Difference between revisions of "What does "unary operator expected" mean"
From Linuxintro
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| − | + | The bash message | |
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unary operator expected | unary operator expected | ||
| − | + | means that you do a comparison where one site is empty for example | |
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if [ $name = "Thorsten" ] | if [ $name = "Thorsten" ] | ||
and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as | and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as | ||
Revision as of 06:02, 19 December 2014
The bash message
unary operator expected
means that you do a comparison where one site is empty for example
if [ $name = "Thorsten" ]
and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as
if [ = "Thorsten" ]
and this is not a valid expression.
The solution is to quote variable names like this:
if [ "$name" = "Thorsten" ]
Then it will work.
Shell scripting tutorial
Try the shell scripting tutorial to avoid this mistake and similar ones in the future.
Debugging bash scripts
You can also debug the script line-by-line using bash -x. bash -x shows all commands that are being executed, just like gdb or strace, but for bash scripts:
tweedleburg:~ # bash -x test.sh + echo 'how is your name? ' how is your name? + read name + '[' = Thorsten ']' test.sh: line 3: [: =: unary operator expected
