DIFF(P) POSIX Programmer's Manual DIFF(P)
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 out-
put 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 characters 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 inter-
preted 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 place.
If both file1 and file2 are directories, diff shall not compare block special files, char-
acter 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 direc-
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 IEEE Std 1003.1-2001, Section 8.2, Inter-
nationalization 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 interna-
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 argu-
ments and input files).
Determine the locale that should be used to affect the format and contents of diag-
nostic messages written to standard error and informative messages written to stan-
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 LC_MESSAGES .
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 directories 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 (path-
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 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, or -C options) diff utility output shall contain lines of
"%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 hyphens:
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-sepa-
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 con-
text, 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 fol-
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
date "+%a %b %e %T %Y"
without the trailing <newline>, executed at the time of last modification of the corre-
sponding 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:
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>
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 directory containing a
directory named x, dir1/x and dir2/x both contain files named date.out, and dir2/x con-
tains 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
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 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
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 his-
torical 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 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 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 asterisks (that is, utilities that cur-
rently 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 sub-
stitute 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 sys-
tems, 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 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 indication to the user that diff
was hung. In most common usage, diff -r should indicate differences in the file hierar-
chies, 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 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.
"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 sen-
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 DIFF(P)