[SOLVED] need some clarity about syntax in following shell script

tkmsr · 04-10-2010, 03:52 AM

Hi
I need some help regarding the following shell script.

Code:

#!/bin/sh
dir=$(dirname "$0")
"$dir/network-bridge" "$@" vifnum=0 netdev=eth0 bridge=xenbr0

Want to know what dirname does I read the man page dirname stores name of parant directory is what I understood is it correct ?

given

Code:

dir=$(dirname "$0")

the above code what does it stores dir $() is executing what "$0"
is it referring to name of script that is passing it as command line parameter if yes then why is it put in double quotes

Code:

""

.

colucix · 04-10-2010, 04:33 AM

Quote:

Originally Posted by tkmsr

Want to know what dirname does I read the man page dirname stores name of parant directory is what I understood is it correct?

Not really: dirname strips the trailing "/something" from a string (path). It is not aware of the actual parent directory of a file. This means it works even on non-existent paths, e.g.

Code:

$ dirname /pippo/pluto/paperino
/pippo/pluto

Suppose you launch the script posted above as

Code:

./script.sh

in this case the dirname command results in

Code:

If you want to retrieve the full path of the script, better to use readlink (despite its name it can be forced to retrieve the full path of a file, not only symbolic links). Using a nested command substitution you can retrieve the full path of the parent directory of the script:

Code:

dir=$(dirname "$(readlink -f "$0")")

Regarding the double quotes, they can be useful in this case if the name of the script (hopefully not) or its path contains blank spaces.

tkmsr · 04-10-2010, 04:48 AM

Quote:

Originally Posted by colucix

Suppose you launch the script posted above as

Code:

./script.sh

in this case the dirname command results in

Code:

Yes you pointed it rightly this is what I am not clear with.As why is it giving

Code:

Thanks I got it.

Quote:

Originally Posted by colucix

Regarding the double quotes, they can be useful in this case if the name of the script (hopefully not) or its path contains blank spaces.

Can you give an example?

colucix · 04-10-2010, 06:28 AM

Well, let's say that using double quotes around variables is a good habit in order to prevent unforeseen problems. Indeed, it is unlikely to have spaces in script names, but suppose you have spaces in some parent directory of the script and you don't put double quotes around the inner command substitution, e.g.

Code:

$ pwd
/home/alex/dir with spaces
$ cat test.sh
#!/bin/bash
dir=$(dirname $(readlink -f "$0"))
echo $dir
$ ./test.sh
dirname: extra operand `with'
Try `dirname --help' for more information.

This is because dirname accepts only one argument. If the argument contains spaces, it's split into two or more fields unless you put it inside quotes.

Actually in my example above I left the inner substitution unquoted (edited now)..

tkmsr · 04-10-2010, 11:28 PM

Quote:

Originally Posted by colucix

Well, let's say that using double quotes around variables is a good habit in order to prevent unforeseen problems. Indeed, it is unlikely to have spaces in script names, but suppose you have spaces in some parent directory of the script and you don't put double quotes around the inner command substitution, e.g.

Code:

$ pwd
/home/alex/dir with spaces
$ cat test.sh
#!/bin/bash
dir=$(dirname $(readlink -f "$0"))
echo $dir
$ ./test.sh
dirname: extra operand `with'
Try `dirname --help' for more information.

This is because dirname accepts only one argument. If the argument contains spaces, it's split into two or more fields unless you put it inside quotes.

Actually in my example above I left the inner substitution unquoted (edited now)..

Thank you.