File: coreutils.info,  Node: md5sum invocation,  Next: sha1sum invocation,  Prev: b2sum invocation,  Up: Summarizing files

6.5 ‘md5sum’: Print or check MD5 digests
========================================

‘md5sum’ computes a 128-bit checksum (or “fingerprint” or
“message-digest”) for each specified FILE.

   Note: The MD5 digest is more reliable than a simple CRC (provided by
the ‘cksum’ command) for detecting accidental file corruption, as the
chances of accidentally having two files with identical MD5 are
vanishingly small.  However, it should not be considered secure against
malicious tampering: although finding a file with a given MD5
fingerprint is considered infeasible at the moment, it is known how to
modify certain files, including digital certificates, so that they
appear valid when signed with an MD5 digest.  For more secure hashes,
consider using SHA-2, or the newer ‘b2sum’ command.  *Note sha2
utilities::.  *Note b2sum invocation::.

   If a FILE is specified as ‘-’ or if no files are given ‘md5sum’
computes the checksum for the standard input.  ‘md5sum’ can also
determine whether a file and checksum are consistent.  Synopsis:

     md5sum [OPTION]... [FILE]...

   For each FILE, ‘md5sum’ outputs by default, the MD5 checksum, a
space, a flag indicating binary or text input mode, and the file name.
Binary mode is indicated with ‘*’, text mode with ‘ ’ (space).  Binary
mode is the default on systems where it’s significant, otherwise text
mode is the default.  The ‘cksum’ command always uses binary mode and a
‘ ’ (space) flag.

   Without ‘--zero’, if FILE contains a backslash, newline, or carriage
return, the line is started with a backslash, and each problematic
character in the file name is escaped with a backslash, making the
output unambiguous even in the presence of arbitrary file names.

   If FILE is omitted or specified as ‘-’, standard input is read.

   The program accepts the following options.  Also see *note Common
options::.

‘-b’
‘--binary’
     Note this option is not supported by the ‘cksum’ command, as it
     operates in binary mode exclusively.  Treat each input file as
     binary, by reading it in binary mode and outputting a ‘*’ flag.
     This is the inverse of ‘--text’.  On systems like GNU that do not
     distinguish between binary and text files, this option merely flags
     each input mode as binary: the MD5 checksum is unaffected.  This
     option is the default on systems like MS-DOS that distinguish
     between binary and text files, except for reading standard input
     when standard input is a terminal.

‘-c’
‘--check’
     Read file names and checksum information (not data) from each FILE
     (or from standard input if no FILE was specified) and report
     whether the checksums match the contents of the named files.  The
     input to this mode of ‘md5sum’ is usually the output of a prior,
     checksum-generating run of ‘md5sum’.

     Three input formats are supported.  Either the default output
     format described above, the ‘--tag’ output format, or the BSD
     reversed mode format which is similar to the default mode, but
     doesn’t use a character to distinguish binary and text modes.

     For the ‘cksum’ command, the ‘--check’ option supports
     auto-detecting the digest algorithm to use, when presented with
     checksum information in the ‘--tag’ output format.

     Also for the ‘cksum’ command, the ‘--check’ option auto-detects the
     digest encoding, accepting both standard hexidecimal checksums and
     those generated via ‘cksum’ with its ‘--base64’ option.

     Output with ‘--zero’ enabled is not supported by ‘--check’.

     For each such line, ‘md5sum’ reads the named file and computes its
     MD5 checksum.  Then, if the computed message digest does not match
     the one on the line with the file name, the file is noted as having
     failed the test.  Otherwise, the file passes the test.  By default,
     for each valid line, one line is written to standard output
     indicating whether the named file passed the test.  After all
     checks have been performed, if there were any failures, a warning
     is issued to standard error.  Use the ‘--status’ option to inhibit
     that output.  If any listed file cannot be opened or read, if any
     valid line has an MD5 checksum inconsistent with the associated
     file, or if no valid line is found, ‘md5sum’ exits with nonzero
     status.  Otherwise, it exits successfully.  Note the ‘cksum’
     command doesn’t support ‘--check’ with the older ‘sysv’, ‘bsd’, or
     ‘crc’ algorithms.

‘--ignore-missing’
     This option is useful only when verifying checksums.  When
     verifying checksums, don’t fail or report any status for missing
     files.  This is useful when verifying a subset of downloaded files
     given a larger list of checksums.

‘--quiet’
     This option is useful only when verifying checksums.  When
     verifying checksums, don’t generate an ’OK’ message per
     successfully checked file.  Files that fail the verification are
     reported in the default one-line-per-file format.  If there is any
     checksum mismatch, print a warning summarizing the failures to
     standard error.

‘--status’
     This option is useful only when verifying checksums.  When
     verifying checksums, don’t generate the default one-line-per-file
     diagnostic and don’t output the warning summarizing any failures.
     Failures to open or read a file still evoke individual diagnostics
     to standard error.  If all listed files are readable and are
     consistent with the associated MD5 checksums, exit successfully.
     Otherwise exit with a status code indicating there was a failure.

‘--tag’
     Output BSD style checksums, which indicate the checksum algorithm
     used.  As a GNU extension, if ‘--zero’ is not used, file names with
     problematic characters are escaped as described above, with the
     same escaping indicator of ‘\’ at the start of the line, being
     used.  The ‘--tag’ option implies binary mode, and is disallowed
     with ‘--text’ mode as supporting that would unnecessarily
     complicate the output format, while providing little benefit.  The
     ‘cksum’ command, uses ‘--tag’ as its default output format.

‘-t’
‘--text’
     Note this option is not supported by the ‘cksum’ command.  Treat
     each input file as text, by reading it in text mode and outputting
     a ‘ ’ flag.  This is the inverse of ‘--binary’.  This option is the
     default on systems like GNU that do not distinguish between binary
     and text files.  On other systems, it is the default for reading
     standard input when standard input is a terminal.  This mode is
     never defaulted to if ‘--tag’ is used.

‘-w’
‘--warn’
     When verifying checksums, warn about improperly formatted MD5
     checksum lines.  This option is useful only if all but a few lines
     in the checked input are valid.

‘--strict’
     When verifying checksums, if one or more input line is invalid,
     exit nonzero after all warnings have been issued.

‘-z’
‘--zero’
     Output a zero byte (ASCII NUL) at the end of each line, rather than
     a newline.  This option enables other programs to parse the output
     even when that output would contain data with embedded newlines.
     Also file name escaping is not used.

   An exit status of zero indicates success, and a nonzero value
indicates failure.