How the below command works?

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# 1  
Old 08-08-2013
How the below command works?

can any one explain the below command. run it and see and if you understand please tell me.
MOD_File=$(echo ${File%.*}_`date +%Y%m%d_%H%M%S%N`.${File##*.})

i asked somedays before and got the above code. i needed to add date time stamp just before the last extension of the filename.

if i use the below command, it removes the last extension from the filename
echo ${File%.*}

output is:

if i use echo ${File%.*.*} , it removes the last two extensions

output is:

echo ${File##*.} gives me the last extension i.e. pdf

echo ${File#*.} gives me all the extension i.e. txt.xls.pdf

how the above command works especially % and #
# 2  
Old 08-08-2013
You pretty much described it yourself. % and # are pattern-matching-and-delete-operators.

${variable#pattern} and ${variable##pattern}... if pattern matches the beginning of the value of variable, then the expression is the value of the variable with the matched portion deleted. If there is no match, the expression is the entire value of the variable. With one #, the smallest matching pattern is deleted, with two ## the largest.

The % operator does the same at the end of the variable.
# 3  
Old 08-08-2013
# - remove prefix pattern
% - remove suffix pattern

see discussion on parameter expansion in man bash or man ksh for more info ...
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