Understanding linux shell script expression

Question

Hi I have a small script (script name is: test1.sh) that looks like this

PRG=$0
data=`expr $PRG : '.*\/.*'`
echo $data

When I run this I see output as

10

I could not understand the regular expression written in the second line of the script. What would that mean?

Zombo · Accepted Answer · 2013-05-13 21:06:53Z

2

From

If the match succeeds the `:'
     expression returns the number of characters
     matched.

So the 10 is likely

./test1.sh
^^^^^^^^^^
||||||||| \
123456789 10

answered May 13, 2013 at 21:06

Zombo

1

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1 Comment

Ryan M Over a year ago

Since the question asked about the meaning of the regular expression, it will match 0 or more of any character, followed by a slash, followed by 0 or more of any character. The expr command matches specifically from the beginning of the input string. So this explains why it would match the entire string, if $0 == "./test1.sh". Also note that $0 in a bash script refers to the script name, as this answer implies.

perreal · Accepted Answer · 2013-05-13 21:14:07Z

1

The expression returns a non-zero value if there is a / within the relative filename of the script ($0 in sh). If you execute the script like this: sh ../../script.sh, it outputs 15, which is the total length of "../../script.sh". It matches "../../" with '.*\/ and matches script.sh with the .* part.

answered May 13, 2013 at 21:14

perreal

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