the following output formats shall be applied for every set
of changes.
First, the range of lines in each file shall be written in the
following format:
"@@ -%s +%s @@", <file1 range>, <file2 range>
Each <range> field shall be of the form:
"%1d", <beginning line number>
or:
"%1d,1", <beginning line number>
if the range contains exactly one line, and:
"%1d,%1d", <beginning line number>, <number of lines>
otherwise. If a range is empty, its beginning line number shall be
the number of the line just before the range, or 0 if the empty
range starts the file.
Next, the affected lines along with lines of context shall be
written. Each non-empty unaffected line shall be written in the
following format:
" %s", <unaffected_line>
where the contents of the unaffected line shall be taken from
file1. It is implementation-defined whether an empty unaffected
line is written as an empty line or a line containing a single
<space> character. This line also represents the same line of
file2, even though file2's line may contain different contents due
to the -b. Deleted lines shall be written as:
"-%s", <deleted_line>
Added lines shall be written as:
"+%s", <added_line>
The order of lines written shall be the same as that of the
corresponding file. A deleted line shall never be written
immediately after an added line.
If -U n is specified, the output shall contain no more than 2n
consecutive unaffected lines; and if the output contains an
affected line and this line is adjacent to up to n consecutive
unaffected lines in the corresponding file, the output shall
contain these unaffected lines. -u shall act like -U3.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 No differences were found.
1 Differences were found.
>1 An error occurred.
Default.
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
directory 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
1c1
< 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 POSIX.1‐2017 can
do, as there is no metric that could be employed to judge the
quality of implementations 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:
1,100c1,100
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 specified. 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 target
hierarchy.
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 difference 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 a 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 <asterisk> characters
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 format. The version of
context described here does not require that multiple 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 <asterisk> characters (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 -u and -U options of GNU diff have been included. Their output
format, designed by Wayne Davison, takes up less space than -c and
-C format, and in many cases is easier to read. The format's
timestamps do not vary by locale, so LC_TIME does not affect it.
The format's line numbers are rendered with the %1d format, not
%d, because the file format notation rules would allow extra
<blank> characters to appear around the numbers.
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 reproducible 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 historical implementations. The specified behavior precludes
the problems arising from running into FIFOs and other files that
would cause diff to hang waiting for input with no indication to
the user that diff was hung. An earlier version of this standard
specified the output format more precisely, but in practice this
requirement was widely ignored and the benefit of standardization
seemed small, so it is now unspecified. 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 POSIX.1‐2017 supports named pipes,
the standard developers decided that such a restriction was
unreasonable. Note also that the allowed filename - almost always
refers to a pipe.
No directory search order is specified for diff. The historical
ordering is, in fact, not optimal, in that it prints out all of
the differences at the current level, including the statements
about all common subdirectories before recursing into those
subdirectories.
The message:
"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.
None.
cmp(1p), comm(1p), ed(1p), find(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 DIFF(1P)
Pages that refer to this page: cmp(1p), comm(1p), delta(1p), patch(1p), xargs(1p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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