Difference between revisions of "What does "unary operator expected" mean"
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| − | + | The bash message | |
| − | |||
unary operator expected | unary operator expected | ||
| − | + | means that you do a comparison where one site is empty for example | |
| − | + | if [ $name = "foo" ] | |
| − | if [ $name = " | + | and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as |
| − | and $name is empty. Then the bash shell | + | if [ = "foo" ] |
| − | if [ = " | ||
and this is not a valid expression. | and this is not a valid expression. | ||
| − | The solution is to quote | + | The solution is to quote variable names like this: |
| − | if [ "$name" = " | + | if [ "$name" = "foo" ] |
Then it will work. | Then it will work. | ||
= Shell scripting tutorial = | = Shell scripting tutorial = | ||
| − | Try the [[ | + | Try the tutorial [[BaBE - Bash By Examples]] to avoid this and similar mistakes in the future. |
= Debugging bash scripts = | = Debugging bash scripts = | ||
| Line 24: | Line 22: | ||
+ read name | + read name | ||
| − | + '[' = | + | + '[' = foo ']' |
test.sh: line 3: [: =: unary operator expected | test.sh: line 3: [: =: unary operator expected | ||
</pre> | </pre> | ||
Latest revision as of 19:15, 24 December 2014
The bash message
unary operator expected
means that you do a comparison where one site is empty for example
if [ $name = "foo" ]
and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as
if [ = "foo" ]
and this is not a valid expression.
The solution is to quote variable names like this:
if [ "$name" = "foo" ]
Then it will work.
Shell scripting tutorial
Try the tutorial BaBE - Bash By Examples to avoid this and similar mistakes 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 + '[' = foo ']' test.sh: line 3: [: =: unary operator expected
