Conditionally omit a command line option in bash

Question

I have a long command which I want to modify in a certain case to either include an option or not.

E.g.

java -jar compiler.jar --externs "$externs" --foo "another" --bar "more"

where I want the --externs "$externs" to be omitted completely if $externs is empty. Is there any way to do this in bash without resorting to building up the whole command with a string?

Yes, it's within a larger script. One other possibility that I'm trying to avoid is simply repeating the command in two if branches. That gets hairy if there are a few different commands which need to be omitted. I was thinking there might be some string substitution command that resulted in bash not even inserting the argument. But I haven't found it yet, if it exists. — Chris Middleton
– Chris Middleton, Commented Dec 7, 2015 at 22:44
Incidentally, this is subquestion 3 of BashFAQ #50: mywiki.wooledge.org/BashFAQ/… — Charles Duffy
– Charles Duffy, Commented Dec 7, 2015 at 22:54
@miken32, you deleted your answer before I could answer your request for clarification. Inspect the output of externs='two words'; if [ ! -z "$externs" ]; then externs_option="--externs \"$externs\""; fi; printf '%q\n' java -jar compiler.jar $externs_option --foo "two words" --bar "etc" -- and compare the two words passed alongside --externs and that passed alongside --foo. — Charles Duffy
– Charles Duffy, Commented Dec 7, 2015 at 23:02

Charles Duffy · Accepted Answer · 2015-12-07 23:06:11Z

12

This is what the ${key:+WORDS} expansion is for:

java ... ${externs:+--externs "$externs"}

It's also a common idiom to use an array for the purpose:

args=( ) if [[ $externs ]]; then args+=( --externs "$externs" ) fi java ... "${args[@]}"

edited Dec 7, 2015 at 23:06

answered Dec 7, 2015 at 22:48

Charles Duffy

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4 Comments

Chris Middleton Over a year ago

This answer is well-written and I like your array approach as well. I think you're missing a : before the + though.

Charles Duffy Over a year ago

@ChrisMiddleton, whether one wants a : before the + depends on how you want to handle empty, as opposed to unset, values; if you want to treat a variable that exists but is empty as if it were unset, then yes, it would be :+.

Charles Duffy Over a year ago

@ChrisMiddleton, ...rereading your question, you did indeed explicitly say "empty"; edited appropriately.

Chris Middleton Over a year ago

Using Google's Closure Compiler, I have to provide each extern file with a separate "--externs" option, so your array approach has become very useful in my case.

Wayne · Accepted Answer · 2019-04-19 02:40:51Z

You could try

java -jar compiler.jar ${externs:+--externs} "${externs:-}" --foo "another" --bar "more"

explanation, from bash docs

${parameter:-word}

Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

${parameter:+word}

Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

You could try it yourself with:

externs="some value" printf '%s\n' ${externs:+--externs} "${externs:-}"

output

--externs
some value

Note: I used printf here test the case of $externs containing multiple words, where if the quoting wasn't being applied after the variable expansion, the output would look like

--externs
some
value

So I believe that should suffice.

externs= echo ${externs:+--externs} "${externs:-}"

output: nil

From a correctness perspective, I have nothing to add to this -- it's a good answer (and a little more paranoid about hewing to the POSIX sh specification than mine, which relies on behavior that bash implements but doesn't document terribly well). That said -- what's up with the formatting? > principally used to indicate that one is quoting text; presumably it has a different meaning here.
Thanks for your answer. I especially like the use of :-. I chose Charles's only because it was first and had the array solution as well, but there's some neat stuff here. From some experimenting, I see that if foo=, then ${foo:-} will be an omitted parameter whereas ${foo:-""} will be the empty string. Cool.
cool , me the > format just helps to visually distinguish between code and output. Of course its always helpful if your code is portable

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