Bash or-equals ||= like Ruby

Question

Does Bash have something like ||= ?

I.e., is there a better way to do the following:

if [ -z $PWD ]; then PWD=`pwd`; fi

I'm asking because I get this error:

$ echo ${`pwd`/$HOME/'~'}
-bash: ${`pwd`/$HOME/'~'}: bad substitution

So, my plan is to do:

if [ -z $PWD ]; then PWD=`pwd`; fi
echo ${PWD/$HOME/'~'}

My real question is: "Is there a better way to do the following?"

# ~/.bash_profile

# Set prompt to RVM gemset, abbr. of current directory & (git branch).
PROMPT_COMMAND='CUR_DIR=`pwd|sed -e "s!$HOME!~!"|sed -E "s!([^/])[^/]+/!\1/!g"`'
PS1='$(~/.rvm/bin/rvm-prompt g) [$CUR_DIR$(__git_ps1)]\$ '

not different (or better) but shorter: `[ -z $PWD ] && PWD=\`pwd\` ` — pje, May 07 '13 at 22:14

score 20 · Accepted Answer · answered Jul 28 '11 at 13:36

20

Bash allows for default values:

a=${b-`pwd`}

If $b is undefined, then pwd is used instead in assigning $a.

answered Jul 28 '11 at 13:36

Manny D

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More often, I see another form: `${PWD:-\`pwd\`}` I wonder if they are equivalent – GregT Jul 14 '15 at 23:38
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@GregT if `b` is unset, they will behave the same. If `b` is set to an empty string, however, then `${b-\`pwd\`}` will result in an empty string, but `${b:-\`pwd\`}` will result in the working directory. – Jun-Dai Bates-Kobashigawa Aug 25 '15 at 14:52

user456584 · Answer 2 · 2020-01-21T22:08:37.200

2

Another solution (which is more akin to Ruby's or-equals in my opinion) would be:

[ -n $PWD ] || PWD=`pwd`

edited Jan 21 '20 at 22:08

answered Jul 20 '13 at 21:46

user456584

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While I like the style, if MyVar is set to a zero length string it will not be set to 'value' in this example, whereas using bash "default values" a la @Manny D, above, would work arguably correctly, as in: `MyVar=${MyVar-'value'} – malcook May 15 '15 at 22:14

score 2 · Answer 3 · answered Jul 28 '11 at 16:32

2

You can set your prompt to be the working directory with this:

PS1='\w '   # Using \W will provide just basename

answered Jul 28 '11 at 16:32

grok12

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Bash or-equals ||= like Ruby

3 Answers3