Difference between revisions of "What does "unary operator expected" mean"
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imported>ThorstenStaerk (Created page with "When you work with Linux scripts on the command line, you will sometimes get an error message saying unary operator expected And you may wonder what this means. To give you an e...") |
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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" ] |
| − | + | 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: | ||
| + | <pre> | ||
| + | tweedleburg:~ # bash -x test.sh | ||
| + | + echo 'how is your name? ' | ||
| + | how is your name? | ||
| + | + read name | ||
| + | |||
| + | + '[' = foo ']' | ||
| + | test.sh: line 3: [: =: unary operator expected | ||
| + | </pre> | ||
| + | |||
| + | = See also = | ||
| + | * [[troubleshooting]] | ||
| + | * [[error messages]] | ||
| + | * [[shell scripting tutorial]] | ||
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
