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What is the meaning of ## in UNIX shell script?

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# 1  
Old 12-30-2016
What is the meaning of ## in UNIX shell script?
Hi All,

I am new to unix shell scripting and I was documenting one of the unix script and encountered below statements -

Code:
for ii in `ls -1rt /oracle/admin/MARSCOPY/ext_files/fpm-ifpm/*.small.txt | tail -1 | awk '{print $1}'`
do
smallssim=${ii##/oracle/admin/MARSCOPY/ext_files/fpm-ifpm/}
prefix=`echo $smallssim | awk -F. '{print $1}'`

I tried to find the meaning of ## on google or other posts but could not find it.

Can anyone please help me understand meaning of the value smallssim?

Thanks in Advance.
Jay Shukla


Moderator's Comments:
Mod Comment Please use CODE/ICODE tags correctly as required by forum rules!
Last edited by RudiC; 12-30-2016 at 03:17 PM.. Reason: Changed CODE tags.
# 2  
Old 12-30-2016
Hello shuklajayb4,

Welcome to forums, hope you will enjoy learning here. It is called Parameter expansion, when you do a man bash you could see the following.
Quote:
${parameter##word}
Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches the
beginning of the value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pat-
tern (the ‘‘#’’ case) or the longest matching pattern (the ‘‘##’’ case) deleted. If parameter is @ or *, the pattern removal operation
is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted
with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
Here is an example of same too.
So let's say we have following string into a variable.
Code:
MYSTRING="Be liberal in what you accept, and conservative in what you send"

So here is the use of # and ##.
Code:
${MYSTRING#* }	Be liberal in what you accept, and conservative in what you send
${MYSTRING##* }	Be liberal in what you accept, and conservative in what you send

As you could see above(selected text will not be shown) # is removing the text till very first space and on other hand ## is printing the text till very last(maximum) match of space. I hope this helps you.

Thanks,
R. Singh
These 2 Users Gave Thanks to RavinderSingh13 For This Post:
rbatte1 shuklajayb4
# 3  
Old 12-30-2016
Thanks Ravinder !! This helps to understand it better.
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