Checking for environment variables
LinuxBashShellLinux Problem Overview
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...
Linux Solutions
Solution 1 - Linux
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.
Solution 2 - Linux
Look at the section titled "Parameter Expansion" you'll find things like:
${parameter:-word}
> Use Default Values. If the parameter is unset or null, the > expansion of word is substituted. Otherwise, the value of > parameter is substituted.
Solution 3 - Linux
In Bash (and ksh and zsh), if you use double square brackets you don't need to quote variables to protect against them being null or unset.
$ if [ $xyzzy == "x" ]; then echo "True"; else echo "False"; fi
-bash: [: ==: unary operator expected
False
$ if [[ $xyzzy == "x" ]]; then echo "True"; else echo "False"; fi
False
There are other advantages.
Solution 4 - Linux
Your test to see if the value is set
if [ -n $TESTVAR ]
actually just tests to see if the value is set to something other than an empty string. Observe:
$ unset asdf
$ [ -n $asdf ]; echo $?
0
$ [ -n "" ]; echo $?
1
$ [ -n "asdf" ]; echo $?
0
Remember that 0
means True.
If you don't need compatibility with the original Bourne shell, you can just change that initial comparison to
if [[ $TESTVAR ]]
Solution 5 - Linux
After interpretation of the missing TESTVAR
you are evaluating [ == "y" ]
. Try any of:
"$TESTVAR"
X$TESTVAR == Xy
${TESTVAR:-''}