在命令执行之前阻止 Bash 扩展通配符

Question

I have a boot script /init that starts a Linux embedded system. In there I have a function that counts partitions from an SD-Card

get_number_of_partitions () { ls ${SDCARD_DEV}p* 2> /dev/null | wc -l; }

It works with some busybox versions, but not all versions. The expectation is to count partitions, like /dev/mmcblk0p1 /dev/mmcblk0p2 . When I traced down, I find that with non-working cases, the p* is expanded to proc before the ls command. Resulting in counting 1 instance of ${SDCARD_DEV} + about 100 items from proc children.

I also tried: ls ${SDCARD_DEV}"p*" which gave the same result.

Answer 1

The only way I can think of that this would happen is if ${SDCARD_DEV} ends with whitespace, eg

SDCARD_DEV="/dev/ "

As a result, the command being executed is

ls /dev/ p*

instead of

ls /dev/p*

Disabling the wildcard expansion won't solve this problem. First of all, ls doesn't do wildcard expansion itself, it depends on the shell expanding the wildcard before invoking it. Second, ls would still see these as two separate arguments, it wouldn't interpret p* as being files to match inside /dev .

Ideally you should figure out why the space is being added to the variable in the first place, and fix that. If not, you can remove it before using the variable:

SDCARD_DEV="$(sed 's/ *$//' <<< "$SDCARD_DEV")"