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Signification of ## in line : device="${i##/sys/bolck/}" ???

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Top Forums Shell Programming and Scripting Signification of ## in line : device="${i##/sys/bolck/}" ???
# 1  
Old 04-09-2013
Signification of ## in line : device="${i##/sys/bolck/}" ???
Dear all,

I am trying to understand this script below:

Code:
#!/bin/bash

# can be executed as simple user?
for i in $(ls -ld /sys/block/?d? 2>/dev/null); do
device="${i##/sys/bolck/}"
DEVICELIST[$((count++))]="$device"
DEVICELIST[$((count++))]="$(cat $i/device/vendor 2>/dev/null) $(cat $i/device/model 2>/dev/null) ($(awk '{print ($1 / 2048) "MB"}' $i/size 2>/dev/null)"

done

dialog --menu "Available Devices:" 10 50 7 "${DEVICELIST[@]}" 2> choice.tmp

What are doing the double ## in line:
Code:
device="${i##/sys/bolck/}"

Many thanks for your help and kind regards,
freddie50
# 2  
Old 04-09-2013
Refer Parameter Substitution or Parameter Expansion in bash manual:
Code:
man bash

This User Gave Thanks to Yoda For This Post:
# 3  
Old 04-09-2013
It basically removes /sys/block/ and gives the device name alone

Code:
[root@Imperfecto_1 test]# echo $i
/sys/block/fd0
[root@Imperfecto_1 test]# echo ${i##/sys/block/}
fd0

Actually, you dont need ##, a single # will work in this case. ## is the longest possible match from the front and # is the shortest match from front.

Know the difference between # and ##
Code:
[root@Imperfecto_1 test]# echo $i
dev/sys/block/fd0
[root@Imperfecto_1 test]# echo ${i#*/}
sys/block/fd0                                                  
[root@Imperfecto_1 test]# echo ${i##*/}
fd0

HTH

--ahamed
This User Gave Thanks to ahamed101 For This Post:
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