Bash Code, what does this do?

Question

This may sound like a very simple question, but what does

ADB=${ADB:-adb}

do?

I've run it and it sets $ADB to adb , I know the assignment of the value to the ADB shell variable part, but what does ${ADB:-adb} do? I haven't seen this syntax/usage before.

It was recently commented in http://stackoverflow.com/questions/15143863/bash-colon-operator — fedorqui, Mar 01 '13 at 21:11
if the variable `ADB` is not set, then it gets set to `adb`, otherwise it keeps it's old value. This is explained in various FAQs and in the bash manual page — Anya Shenanigans, Mar 01 '13 at 21:11

Gilles Quénot · Accepted Answer · 2013-03-04T22:08:20.647

4

If ADB variable is not set, bash will displays the string adb.

Another trick is to type :

echo ${ADB:=adb}

In this case, ADB variable is assigned with the string adb (still if ADB is not set)

All of these nice features are bash parameter expansions

edited Mar 04 '13 at 22:08

answered Mar 01 '13 at 21:11

Gilles Quénot

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A shorter alternative to that trick is `: ${ADB:=adb}`; you don't have to assign the value twice. – chepner Mar 01 '13 at 22:45
post edited accordingly, thanks, typed too fast – Gilles Quénot Mar 03 '13 at 21:46

Bash Code, what does this do?

1 Answers1