unary operator expected

问题:

I've been trying to figure out whats wrong with this but just can't figure it out..

This is the part seems to be 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

回答1:

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.

回答2:

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

回答3:

Try assigning a value to $aug1 before use it in if[] statements; the error message will disappear afterwards.

回答4:

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".