Getting no output from command substitution?

Question

I've written a few shell scripts, but have never seen this behavior and am at a loss. I've got the following simple script which runs in a bash shell:

LOGFILE="/var/log/constructor-events.txt"

SUBSYSTEM="$1"
DEVTYPE="$2"
DEVICE="$3"

VENDOR=$(lsusb -D "$DEVICE" | grep idVendor 2>&1)

REQUEST=$(cat <<EOF
{"data": {
    "action": "add",
    "port": {
        "type": "$SUBSYSTEM",
    },
    "drive": {
        "vendor_id": "$VENDOR",
    }
}}
EOF
)

printf "output: $VENDOR" >> $LOGFILE
printf "%s\n" "`date +%x\ %r\ %Z` $REQUEST"  >> $LOGFILE

This is executed from a udev rule. The positional parameters (coming from udev) have the values I expect, and I have no problem printing them to the log file. But for some reason, the $VENDOR variable contains no output from the lsusb command.

Here is the debugging I've done.

• Added stderr redirection to stdout to capture any error that may be occurring.
• Added the line to send the $VENDOR variable directly to the log, which is empty.
• Executed the script manually in the shell and printed the $VENDOR variable to the terminal and it's empty.

• Took the string that is contained in the $DEVICE variable and executed it directly in the shell, with this result.

  [root@host ~]# lsusb -D /dev/bus/usb/016/030 | grep idVendor
  Cannot open /dev/bus/usb/016/030
  

Why is the $VENDOR variable empty when outputting to the log file?

EDIT:

Here is an update after removing the redirection from the command substitution.

• Added a line to print the $VENDOR variable to the terminal.
• Statically assigned the value instead of using the $DEVICE variable.

Printing $VENDOR to the terminal output the error, but it still did not append to the log file! The value of $DEVICE IS in the log file. I believe it has something to do with the output redirection inside the command substitution.

Answer 1

Here, if lsusb drops an error, the error message is not redirected, only errors from grep. Errors from lsusb still go the the usual stderr, i.e. the terminal (or whatever stderr was when the script started.

VENDOR=$(lsusb -D "$DEVICE" | grep idVendor 2>&1)

E.g.

$ blah=$(ls -l /nonexisting | grep foo 2>&1)
ls: cannot access '/nonexisting': No such file or directory
$ echo "blah: '$blah'"
blah: ''

You'll need to redirect both of their outputs as a group to have the command substitution capture the error, too:

$ blah=$( { ls -l /nonexisting | grep foo; } 2>&1)
$ echo "blah: '$blah'"
blah: 'ls: cannot access '/nonexisting': No such file or directory'

Using separate redirections like $( foo 2>&1 | grep 2>&1 ) would send the errors of the first command to the pipe, and for grep to filter, which in the above example would again produce an empty output.