explain file in /bin

trollog · 10-20-2014, 04:39 AM

slack 14.1

ls /bin/

shows a file.. (literally) '['

Second file, after the symlink to 'Mail' and before 'arch'

What is this file?
Executable, but does nothing.
Looks evil.
Does it have a valid purpose?

ls -l /bin
total 10784

lrwxrwxrwx 1 root root 14 Dec 3 2013 Mail -> /usr/bin/mailx*
-rwxr-xr-x 1 root root 36648 Feb 26 2013 [*
-rwxr-xr-x 1 root root 6896 Aug 7 2013 arch*
lrwxrwxrwx 1 root root 4 Dec 3 2013 awk -> gawk*
-rwxr-xr-x 1 root root 33512 Feb 26 2013 base64*
-rwxr-xr-x 1 root root 28008 Feb 26 2013 basename*
-rwxr-xr-x 1 root root 973688 Sep 26 15:05 bash*

..

ReaperX7 · 10-20-2014, 04:48 AM

It's part of coreutils.

[ - A synonym for test; this program permits expressions like [ expression ].

trollog · 10-20-2014, 04:57 AM

Thanks.

ruario · 10-20-2014, 06:31 AM

Next time you want to know from which package a binary or library comes grep /var/log/packages, e.g.

Code:

$ grep -F "bin/[" /var/log/packages/*
/var/log/packages/coreutils-8.19-i486-1:bin/[

////// · 10-20-2014, 07:06 AM

i remember first time i saw it, i was kinda sure i was compromised / one of my scripts were malfunctioning and created it. that was several years ago.

pan64 · 10-20-2014, 07:11 AM

it still has a man page.

wadsworth · 10-20-2014, 11:06 AM

I don't have a man page for it here.

But I'm running Slackware64 14.0. Maybe that's why?

Stephen Morgan · 10-20-2014, 04:15 PM

My -current also has no manpage, although there is of course a manpage for test.

the3dfxdude · 10-20-2014, 10:13 PM

It's a bash built-in: do 'help [' and you'll get a complete description. The /bin program is for some other context?

ponce · 10-20-2014, 11:47 PM

Code:

$ /bin/[ --help
Usage: test EXPRESSION
  or:  test
  or:  [ EXPRESSION ]
  or:  [ ]
  or:  [ OPTION
Exit with the status determined by EXPRESSION.

      --help     display this help and exit
      --version  output version information and exit

An omitted EXPRESSION defaults to false.  Otherwise,
EXPRESSION is true or false and sets exit status.  It is one of:

  ( EXPRESSION )               EXPRESSION is true
  ! EXPRESSION                 EXPRESSION is false
  EXPRESSION1 -a EXPRESSION2   both EXPRESSION1 and EXPRESSION2 are true
  EXPRESSION1 -o EXPRESSION2   either EXPRESSION1 or EXPRESSION2 is true

  -n STRING            the length of STRING is nonzero
  STRING               equivalent to -n STRING
  -z STRING            the length of STRING is zero
  STRING1 = STRING2    the strings are equal
  STRING1 != STRING2   the strings are not equal

  INTEGER1 -eq INTEGER2   INTEGER1 is equal to INTEGER2
  INTEGER1 -ge INTEGER2   INTEGER1 is greater than or equal to INTEGER2
  INTEGER1 -gt INTEGER2   INTEGER1 is greater than INTEGER2
  INTEGER1 -le INTEGER2   INTEGER1 is less than or equal to INTEGER2
  INTEGER1 -lt INTEGER2   INTEGER1 is less than INTEGER2
  INTEGER1 -ne INTEGER2   INTEGER1 is not equal to INTEGER2

  FILE1 -ef FILE2   FILE1 and FILE2 have the same device and inode numbers
  FILE1 -nt FILE2   FILE1 is newer (modification date) than FILE2
  FILE1 -ot FILE2   FILE1 is older than FILE2

  -b FILE     FILE exists and is block special
  -c FILE     FILE exists and is character special
  -d FILE     FILE exists and is a directory
  -e FILE     FILE exists
  -f FILE     FILE exists and is a regular file
  -g FILE     FILE exists and is set-group-ID
  -G FILE     FILE exists and is owned by the effective group ID
  -h FILE     FILE exists and is a symbolic link (same as -L)
  -k FILE     FILE exists and has its sticky bit set
  -L FILE     FILE exists and is a symbolic link (same as -h)
  -O FILE     FILE exists and is owned by the effective user ID
  -p FILE     FILE exists and is a named pipe
  -r FILE     FILE exists and read permission is granted
  -s FILE     FILE exists and has a size greater than zero
  -S FILE     FILE exists and is a socket
  -t FD       file descriptor FD is opened on a terminal
  -u FILE     FILE exists and its set-user-ID bit is set
  -w FILE     FILE exists and write permission is granted
  -x FILE     FILE exists and execute (or search) permission is granted

Except for -h and -L, all FILE-related tests dereference symbolic links.
Beware that parentheses need to be escaped (e.g., by backslashes) for shells.
INTEGER may also be -l STRING, which evaluates to the length of STRING.

NOTE: [ honors the --help and --version options, but test does not.
test treats each of those as it treats any other nonempty STRING.

NOTE: your shell may have its own version of test and/or [, which usually supersedes
the version described here.  Please refer to your shell's documentation
for details about the options it supports.

Report [ bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils '[ invocation'

note in particular the part after the Synopses in the following extract from the info file

Code:

File: coreutils.info,  Node: test invocation,  Next: expr invocation,  Prev: true invocation,  Up: Conditions

16.3 `test': Check file types and compare values
================================================

`test' returns a status of 0 (true) or 1 (false) depending on the
evaluation of the conditional expression EXPR.  Each part of the
expression must be a separate argument.

   `test' has file status checks, string operators, and numeric
comparison operators.

   `test' has an alternate form that uses opening and closing square
brackets instead a leading `test'.  For example, instead of `test -d
/', you can write `[ -d / ]'.  The square brackets must be separate
arguments; for example, `[-d /]' does not have the desired effect.
Since `test EXPR' and `[ EXPR ]' have the same meaning, only the former
form is discussed below.

   Synopses:

     test EXPRESSION
     test
     [ EXPRESSION ]
     [ ]
     [ OPTION

   Due to shell aliases and built-in `test' functions, using an
unadorned `test' interactively or in a script may get you different
functionality than that described here.  Invoke it via `env' (i.e.,
`env test ...') to avoid interference from the shell.

   If EXPRESSION is omitted, `test' returns false.  If EXPRESSION is a
single argument, `test' returns false if the argument is null and true
otherwise.  The argument can be any string, including strings like
`-d', `-1', `--', `--help', and `--version' that most other programs
would treat as options.  To get help and version information, invoke
the commands `[ --help' and `[ --version', without the usual closing
brackets.  *Note Common options::.

   Exit status:

     0 if the expression is true,
     1 if the expression is false,
     2 if an error occurred.

* Menu:

* File type tests::             -[bcdfhLpSt]
* Access permission tests::     -[gkruwxOG]
* File characteristic tests::   -e -s -nt -ot -ef
* String tests::                -z -n = == !=
* Numeric tests::               -eq -ne -lt -le -gt -ge
* Connectives for test::        ! -a -o

gnashley · 10-21-2014, 02:02 AM

No, /bin/[ is not a shell builtin. bash does have both '[' and '[[' as builtins -and they are the ones that usually get used in scripts, unless /bin/[ gets called explicitly with the full path. The same is true for 'test' and /bin/test. And, /bin/[ is not simply a copy of /bin/test -although some systems might have /bin/[ as a link to /bin/test.
If you run '/bin/[ --help' you will not get the same results as '/bin/test' or 'help [' (from the bash command line).
So, in short, '[' & 'test' (from shell) and '/bin/[' & '/bin/test' are four separate things -each with slightly different features. And, of course, the double '[[' is only available as a shell builtin. One other helpful thing is to use the shell built-in 'type' instead of 'which' in order to identify which binary is really being used.