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[bash] Parameter expansions

08 Oct

First set a parameter, param1:
$ param1=hello

Now let’s see the result of parameter expansions

$ echo ${param1}  # hello     i.e., same as $param1)
$ echo ${param1}a #
helloa i.e., the braces separate the name
$ echo ${param2}  # (nothing) i.e., nothing there


$ echo ${param2:-file*}    #           lists all files int the working directory starting with file.
$ echo ${param2:-$param1} # hello        uses $param1's value...
$ echo $param2            #(nothing)      ... but didn't change $param2
$ echo ${param3:=$param1} # hello         assigns param1's value to param3
$ echo $param3            #hello 


$ echo ${param1:2}    # llo  i.e., substring from 2
$ echo ${param1:2:2}  # ll   i.e., substring from 2, len 2
$ echo ${param1#he}   # llo  i.e., strip shortest match from start
$ echo ${param1#hel*} # lo   i.e., strip shortest match from start
$ echo ${param1#he*l} # lo   i.e., strip shortest match from start
$ echo ${param1##he*l}# o    i.e., strip longest match from start
$ echo ${param1%l*o}  # hel  i.e., strip shortest match from end
$ echo ${param1%%l*o} # he   i.e., strip longest match from end
$ echo ${param1/l/p}  # heplo i.e., replace as few as possible
$ echo ${param1//l/p} # heppo i.e., replace as many as possible


$ echo ${!param*} # list parameter names starting with param
$ echo ${#param1} # 5 i.e., the length of param

Examples

1. Rename all .GIF files to .gif

$ for file in *.GIF; do mv $file ${file%GIF}gif; done

2. Number the files sequentially

$ cnt=0; for file in *.gif; do mv $file $cnt$file; let cnt=cnt++; done

3. Get rid of the numbers’

$ for file in *.gif; do mv $file ${file##[0-9]}; done

Reference: Directly copied from here with minor changes (as it was in a very-easy-to-understand form in the original).

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Posted by on October 8, 2009 in bash

 

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