Setting a variable inside a conditional

Question

Is it possible to set a variable from the output of a command inside of a conditional where the conditional is false if nothing gets assigned to the variable.

If I set the variable to a grep with no return and then test:

test=$(echo hello | grep 'helo')
if [[ ! -z $test ]]; then
  echo "is set"
else
  echo "not set"
fi

Output: not set (this is expected)

But I'm trying to put it all into one statement like this:

test=
if [[ ! -z test=$(echo hello | grep 'helo') ]]; then
  echo "is set"
else
  echo "not set"
fi

output: "is set" (expected not set)

test= is the same as test="", so the variable is not unset, but set to the empty string. — Socowi
– Socowi, Commented Feb 22, 2018 at 20:22
@Socowi, ...the variable isn't being set at all; the string test= is being compared against the empty string, not evaluated as an assignment. — Charles Duffy
– Charles Duffy, Commented Feb 22, 2018 at 20:52
@ArchieArchbold : In your second example, the variable test doesn't get assigned inside the if statement. Instead, -z looks at the string test=SOMETHING, and this string is always non-empty, because no matter what SOMETHING evaluates to, the argument of the -z operator always starts with test=. Side note: If you replace ! -z by -n, the code gets a bit more readable. — user1934428
– user1934428, Commented Feb 23, 2018 at 7:20

that other guy · Accepted Answer · 2018-02-22 20:50:44Z

4

grep returns success if there is a match, so you can just do:

if test=$(echo hello | grep 'helo')
then
  echo "Match: $test"
else
  echo "No match"
fi

If you're running something that doesn't differentiate by exit code, you can assign and check in two statements on the same line:

if var=$(cat) && [[ -n $var ]] 
then
  echo "You successfully piped in some data."
else
  echo "Error or eof without data on stdin."
fi

(or ; instead of && if you want to inspect the result even when the command reports failure)

edited Feb 22, 2018 at 20:50

answered Feb 22, 2018 at 20:43

that other guy

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1 Comment

Archie Archbold Over a year ago

exactly what I was looking for. Thank you much!

agc · Accepted Answer · 2018-02-23 06:59:07Z

0

Bit of a hack, using the shell's parameter expansion alternate value syntax, echo -e and some backspaces:

test=$(echo hello | grep 'helo'); echo -e not${test:+\\b\\b\\bis} set

Which outputs is set or not set depending on what grep finds.

edited Feb 23, 2018 at 6:59

answered Feb 23, 2018 at 6:24

agc

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