Use of ":" (was: Re: mtab very long)

From: Paul Smith (psmith_at_nortel.com)
Date: 05/25/05

  • Next message: Eric van der Paardt: "RE: Small issues, but still annoying" 
    To: debian-user@lists.debian.org
Date: 25 May 2005 08:28:40 -0400

    %% Maurits van Rees <maurits@vanrees.org> writes:

      mvr> maurits@mauritsvanrees:~$ : > temp

      mvr> Does anyone know any other uses for this ':' command?

    The most common usage these days is in conjunction with :=, like this:

        : ${TMPDIR:=/tmp}

    The := form assigns the value if the variable is empty, but it _also_
    substitutes the value. Adding ":" to the front makes that value a
    no-op.

    If you need to write scripts compatible with older versions of the
    shell, then there are places where you need empty statements. For
    example, in traditional Bourne shells you can't have an empty case
    entry, so if you need it to do nothing, like:

        case $foo in
            bar) : do nothing ;;
            *) echo "bad value: $foo"; exit 1 ;;
        esac

    Also, older shells didn't have the "not" operator for program exit
    codes, and you can't have an empty clause in an if-statement, so if you
    wanted to do something when a program failed you needed to write it:

        if someprogram; then
            : success
        else
            echo "someprogram failed!"
            exit 1
        fi

    HTH! 

    -- 
-------------------------------------------------------------------------------
 Paul D. Smith <psmith@nortel.com>           HASMAT--HA Software Mthds & Tools
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist
-------------------------------------------------------------------------------
   These are my opinions---Nortel Networks takes no responsibility for them.
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