Question

I am trying to check the value of an environment variable and depending on the value do certain things and it works fine as long as the variable is set. When it isn’t though I get a whole bunch of errors (as BASH is trying to compare the string I specify with an undefined variable, I guess)

I tried implementing an extra check to prevent it happening but no luck. The block of code I am using is:

#!/bin/bash

if [ -n $TESTVAR ]
then
  if [ $TESTVAR == "x" ]
  then
    echo "foo"
    exit
  elif [ $TESTVAR == "y" ]
  then
    echo "bar"
    exit
  else
    echo "baz"
    exit
  fi
else
  echo -e "TESTVAR not set\n"
fi

And this is the output:

$ export TESTVAR=x
$ ./testenv.sh 
foo
$ export TESTVAR=y
$ ./testenv.sh 
bar
$ export TESTVAR=q
$ ./testenv.sh 
baz
$ unset TESTVAR
$ ./testenv.sh 
./testenv.sh: line 5: [: ==: unary operator expected
./testenv.sh: line 9: [: ==: unary operator expected
baz

My question is, shouldn’t ‘unset TESTVAR’ nullify it? It doesn’t seem to be the case…

Answer 1

Enclose the variable in double-quotes.

if [ "$TESTVAR" == "foo" ]

if you do that and the variable is empty, the test expands to:

if [ "" == "foo" ]

whereas if you don’t quote it, it expands to:

if [  == "foo" ]

which is a syntax error.