w64devkit/man/man1/cmp.1

.TH CMP 1 2026-10-19 "w64devkit" "w64devkit Manual"
.SH NAME
cmp \- compare two files byte by byte
.SH SYNOPSIS
.B cmp
.RB [ \-l | \-s ]
.RB [ \-n
.IR count ]
.I file1
.RI [ file2
.RI [ skip1
.RI [ skip2 ]]]
.SH DESCRIPTION
.B cmp
compares
.I file1
with
.I file2
byte by byte and reports the first difference.
A file named
.B \-
is standard input, and
.I file2
defaults to
.BR \- .
If both files are
.BR \- ,
.B cmp
exits with status 0 without reading.
.PP
.I skip1
and
.I skip2
are numbers of bytes to skip at the start of
.I file1
and
.IR file2 ,
and
.I skip2
defaults to 0.
Each is a decimal number, optionally followed by a suffix that
multiplies it:
.BR k ,
.BR K ,
.BR kiB ,
or
.B KiB
for 1024;
.BR m ,
.BR M ,
.BR miB ,
or
.B MiB
for 1048576;
.BR g ,
.BR G ,
.BR giB ,
or
.B GiB
for 1073741824; and
.BR KB ,
.BR MB ,
or
.B GB
for powers of 1000.
Skipped bytes are read and discarded, so skipping works on pipes, and a
file shorter than its skip is compared as empty.
.PP
At the first differing byte,
.B cmp
writes a line to standard output and stops:
.PP
.RS 4
.EX
\fIfile1\fR \fIfile2\fR differ: byte \fIn\fR, line \fIm\fR
.EE
.RE
.PP
.I n
is the position of the byte, counting from 1, and
.I m
is 1 plus the number of LF bytes before it.
Both count from the skip points.
If one file ends before a difference is found,
.B cmp
instead writes
.PP
.RS 4
.EX
cmp: EOF on \fIfile\fR
.EE
.RE
.PP
to standard error, naming the shorter file.
Files are named in messages as given on the command line, and an absent
.I file2
as
.BR \- .
.SH OPTIONS
Options may be grouped, as in
.BR "\-ln 100" ,
and the argument of
.B \-n
may be attached, as in
.BR \-n100 .
Long options may be abbreviated to a unique prefix, and the argument of
.B \-\-bytes
may follow
.B =
or be the next argument.
Options may follow operands, unless
.B POSIXLY_CORRECT
is set.
.B \-\-
ends the options.
.TP
.BR \-l ", " \-\-verbose
Report every differing byte, not only the first.
For each, write a line to standard output with its position in decimal
and its values in
.I file1
and
.I file2
in octal, separated by spaces, with each octal value padded on the left
with spaces to 3 characters.
If one file is shorter, the EOF message follows the lines for the bytes
the files have in common.
Incompatible with
.BR \-s .
.TP
.BR \-s ", " \-\-quiet ", " \-\-silent
Write no difference or EOF message, and no message when a file cannot
be opened, so only the exit status reports the result.
Read errors and command line errors are still reported.
Incompatible with
.BR \-l .
.TP
\fB\-n\fR \fIcount\fR, \fB\-\-bytes=\fIcount\fR
Compare at most
.I count
bytes after the skips.
.I count
is a decimal number from 0 to 2147483647, with no suffix.
If given more than once, the last is used.
.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 cmp
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.
.PP
Variables that affect every busybox applet are described in
.BR busybox (1).
.SH EXIT STATUS
.TP
.B 0
The files are identical, or identical in their first
.I count
bytes, or both are
.BR \- .
.TP
.B 1
A difference was found, or one file ended before the other.
Also an invalid command line, including an invalid
.IR count ,
.IR skip1 ,
or
.IR skip2 ,
with a message on standard error.
.TP
.B 2
A file could not be opened or read, or standard output could not be
written, as when it is a pipe whose reader has exited.
.TP
.B 141
With
.BR \-l ,
standard output is a pipe whose reader exited before
.B cmp
finished writing a long listing.
.SH EXAMPLES
Compare two files:
.PP
.RS 4
.EX
$ cmp old.txt new.txt
old.txt new.txt differ: byte 15, line 2
.EE
.RE
.PP
List every differing byte:
.PP
.RS 4
.EX
$ cmp \-l old.txt new.txt
15 107 147
22  56  41
.EE
.RE
.PP
Check in a script whether a rebuilt program is unchanged:
.PP
.RS 4
.EX
$ if cmp \-s old/prog.exe prog.exe; then echo unchanged; fi
.EE
.RE
.PP
Compare the output of a program with the expected output:
.PP
.RS 4
.EX
$ ./prog | cmp \- expected.txt
.EE
.RE
.PP
Compare a 4 KiB file with the data at offset 1 MiB of a disk image:
.PP
.RS 4
.EX
$ cmp \-n 4096 part.bin disk.img 0 1M
.EE
.RE
.SH NOTES
Differences from GNU cmp:
.IP \(bu 2
There is no
.BR \-b ,
.BR \-i ,
or
.BR \-v ,
nor their long forms.
.IP \(bu 2
.I count
takes no suffix and is at most 2147483647.
.IP \(bu 2
The EOF message does not give the byte and line number where the shorter
file ends.
.IP \(bu 2
.B \-l
does not pad positions to a common width.
.IP \(bu 2
An invalid command line exits with status 1, the same as for differing
files, not 2.
.PP
Files and standard input are read in binary mode, so text with CRLF line
endings differs from the same text with LF line endings.
Output lines end with LF, not CRLF.
.PP
.I file1
and
.I file2
may be Windows paths, with a drive letter and either kind of slash, or
busybox-w32 device names such as
.I /dev/null
and
.IR /dev/zero .
Since
.I /dev/zero
never ends,
.BI "cmp " file " /dev/zero"
writes only the EOF message, and exits with status 1, when
.I file
contains nothing but zero bytes.
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.
.SH SEE ALSO
.BR busybox (1),
.BR diff (1),
.BR xxd (1)