What is the purpose of the colon GNU Bash builtin

Question

What is the purpose of a command that does nothing  being little more than a comment leader  but is actually a shell builtin in and of itself   It s slower than inserting a comment into your scripts by about 40  per call  which probably varies greatly depending on the size of the comment  The only possible reasons I can see for it are these     poor man s delay function for   x 0 x lt 100000   x     do     done    inserting comments into string of commands command   command     we need a comment in here for some reason   command    an alias for  true   lazy programming  while     do command   done   I guess what I m really looking for is what historical application it might have had

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You could use it in conjunction with backticks (``) to execute a command without displaying its output, like this:

: `some_command`

Of course you could just do some_command > /dev/null, but the :-version is somewhat shorter.

That being said I wouldn't recommend actually doing that as it would just confuse people. It just came to mind as a possible use-case.

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Another way  not yet mentioned here is the initialisation of parameters in infinite while-loops  Below is not the cleanest example  but it serves it s purpose     usr bin env bash    quot  1 quot     amp  amp  foo 0  amp  amp  bar  quot baz quot  while    quot   foo 2  quot   quot   bar  qux  quot   do     echo  quot  foo quot         foo    3     amp  amp  echo  quot  bar quot   amp  amp  break        foo foo 1    done

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can also be for block comment  similar to       in C language   For example  if you want to skip a block of code in your script  you can do this      lt  lt   SKIP   your code block here  SKIP

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I saw this usage in a script and thought it was a good substitute for invoking basename within a script   oldIFS  IFS   IFS     for basetool in  0   do     done   IFS  oldIFS         this is a replacement for the code  basetool   basename  0

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It s similar to pass in Python   One use would be to stub out a function until it gets written   future function

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A useful application for   is if you re only interested in using parameter expansions for their side-effects rather than actually  passing their result to a command  In that case you use the PE as an argument to either   or false depending upon whether you want an exit status of 0 or 1  An example might be      var   1    Since   is a builtin it should be pretty fast

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If you d like to truncate a file to zero bytes  useful for clearing logs  try this     gt  file log

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I use it to easily enable disable variable commands      bin bash if      VERBOSE             VERBOSE      0      then     vecho           no  verbose echo  else     vecho echo      enable  verbose echo  fi   vecho  Verbose echo is ON    Thus      vecho   VERBOSE 1   vecho Verbose echo is ON   This makes for a clean script  This cannot be done with       Also      gt afile   is one of the simplest ways to guarantee that  afile  exists but is 0 length

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It s also useful for polyglot programs      usr bin env sh         exec    command -v node     0        function            This is now both an executable shell-script and a JavaScript program  meaning   filename js  sh filename js  and node filename js all work    Definitely a little bit of a strange usage  but effective nonetheless      Some explication  as requested    Shell-scripts are evaluated line-by-line  and the exec command  when run  terminates the shell and replaces it s process with the resultant command  This means that to the shell  the program looks like this      usr bin env sh         exec    command -v node     0        As long as no parameter expansion or aliasing is occurring in the word  any word in a shell-script can be wrapped in quotes without changing its  meaning  this means that     is equivalent to    we ve only wrapped it in quotes here to achieve the JavaScript semantics described below      and as described above  the first command on the first line is a no-op  it translates to       or if you prefer to quote the words            Notice that the    carries no special meaning here  as it does in JavaScript  it s just a meaningless word that s being thrown away   Finally  the second command on the first line  after the semicolon   is the real meat of the program  it s the exec call which replaces the shell-script being invoked  with a Node js process invoked to evaluate the rest of the script  Meanwhile  the first line  in JavaScript  parses as a string-literal        and then a comment  which is deleted  thus  to JavaScript  the program looks like this        function            Since the string-literal is on a line by itself  it is a no-op statement  and is thus stripped from the program  that means that the entire line is removed  leaving only your program-code  in this example  the function          body

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Self-documenting functions  You can also use   to embed documentation in a function   Assume you have a library script mylib sh  providing a variety of functions  You could either source the library    mylib sh  and call the functions directly after that  lib function1 arg1 arg2   or avoid cluttering your namespace and invoke the library with a function argument  mylib sh lib function1 arg1 arg2    Wouldn t it be nice if you could also type mylib sh --help and get a list of available functions and their usage  without having to manually maintain the function list in the help text       bin bash    all  public  functions must start with this prefix LIB PREFIX  lib       public  library functions lib function1           This function does something complicated with two arguments              Parameters            arg1 - first argument   1             arg2 - second argument              Result            it s complicated         actual function code starts here    lib function2           Function documentation        function code here      help function --help         echo MyLib v0 0 1     echo     echo Usage  mylib sh  function name  args       echo     echo Available functions      declare -f   sed -n -e      LIB PREFIX               LIB PREFIX           t                s      LIB PREFIX          n     1      s     t                      s                 p         main code if      BASH SOURCE 0         0      then       the script was executed instead of sourced       invoke requested function or display help     if      type -t -   1  2  dev null     function    then                  else         --help     fi fi   A few comments about the code    All  public  functions have the same prefix  Only these are meant to be invoked by the user  and to be listed in the help text  The self-documenting feature relies on the previous point  and uses declare -f to enumerate all available functions  then filters them through sed to only display functions with the appropriate prefix  It is a good idea to enclose the documentation in single quotes  to prevent undesired expansion and whitespace removal  You ll also need to be careful when using apostrophes quotes in the text  You could write code to internalize the library prefix  i e  the user only has to type mylib sh function1 and it gets translated internally to lib function1  This is an exercise left to the reader  The help function is named  --help   This is a convenient  i e  lazy  approach that uses the library invoke mechanism to display the help itself  without having to code an extra check for  1  At the same time  it will clutter your namespace if you source the library  If you don t like that  you can either change the name to something like lib help or actually check the args for --help in the main code and invoke the help function manually

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Historically  Bourne shells didn t have true and false as built-in commands  true was instead simply aliased to    and false to something like let 0     is slightly better than true for portability to ancient Bourne-derived shells  As a simple example  consider having neither the   pipeline operator nor the    list operator  as was the case for some ancient Bourne shells   This leaves the else clause of the if statement as the only means for branching based on exit status   if command  then    else      fi   Since if requires a non-empty then clause and comments don t count as non-empty    serves as a no-op   Nowadays  that is  in a modern context  you can usually use either   or true  Both are specified by POSIX  and some find true easier to read  However there is one interesting difference    is a so-called POSIX special built-in  whereas true is a regular built-in    Special built-ins are required to be built into the shell  Regular built-ins are only  typically  built in  but it isn t strictly guaranteed  There usually shouldn t be a regular program named   with the function of true in PATH of most systems  Probably the most crucial difference is that with special built-ins  any variable set by the built-in - even in the environment during simple command evaluation - persists after the command completes  as demonstrated here using ksh93     unset x    x hi    echo   x    hi     x hi true  echo   x         Note that Zsh ignores this requirement  as does GNU Bash except when operating in POSIX compatibility mode  but all other major  POSIX sh derived  shells observe this including dash  ksh93  and mksh  Another difference is that regular built-ins must be compatible with exec - demonstrated here using Bash       exec     -bash  exec     not found     exec true      POSIX also explicitly notes that   may be faster than true  though  this is of course an implementation-specific detail

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Two more uses not mentioned in other answers   Logging  Take this example script   set -x   Logging message here example command   The first line  set -x  makes the shell print out the command before running it  It s quite a useful construct  The downside is that the usual echo Log message type of statement now prints the message twice  The colon method gets round that  Note that you ll still have to escape special characters just like you would for echo   Cron job titles  I ve seen it being used in cron jobs  like this   45 10         Backup for database    opt backup sh   This is a cron job that runs the script  opt backup sh every day at 10 45  The advantage of this technique is that it makes for better looking email subjects when the  opt backup sh prints some output

[bash] What is the purpose of the : (colon) GNU Bash builtin?

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