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
characters preceding the <newline> are ignored) and other strings
of white-space characters, not including <newline>s,
to compare equal.
-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.
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 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 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 the -C and -c options. If TZ is
unset or null, an unspecified default timezone shall be used.
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, 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:
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).
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.
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 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:
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.
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:
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:
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 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 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 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 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 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 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 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.
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
DIFF (P)
2003
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