[SOLVED] Pipe within variable for subshell with $()

zhjim · 12-10-2013, 05:20 AM

Hi folks,

im writing a bash script that hdparms some block devices. I want to use a variable like CMD="lsblk /dev/sda | grep -v sda" to only get the partitions of the devices. But bash always ignores the pipe symbol and so lsblk complains about unkown option -v. Any idea how I can go about this?

Code:

#!/bin/bash
#set -x

# Get list of device
CMD="lsblk -e "11,1" -n -l -i -d -o NAME"
read -n 1 -p "Gather all block device data (y/n): " line
echo ""
if [ "$xline" == "xy" ]; then
        echo > ./LAST
else
        x=0
        for i in $($CMD); do
                x=$((x+1))
                echo "$x) $i"
        done
        read -n 1 -p "Gather data of partition from ($(seq -s '/' 1 $x)): " line
        echo ""
        DEV=$($CMD | awk "NR==$line { print $1 }")
        #CMD="lsblk -n -l -i -o NAME /dev/$DEV \| grep -v $DEV$"
        CMD='lsblk -n -l -i -o NAME /dev/'
        CMD+="$DEV "
        CMD+='| grep -v "'
        CMD+="$DEV"
        CMD+='$"'
fi

set -x
echo $CMD
for i in $($CMD); do
        echo some
        echo $i
done

set +x
exit 0

What I can see from the debug output is that | is escaped with '' when the command should be run. So how can I convince the shell to interpret the pipe symbol. I also tried to escape it with \ but to no avail.

Code:

Gather all block device data (y/n): n
1) sda
2) sdb
3) sdc
Gather data of partition from (1/2/3): 1
+ echo lsblk -n -l -i -o NAME /dev/sda '|' grep -v '"sda$"'
lsblk -n -l -i -o NAME /dev/sda | grep -v "sda$"
++ lsblk -n -l -i -o NAME /dev/sda '|' grep -v '"sda$"'
lsblk: invalid option -- 'v'

Best Regards

colucix · 12-10-2013, 06:31 AM

To avoid this behaviour (the pipe is interpreted literally) you can either avoid the CMD assignment and put the command directly inside the command substitution or use a shell function instead. For example this works:

Code:

for i in $(lsblk -n -l -i -o NAME /dev/$DEV | grep -v "$DEV$")
do

This works as well:

Code:

function list_partitions () {
  lsblk -n -l -i -o NAME /dev/$1 | grep -v "$1$"
}

for i in $(list_partitions $DEV)
do

Another option is the usage of eval (but it is not recommended for some reasons, as explained here). In any case this should work as well:

Code:

for i in $(eval $CMD)
do

Hope this helps.

zhjim · 12-10-2013, 06:44 AM

I go for the evil with eval. I guess should start using functions from the beginnings and not after setting me skeleton up.

Thanks for the input. Working now.