How to understand the following shell script?

Question

I would be glad if somebody could explain me what this part of the code does

[ -n "${0%/*}" ] && cd ${0%/*}
. ./environment.sh
. ./readParams.sh

I understand that this script is reading from ./enviroment.sh and ./readparams.sh

But i am really struggling to understand what the first line of the code does.So i can understand the whole process after this.

I would be glad if you could give me a site or anything that will help me understand such a syntax. I am a beginner in shell script so any help is appreciated.

maybe i am mistaken,and is a shell script,but the question remains.If you have any information about where should i look to understand what this syntax does.Sorry for my misunderstanding — stratis, May 16 '13 at 12:16
It would be better if you could also edit my title so it not confuses the users — stratis, May 16 '13 at 12:17

score 5 · Accepted Answer · edited May 23 '17 at 12:05

That's a shell script, not python.

[ -n "${0%/*}" ] && cd ${0%/*}

That changes the current working directory to the directory where the shell script resides.

. ./environment.sh
. ./readParams.sh

That sources the two listed shell script, presumable stored within the same directory as the script being called.

To break down the first line, $0 will contain the first argument of the command i.e. the script being called. ${0%/*} performs a parameter substitution to remove the trailing portion (from the last / onwards) so this effectively gives you the directory name.

[ -n "${0%/*}" ] translates to "if "${0%/*}" results in a string that is not empty".

The whole line therefore translates to "if the executed script is prefixed with a path, then cd to that directory".

p.s. I'm not convinced that's the best way to achieve that, but that's pretty much what it does. Personally, I would opt for the more readable cd $(dirname "$0"), or for a more robust solution, see Getting the source directory of a Bash script from within.

Thank you very much!So that is i think the reason that i am reading all day python and i could not find anything that will make me understand it. — stratis, May 16 '13 at 12:24

score 1 · Answer 2 · answered May 16 '13 at 12:20

first, this is not python at all, even not close!

[ -n "${0%/*}" ] && cd ${0%/*}

it used parameter expansion, you can man bash to see it.

basically, it cd to the directory, which your script is located. for example:

if your script is at:

/foo/bar/baz/xxx.sh

the above line does cd /foo/bar/baz

the files:

environment.sh
readParams.sh

are in same directory of xxx.sh

How to understand the following shell script?

2 Answers2