Pass Variable Name and Value in Bash Script

Question

I just want to verify that the script I wrote is doing what I think it's doing, and that it's doing it properly.

I wanted to write a script that takes an environment variable and a string value, and then sets that variable to the given value. So I can do something like setvar BOOST_HOME /home/me/boost/boost_1.52.0 and the script will export BOOST_HOME=/home/me/boost/boost_1.52.0

Something like:

#!/bin/bash
# Usage: setvar VAR VAR_VALUE
function setvar()
{
    VAR=${1}
    VAR_VALUE=${2}
    if [ -d $2 ]
    then
        eval export $VAR=$2
    fi
}

This seems to work, at least judging from a echo echo tests, but I am still not very comfortable with shell scripting, and would like someone to either verify what I am doing or point out what I am doing wrong / less correct.

I rather think it should work without the eval, and that it would be safer without the eval (and I'd put double quotes around the $2, but I may be old-fashioned). You set VAR_VALUE but don't use it. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 13, 2014 at 17:10
yep its bad practice to use eval. but if you know you can make it secure, then id dosent matter — Hackaholic
– Hackaholic, Commented Nov 13, 2014 at 17:10
With export you need quotes around both $1 and $2 to be safe. — Etan Reisner
– Etan Reisner, Commented Nov 13, 2014 at 17:11
A comment on the purpose of the function itself: if $2 isn't a directory, this function gives no indication that BOOST_HOME won't actually be set. — chepner
– chepner, Commented Nov 13, 2014 at 17:13

rici · Accepted Answer · 2014-11-13 17:46:33Z

5

You don't need the eval.

setvar() {
  if [[ -d $2 ]]; then
    export "$1=$2"
  fi
}

Using [[ instead of [ avoids the need to quote $2, since the bash (and other shell) extension [[ does not word-split interior parameter expansions. If I'd stuck with the old-fashioned [ -d "$2" ], I would have had to quote the $2 in case its value included whitespace.

edited Nov 13, 2014 at 17:46

answered Nov 13, 2014 at 17:09

rici

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4 Comments

Etan Reisner Over a year ago

Might be worth explaining why you need quotes around the variables in the export but not in [[ ]] since it isn't at all obvious (and in quick testing export surprised me in ways I wasn't expecting).

rici Over a year ago

@EtanReisner: What suprised you about export?

rici Over a year ago

@EtanReisner: Oh, yeah, that. export/typeset/local/declare (all the same builtin) partially parse the command line; if it is recognizable as an assignment, the part after the = is expanded roughly according to [[ rules. If it's not, then regular expansion is followed, and the result is processed. This is particularly wierd when you're using it to assign/declare array elements.

Etan Reisner Over a year ago

Right. I mostly understood what must have been going on there it was just surprising. Naively, I'd expected it to not be smart about spaces at all (forcing quotes on the rhs) when that wasn't necessary that was surprising (in the good, and understandable, way) then I tested more and was surprised (in the actually surprised way).

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