.TH DIFF 1 2026-10-20 "w64devkit" "w64devkit Manual"
.SH NAME
diff \- compare files line by line
.SH SYNOPSIS
.B diff
.RB [ \-abBdiNqrTstw ]
.RB [ \-L
.IR label ]
.RB [ \-S
.IR file ]
.RB [ \-U
.IR lines ]
.RB [ \-\-binary ]
.I file1
.I file2
.SH DESCRIPTION
.B diff
compares
.I file1
with
.I file2
line by line and writes the differences as a unified diff, the format
that
.BR patch (1)
reads.
It writes nothing when the files are the same.
There must be exactly two operands.
A file named
.B \-
is standard input.
.PP
Lines are compared as octets, and a line ends at LF.
Options can make the comparison ignore case or white space, but they do
not change how lines are written.
A CR before an LF is also ignored, unless
.B \-\-binary
is given; see
.BR NOTES .
.PP
If the files differ and a NUL octet appears in either of them, within
the length of the shorter file,
.B diff
does not compare lines, and writes
.PP
.RS 4
.EX
Files \fIfile1\fR and \fIfile2\fR differ
.EE
.RE
.PP
on standard output instead.
.B \-a
compares them as text.
.SS "Output format"
Output begins with two header lines naming the files:
.PP
.RS 4
.EX
\-\-\- \fIfile1\fR
+++ \fIfile2\fR
.EE
.RE
.PP
The names are the operands as given, or the
.B \-L
labels, with no timestamps.
Each hunk, a group of changes with their surrounding lines, follows as a
line
.PP
.RS 4
.EX
@@ \-\fIstart\fR,\fIcount\fR +\fIstart\fR,\fIcount\fR @@
.EE
.RE
.PP
and the lines it covers.
The first range is in
.IR file1 ,
the second in
.IR file2 .
A
.I count
of 1 is omitted.
A range of no lines is written as the number of the line before it, or
0 at the start of the file, and a
.I count
of 0.
.PP
Each line of a hunk starts with one character.
A space marks a context line, which is in both files.
A
.B \-
marks a line only in
.IR file1 ,
and a
.B +
marks a line only in
.IR file2 .
A hunk has
.B 3
context lines around each change, or the number given by
.BR \-U .
Changes separated by no more than twice that many unchanged lines share
a hunk.
A line that ends without a newline, which only the last line of a file
can, is followed by
.PP
.RS 4
.EX
\e No newline at end of file
.EE
.RE
.SS Directories
If both operands are directories,
.B diff
compares the files that have the same name in both, and writes the diff
of each pair that differs.
Names are sorted in byte order and match case-sensitively.
In messages and headers a file is named by the operand as given, a
slash unless the operand ends in one, and the name.
Other output is one of these lines:
.TP
.BI "Only in " dir ": " name
.I name
exists only in
.IR dir .
With
.BR \-r ,
.I name
is a path relative to the operand.
.B \-N
suppresses this line.
.TP
.BI "Common subdirectories: " path1 " and " path2
Without
.BR \-r ,
a subdirectory in both is not entered.
.TP
.BI "File " path1 " is a directory while file " path2 " is a regular file"
Without
.BR \-r .
The reverse, with the kinds exchanged, also occurs.
.TP
.BI "File " path " is not a regular file or directory and was skipped"
For a device, or another special file.
.TP
.BI "Files " path1 " and " path2 " differ"
With
.BR \-q ,
or for binary files.
.TP
.BI "Files " path1 " and " path2 " are identical"
With
.BR \-s .
.PP
Without
.BR \-r ,
only the entries of the two directories are compared.
With
.BR \-r ,
every subdirectory is entered, through symbolic links too, and files are
matched by path relative to the operands.
A subdirectory that is missing from the other operand, or is not a
directory there, is not entered and is not reported, but sets exit
status 1.
With
.BR \-N ,
it is entered, and its files are compared with empty files.
An entry that cannot be examined, such as a broken symbolic link, is
reported on standard error and left out, and the exit status does not
change.
.PP
If only one operand is a directory,
.B diff
compares the other operand with the file in that directory that has its
name: the part after the last forward slash, or all of the operand if it
has none.
That file must exist, even with
.BR \-N .
Standard input cannot be compared with a directory.
.SH OPTIONS
Options may be grouped, as in
.BR \-rN ,
and may follow the operands, unless
.B POSIXLY_CORRECT
is set.
An option's argument may be attached, as in
.BR \-U5 ,
or be the next argument.
Long options may be abbreviated to a unique prefix, and the argument of
a long option may follow
.B =
or be the next argument.
.B \-\-
ends the options.
.TP
.BR \-a ", " \-\-text
Treat all files as text, even if they contain NUL octets.
.TP
.BR \-b ", " \-\-ignore\-space\-change
Ignore changes in the amount of white space: compare each run of white
space as a single space.
White space at the end of a line, and a missing newline at the end of
the file, are ignored.
A line with white space at its start differs from one without.
White space is space, tab, CR, LF, VT, and FF.
.TP
.BR \-B ", " \-\-ignore\-blank\-lines
Omit a hunk whose changed lines, in both files, are all empty.
A line is empty when it contains only its newline.
A line of white space, or one ending in CR LF, is not empty.
If every hunk is omitted, the files count as the same.
.TP
.BR \-d ", " \-\-minimal
Try harder to find a small set of changes.
Without
.BR \-d ,
.B diff
limits the effort spent on a line that occurs many times in the other
file, so the result can be larger than necessary.
.TP
.BR \-i ", " \-\-ignore\-case
Ignore differences in case.
Only the ASCII letters are folded.
.TP
\fB\-L\fR \fIlabel\fR, \fB\-\-label=\fIlabel\fR
Use
.I label
instead of the file name in a header line.
Give
.B \-L
twice to label both files: the first labels
.IR file1 ,
the second
.IR file2 .
A single label applies to
.IR file1 .
Of more than two, only the first and last are used.
When directories are compared, the labels apply to every pair of files
that differs.
.TP
.BR \-N ", " \-\-new\-file
Treat absent files as empty.
An operand or directory entry that does not exist, or cannot be opened,
is compared as an empty file.
In a directory, a file that is in only one of the two is diffed against
an empty file named
.I /dev/null
instead of being reported as
.BR "Only in" .
Without
.BR \-r ,
a subdirectory that is in only one of the two is reported as common to
both.
A missing operand keeps its name in the header.
.TP
.BR \-q ", " \-\-brief
Write only
.BI "Files " file1 " and " file2 " differ"
for files that differ, and no diff.
Binary files are always reported this way.
.TP
.BR \-r ", " \-\-recursive
Compare directories recursively.
Has no effect unless both operands are directories.
.TP
\fB\-S\fR \fIfile\fR, \fB\-\-starting\-file=\fIfile\fR
When comparing directories, start with
.IR file :
skip each entry whose name sorts before
.I file
in byte order.
With
.BR \-r ,
an entry's name is its path relative to the operand.
.I file
need not exist.
.TP
.BR \-T ", " \-\-initial\-tab
Write a tab after the space,
.BR \- ,
or
.B +
that starts each line of a hunk, so that tab stops in the text keep
their columns.
.TP
.BR \-s ", " \-\-report\-identical\-files
Write
.BI "Files " file1 " and " file2 " are identical"
when two files compared are the same.
.TP
.BR \-t ", " \-\-expand\-tabs
Expand tabs to spaces in the output, to tab stops every 8 columns.
Columns count octets from the start of the text of the line.
.TP
\fB\-U\fR \fIlines\fR, \fB\-\-unified=\fIlines\fR
Use
.I lines
as the number of lines of context around each change.
The default is 3.
.I lines
is a decimal number from 0 to 2147483647, and anything else is an error.
Values of 1073741824 or more overflow and give wrong hunks.
.TP
.BR \-w ", " \-\-ignore\-all\-space
Ignore white space: compare lines with all white space removed, so that
.B "a b"
and
.B ab
are equal.
A missing newline at the end of the file is also ignored.
.TP
.B \-\-binary
Compare lines exactly, without ignoring a CR before an LF.
See
.BR NOTES .
.TP
.BR \-E ", " \-\-ignore\-tab\-expansion
Accepted for compatibility and ignored.
.TP
.BR \-p ", " \-\-show\-c\-function
Accepted for compatibility and ignored.
The hunk headers never name a function.
.TP
.B \-u
Accepted for compatibility and ignored.
The output is always unified.
.TP
.B \-\-help
As the first argument, print a usage message on standard output and
exit with status 0.
.SH ENVIRONMENT
.TP
.B BB_GLOBBING
Unless it is
.BR 0 ,
wildcards in the operands are expanded when
.B diff
starts, as needed when it is run from
.BR cmd.exe .
A busybox process started with it unset sets it to
.B 0
for its children, so commands run from the kit's shell receive their
operands as the shell expanded them.
.TP
.B POSIXLY_CORRECT
If set, the first operand ends the options.
.TP
.B TMPDIR
The directory for the temporary copy of an input that cannot seek, such
as a pipe.
When it is unset,
.B TMP
and then
.B TEMP
are used.
If none is set, see
.BR NOTES .
.PP
Variables that affect every busybox applet are described in
.BR busybox (1).
.SH EXIT STATUS
.TP
.B 0
No differences were found, including when the files differ only in what
an option ignores, or in a CR before an LF (see
.BR NOTES ).
.TP
.B 1
The files differ, including binary files, an
.B Only in
line, and a subdirectory skipped under
.BR \-r .
Also an invalid command line, with a message on standard error, and any
error other than a missing operand, such as a temporary file that cannot
be created, a file missing from the directory when only one operand is a
directory, or standard input compared with a directory.
.TP
.B 2
An operand does not exist, and
.B \-N
was not given.
.TP
.B 141
Standard output is a pipe whose reader has exited.
.PP
Messages about common subdirectories, files of different kinds, and
skipped files do not change the status.
Nor does a file that cannot be opened, which is reported on standard
error as
.PP
.RS 4
.EX
diff: can't open '\fIfile\fR': \fIreason\fR
.EE
.RE
.PP
The pair of files then counts as the same.
.SH EXAMPLES
Compare two files:
.PP
.RS 4
.EX
$ diff old.txt new.txt
\-\-\- old.txt
+++ new.txt
@@ \-1,3 +1,3 @@
 one
\-two
+2
 three
.EE
.RE
.PP
Test in a script whether two files are the same:
.PP
.RS 4
.EX
$ if diff \-q old.txt new.txt >/dev/null; then echo same; fi
.EE
.RE
.PP
Ignore changes in white space, and show 10 lines of context:
.PP
.RS 4
.EX
$ diff \-w \-U 10 old.c new.c
.EE
.RE
.PP
Compare the output of a program with the expected output:
.PP
.RS 4
.EX
$ ./prog | diff \- expected.txt
.EE
.RE
.PP
List the files that differ between two build trees:
.PP
.RS 4
.EX
$ diff \-rq build1 build2
.EE
.RE
.PP
Write a patch that turns tree
.I a
into tree
.IR b ,
including new files, and apply it to a copy of
.IR a :
.PP
.RS 4
.EX
$ diff \-rN a b >fix.patch
$ cp \-r a c
$ cd c
$ patch \-p1 <../fix.patch
.EE
.RE
.PP
Compare files with their CR LF line endings significant:
.PP
.RS 4
.EX
$ diff \-\-binary dos.txt unix.txt
.EE
.RE
.SH NOTES
Differences from GNU diff:
.IP \(bu 2
Only the unified format is written.
Options such as
.BR \-c ,
.BR \-e ,
.BR \-y ,
.BR \-x ,
.BR \-X ,
and
.B \-I
are not accepted.
.IP \(bu 2
The header lines have no timestamps.
.IP \(bu 2
The message for binary files begins with
.BR Files ,
not
.BR "Binary files" .
.IP \(bu 2
With
.BR \-r ,
a file in only one tree is reported as
.BI "Only in " dir ": " sub/name
with the subdirectory in the file part, where GNU diff puts it in the
directory part, and a subdirectory in only one tree is not reported.
.IP \(bu 2
Status 2 is for a missing operand only, and other errors give status 1.
.PP
Files and standard input are read in binary mode.
Unless
.B \-\-binary
is given, a CR directly before an LF is removed when lines are compared,
so text with CR LF line endings and the same text with LF line endings
are the same: nothing is written, and the exit status is 0.
Lines that are written keep their CRs.
Output lines that
.B diff
writes itself end with LF, not CR LF.
.PP
Operands may be Windows paths, with a drive letter and either kind of
slash, and are named in output as given.
.I /dev/null
is accepted as an operand, but the Windows name
.I nul
is not.
When directories are compared, names are joined to the directory operand
with a forward slash, which is added even after a trailing backslash.
When only one operand is a directory, only a forward slash ends the
directory part of the other operand, so
.B diff
appends all of an operand such as
.I sub\ef.txt
to the directory, and looks for
.IR dir/sub\ef.txt .
A bare drive such as
.I c:
names the current directory of that drive, and names are joined to it
with no slash, as
.IR c:name .
File names may contain any Unicode characters on Windows 10 version 1903
and later, and are limited to the ANSI code page on earlier versions.
.PP
An input that cannot seek, such as a pipe, is first copied to a
temporary file in
.BR TMPDIR ,
named
.B dif
and six more characters, which
.B diff
removes when it finishes.
It reads such an input to its end before it writes anything.
If none of
.BR TMPDIR ,
.BR TMP ,
and
.B TEMP
is set,
.B diff
does not compare the pair of files, writes no message, and counts them
as the same: the exit status is 0, and
.B \-s
reports them as identical.
.PP
If both operands name the same file, such as one path spelled two ways,
.B diff
exits with status 0 at once, and writes nothing even with
.BR \-s .
Two
.B \-
operands also count as the same.
.SH SEE ALSO
.BR busybox (1),
.BR cmp (1),
.BR comm (1),
.BR patch (1)