busybox shell error: line 9 (left square bracket): not found

Question

I have the following code and I get the Error "line 9: [: not found":

#!/bin/sh
msg=$(dmesg | tail -n1)
echo "$msg"
if [ "$msg" = "Tasklet grp12" ]
then
    echo "Test was successful, Strings are equal."
else
    echo "Test failed, Strings are not equal."
fi

That doesn't seem to be the code that causes your error, seeing that the lines don't match. — Benjamin W., Jan 08 '21 at 14:55
`[` is, like every other command busybox can build into its single self-call binary, a compile-time option. — Charles Duffy, Jan 08 '21 at 14:58
Make sure your script doesn't contain any hidden characters and isn't using unicode lookalikes; but once that's done it may be time to investigate how your specific copy of busybox was compiled. — Charles Duffy, Jan 08 '21 at 14:59
@CharlesDuffy busybox lets you disable the command (or at least, require it to appear as an external command)? I'm going to stop facetiously suggesting that `[` shouldn't exist. — chepner, Jan 08 '21 at 15:52
@SimonRechermann, btw, as an aside, `dmesg | tail -n1` requires `dmesg` to write all its output to a FIFO, whereafter `tail` reads all those lines and copies out the last one. If by contrast you have something like `/var/log/dmesg` (as a seekable regular file), `tail -n1 /var/log/dmesg` can skip right to the end of the file and only read the last block, and thus can be faster to run. — Charles Duffy, Jan 08 '21 at 17:02
*'unlink $(which [)'* should disable busybox applet on runtime — alecxs, Jan 19 '21 at 22:53

score -1 · Accepted Answer · answered Jan 09 '21 at 13:26

thanks to Charles Duffy! I had to check the box "Builtin version of 'test'" in the shell section of the make menuconfig menu of busybox, to enable string comparison. Now the code works. Since I had grep activated I first tried another solution which also worked but has a bad performance for sure:

#!/bin/sh
msg=$(dmesg | tail -n1)
echo "$msg"
tasklet="Tasklet grp12"
if ( echo "$msg" | grep "^$tasklet$" )
then
    echo "Test was successful, Strings are equal."
else
    echo "Test failed, Strings are not equal."
fi

busybox shell error: line 9 (left square bracket): not found

1 Answers1