Running variable commands in BASH

Question

I have a BASH script called script.sh that takes 3 arguments and runs an executable file with them. The first two are just numbers, but the last is an argument giving the input file. I would like the script to run the executable with the input as an argument of the executable and using the "<" as a replacement for stdin. (i.e.

bash script.sh 5 1 input.txt

calls the BASH script, and the contents of script.sh are as follows:

#!/bin/bash

command1="./l${1}t${2} $3"
command2="./l${1}t${2} < $3"

echo + ${command1}
${command1}

echo + ${command2}
${command2}

When I echo command1 I get

./l5t1 input.txt

which is exactly what I want and it runs just fine.

When I echo command2 I get

./l5t1 < input.txt

which is again what I want. The problem is the actual command the script runs is

./l5t1 '<' input.txt

which of course causes a segmentation fault in my program.

I'd like to know if there is a way I can run command 2 so that it runs the string exactly as it is printed in the echo output. Honestly, I have no idea why the single quotes are even inserted around the < character.

Anyways, you should be able to eval the contents of the variable. — Rubens
– Rubens, Commented Oct 2, 2013 at 18:56

John Kugelman · Accepted Answer · 2013-10-02 19:10:49Z

4

If you want to store commands it's better to use functions than variables. As you've found out, redirections don't work when stored in variables (nor do |, ;, or &).

command1() {
    "./l${1}t${2}" "$3"
}

command2() {
    "./l${1}t${2}" < "$3"
}

command1 "$@"
command2 "$@"

Here I've defined two functions, which are called with the arguments from the array $@. "$@" forwards the script's arguments to the functions.

Notice also that I've put quotes around "./${1}t${2}" and "$3". Using double quotes allows these parameters to contain spaces. Liberal quoting is a good defensive scripting technique.

(I strongly recommend not doing eval "$command2". Using eval is a really dangerous habit to get into.)

edited Oct 2, 2013 at 19:10

answered Oct 2, 2013 at 19:01

John Kugelman

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