问题
I have the following line as part of a much bigger bash script:
if [ `packages/TinySVM-0.09/bin/svm_learn 2>&1| grep TinySVM | wc -l | cut -c0-7 | sed 's/^ *//g'` -eq 1 ]
upon running the script, I get:
./install.sh: line 219: [: -eq: unary operator expected
Where line 219 is the line above. Any suggestions for a fix?
回答1:
This happens when you are using the test builtin via [ and your left side expression returns NUL. You can fix this by the use of:
if [ x`some | expression | here` = x1 ]; then
Or, since you're already using bash you can use its much nicer (( )) syntax which doesn't have this problem and do:
if (( $(some | expression | here) == 1 )); then
Note that I also used $() for command substitution over backticks `` as the latter is non-POSIX and deprecated
回答2:
You can run your command without any additional syntax. For example, the following checks the exit code of grep to determine whether the regular expression matches or not:
if ! grep -q "$word" /usr/share/dict/words
then
echo "Word $word is not valid word!"
fi
回答3:
The error occurs because your command substitution returns nothing effectively making your test look like:
if [ -eq 1 ]
A common way to fix this is to append some constant on both sides of the equation, so that no operand becomes empty at any time:
if [ x`packages/TinySVM-0.09/bin/svm_learn 2>&1| grep TinySVM | wc -l | cut -c0-7 | sed 's/^ *//g'` = x1 ]
Note that = is being used as we are now comparing strings.
回答4:
You could add an "x" to both sides of the comparison or you could just quote the left side:
[ "$(command | pipeline)" = 1 ]
I don't understand what the cut and sed at the end are for. The output of wc -l in a pipeline is simply a number.
回答5:
Try [[ test_expression ]]; instead of [ test_expression ];
来源：https://stackoverflow.com/questions/5276393/command-inside-if-statement-of-bash-script