Checking variables in bash

sharky · 02-13-2009, 12:23 PM

Code:

#!/bin/bash
export MYVAR="apple"
if [ ! $?MYVAR ]
then 
export MYVAR="orange"
echo "MYVAR was not previously set to $MYVAR"
else
echo "MYVAR is set to $MYVAR" 
fi

The output of the above simple script is "MYVAR is set to apple", no surprise.

However if I comment out the first export statement so that MYVAR is not set I would expect the 'then' statement to execute. MYVAR would get set to "orange" and the output should be "MYVAR was not previously set to orange". Instead it outputs "MYVAR is set to ". So whether MYVAR is set or not it executes the else statement.

To work around this I can change the if statement as follows.

Code:

if [ ! "$MYVAR" ]

Taking out the '?' and wrapping double quotes around the variable works.

Shouldn't 'then' execute if MYVAR is not set in the original script?

frieza · 02-13-2009, 12:57 PM

you cant comment out an if statement and expect a then statement to still work for one
or are you commenting out the if then else and fi
second of all if you comment out the export myvar=apple instead of the if, THEN you should see your then work
if [ ! "$MYVAR" ] only checks if the variable is null
try if [ if $myvar != "orange" ] instead

colucix · 02-13-2009, 03:23 PM

Code:

if [ ! $?MYVAR ]

This is not a valid BASH syntax - or better - it is not what you expect to be. The notation $?varname expands to 1 if varname is set, expands to 0 if varname is not set, but only in C-shells!

To check if a variable is set or not in Bash, you have to use

Code:

if [ -z "$MYVAR" ]

where the -z test means "the length of string is zero".

When you use the $?MYVAR notation in Bash, it is interpreted as the concatenation of $? (the exit status of the previous command) plus the literal string "MYVAR". You can easily demonstrate it running the script with bash -x. This will report a trace of the actual commands executed by the shell:

Code:

$ bash -x test.sh
+ export MYVAR=apple
+ MYVAR=apple
+ '[' '!' 0MYVAR ']'
+ echo 'MYVAR is set to apple'
MYVAR is set to apple

As you can see, the string in red is the result of the expansion performed by the shell, as mentioned above. In summary, the correct Bash version of your script should be:

Code:

#!/bin/bash
#export MYVAR="apple"
if [ -z $MYVAR ]
then
  export MYVAR="orange"
  echo "MYVAR was not previously set to $MYVAR"
else
  echo "MYVAR is set to $MYVAR"
fi

sharky · 02-13-2009, 07:09 PM

thanks colucix. Your detailed explanation went above and beyond the call of duty.

frieza, your misunderstanding is ...er.. understandable considering my poor explanation of the issue. I did not comment out the 'if' statement. I commented out the first 'export' statement.

Regards all,

jlinkels · 02-13-2009, 08:07 PM

It seems the best hidden secret of shell programming, but run your script with:

Code:

sh -x myscript

It becomes immediately apparent what all variables and statements are expanded to and solves 80% of your problems.

jlinkels