shell: build variable with multiple paths and commands

Question

Is there a way to build a varible like this:

var="-i \"/path to/file 1.mp4\" -i \"/path to/file 2.mp4\""

And then use this later in a program:

ffmpeg $var

I know \" doesn't work, and eval works in a way, but eval can be ugly.

Is there a solution without eval?

Does the way you've built the variable in your question not work? It looks fine to me. Also, what shell? POSIX? bash? If you're using a more advanced shell like bash/zsh/ksh/fish, you might get useful results by reading the documentation about Arrays. — ghoti
– ghoti, Commented Jun 8, 2016 at 20:09
@ghoti This is attempt is not fine. The intention is for $var to expand to 4 arguments, but in this example it expands to 8 in a POSIX-compliant shell: -i, "/path, to/file, 1.mp4", -i, "/path, to/file, and 2.mp4". The embedded quotes are literal characters that do not protect the surrounded whitespace from word-splitting. — chepner
– chepner, Commented Jun 9, 2016 at 4:18
@chepner Right, of course, I should have been more explicit. The expansion works with word splitting as expected using eval; my test was to do the assignment as shown, then eval set -i $var, then inspect $1, $2, etc. If you can trust your input not to break things, there's little harm in using eval. If the OP's shell supports it, I still think a simple array might be the way to go. — ghoti
– ghoti, Commented Jun 9, 2016 at 17:28

Dennis B. · Accepted Answer · 2016-06-08 20:25:38Z

1

The next best thing to do is to build an array. When you put ${var[@]} somewhere, it will replace it with the complete array contents. If you put "${var[@]}" somewhere (with quotes), the elements of the array will be quoted, so elements with spaces will not be split at the whitespace into multiple elements:

#!/bin/bash

function test() {
    for ((i=0; i<$#; i++)) {
        echo "Param $i: ${!i}"
    }
}

var[0]="-i"
var[1]="/a/path"
var[2]="-i"
var[3]="/second/path with space/"

test "${var[@]}"

Will output:

Param 0: test.sh
Param 1: -i
Param 2: /a/path
Param 3: -i

and that is exactly what you require. I only tried this in bash, other shells may behave differently.

answered Jun 8, 2016 at 20:25

Dennis B.

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2 Comments

jb_alvarado Over a year ago

Thank you Dennis! This is exactly what I need! And the array fits also perfect in my script.

Dennis B. Over a year ago

Nice to see I could help!

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