This script is getting an error:
elif [ $operation = "man" ]; then
if [ $aug1 = "add" ]; then # <- Line 75
echo "Man Page for: add"
echo ""
echo "Syntax: add [number 1] [number 2]"
echo ""
echo "Description:"
echo "Add two different numbers together."
echo ""
echo "Info:"
echo "Added in v1.0"
echo ""
elif [ -z $aug1 ]; then
echo "Please specify a command to read the man page."
else
echo "There is no manual page for that command."
fi
I get this error:
calc_1.2: line 75: [: =: unary operator expected
5 Answers
If you know you're always going to use Bash, it's much easier to always use the double bracket conditional compound command [[ ... ]], instead of the POSIX-compatible single bracket version [ ... ]. Inside a [[ ... ]] compound, word-splitting and pathname expansion are not applied to words, so you can rely on
if [[ $aug1 == "and" ]];
to compare the value of $aug1 with the string and.
If you use [ ... ], you always need to remember to double quote variables like this:
if [ "$aug1" = "and" ];
If you don't quote the variable expansion and the variable is undefined or empty, it vanishes from the scene of the crime, leaving only
if [ = "and" ];
which is not a valid syntax. (It would also fail with a different error message if $aug1 included white space or shell metacharacters.)
The modern [[ operator has lots of other nice features, including regular expression matching.
It took me a while to find this, but note that if you have a spacing error, you will also get the same error:
[: =: unary operator expected
Correct:
if [ "$APP_ENV" = "staging" ]
vs
if ["$APP_ENV" = "staging" ]
As always, setting -x debug variable helps to find these:
set -x
Try assigning a value to $aug1 before use it in if[] statements; the error message will disappear afterwards.
This error can also occur reading numerical input that could possibly be blank (to accept a default option).
The solution is to structure the if statement with multiple conditions & test for an empty variable first.
For example:
# sanitise input
var=$(echo $ans | tr -cd "[:digit:]")
if [ -z "$var" ] || [ "$var" -lt 1 ]; then
do_something
fi
I had to solve this unary operator expected issue in remove_old_pkgs() of the helper script abk for Arch Sign Modules.
See also 6.4 Bash Conditional Expressions
You can also set a default value for the variable, so you don't need to use two "[", which amounts to two processes ("[" is actually a program) instead of one.
It goes by this syntax: ${VARIABLE:-default}.
The whole thing has to be thought in such a way that this "default" value is something distinct from a "valid" value/content.
If that's not possible for some reason you probably need to add a step like checking if there's a value at all, along the lines of "if [ -z $VARIABLE ] ; then echo "the variable needs to be filled"", or "if [ ! -z $VARIABLE ] ; then #everything is fine, proceed with the rest of the script".
