the meaning of builtin colon

[rubinglen]

I'm having trouble understanding what the colon is used for in bash? Can somebody explain it to me with an example? thanks!

[Cabhan]

Well first, from the bash man page:

       : [arguments]
              No  effect;  the command does nothing beyond expanding arguments
              and performing any specified redirections.  A zero exit code  is
              returned.

Basically, : does nothing, and produces no output. So why is it useful? It allows us to do stuff like this:

#!/bin/bash

if [ $DEBUG_DEFINED ]; then
    DEBUG_COMMAND=echo
else
    DEBUG_COMMAND=:
fi

DEBUG_COMMAND "This is a debug statement."

Now, we can enable or disable debug statements just by changing the value of $DEBUG_DEFINED.

: is also useful for clearing files:

: > file

file still exists, but it has no content.

Does this make sense?