: colon command for bash

Speaking as a bash newbie I have been upgrading my .bashrc via copy/paste + github and I have come across the : command that stumps both me and google. e.g. : ${USER_BASH_COMPLETION_DIR:=~/.bash_completion.d}.

Without this statement originally in my .bashrc, and typing this stuff into my terminal (-> indicates relevant output):

: ${USER_BASH_COMPLETION_DIR=~/.bash_completion.d}
echo $USER_BASH_COMPLETION_DIR
-> /Users/sh/.bash_completion.d

And:

: ${USER_BASH_COMPLETION_DIR=~/.bash_completion.d}
export USER_BASH_COMPLETION_DIR=asdf
echo $USER_BASH_COMPLETION_DIR
-> asdf

But:

: ${USER_BASH_COMPLETION_DIR=~/.bash_completion.d}
export USER_BASH_COMPLETION_DIR=asdf
: ${USER_BASH_COMPLETION_DIR=~/.bash_completion.d}
echo $USER_BASH_COMPLETION_DIR
-> asdf

I don't get it!

1) How does the colon command set a variable but cannot overwrite one set by export?

2) What is the logic behind using : in some .bashrc?

Using Mac 10.6.8

(out of context include of keyword colon just to help others like me who tried to search for that term)

1 Answer

: is a shell builtin that is basically equivalent to the true command. It is often used as a no-op eg after an if statement. You can read more about it in this question from stack overflow.

The ${varname=value} basically means set the value of $varname to value if $varname is not already set, and then return the value of $varname. Though if you try to run that at the command line it will try to run the value returned. Putting the : in front as a no-op prevents bash from trying to run the value.

Note there are two slightly different forms:

${varname:=value}

sets varname to value if varname is either unset or null.

${varname=value}

only sets the value of varname if varname is currently unset (i.e., it will not change varname from "" to value)

(Thank you to chepner for clarifying that in a comment).

Someone else referencing this method