Combine two command defined as String in variable and run as one

Question

Can I join two commands defined as String and run as one. E.g.

var="ls -alt"
var2="| grep usersfile"
var3="| grep usersfolder"

Following example for joining commands does not work.

a. '{$var & $var2;}'

b. '{$var & $var3;}'

What I what is:

a. run "$var $var2" and get "ls -alt | grep usersfile"

or

b. run "$var $var3" and get "ls -alt | grep usersfolder"

Welcome to the site. Could you elaborate what you mean with the statements a. '{ etc. }' and b. '{ etc. }'. Do you want to assign the "joint command" to a variable a and b respectively which you then want to "execute"? — AdminBee, Jan 31 '20 at 12:37
What is it that you actually want to do? It looks as if you want to get the ls -lt output for something containing the string userfolder and/or the string userfile. Could you rephrase your question to show what the underlying issue is. Grepping the output of ls is very rarely the correct approach. For example, why would you need to store the ls command in a variable? Why not create a shell function instead? — Kusalananda, Jan 31 '20 at 12:49
seems like an xy problem and if you are trying to do what I think you are, you should make a function instead. — jesse_b, Jan 31 '20 at 14:24

score 2 · Answer 1 · edited Jan 31 '20 at 12:48

2

Yes, you can do this by using eval.

$ var="ls -alt"
$ var2="| grep usersfile"
$ var3="| grep usersfolder"
$ eval "$var$var2"
< Output of ls -alt | grep userfile >
$ eval "$var$var3"
< Output of ls -alt | grep usersfolder >

edited Jan 31 '20 at 12:48

Stéphane Chazelas

544,893

answered Jan 31 '20 at 12:27

Panki

6,664

works fine. Thnx! – m19v Jan 31 '20 at 12:38
@iodev Welcome to SE. If you find that this answers your question, please "accept" it as your answer. For "thanks", use the upvote / up arrow by the answer to say "this answer is useful". – L. Scott Johnson Jan 31 '20 at 12:45

score 1 · Accepted Answer · answered Jan 31 '20 at 15:16

You probably shouldn't do it this way. Depending on what you are actually trying to accomplish there are likely other options:

If you are just trying to have a dynamic option to select your search string you could do something like this instead:

#!/bin/bash
pattern=$1

ls -alt | grep "${pattern:-.}"

This will search for whatever your first positional parameter is, or anything if none is provided.

Or if you're trying to act on some condition:

#!/bin/bash

cmd=(ls -alt)
p1=usersfile
p2=usersfolder

if [[ condition1 ]]; then
    p=$p1
elif [[ condition2 ]]; then
    p=$p2
fi

"${cmd[@]}" | grep "$p"

Or in a function:

#!/bin/bash

ls_func () {
    local p=$1
    ls -alt | grep "${p:-.}"
}

while getopts p:c: opt; do
    case $opt in
        c)  command=$OPTARG;;
        p)  pattern=$OPTARG;;
    esac
done

if [[ $command == 'ls' ]]; then
    ls_func "$pattern"
fi

Combine two command defined as String in variable and run as one

2 Answers2