help for explanation for two lines code in bash

cooky_jun · 05-21-2010, 04:47 AM

if [ -d $d ]; then
if [ x$dm = x ]; then dm=$d; else dm=$dm:$d; fi

first line is search directory $d ?
what does [x$dm = x] mean?? $dm is empty ?

colucix · 05-21-2010, 05:25 AM

Hi and welcome to LinuxQuestions!

The first line, checks if $d is a directory. The second line checks if $dm is an empty string: in this case the concatenation x$dm brings to x. Exactly as you said.

druuna · 05-21-2010, 05:25 AM

Hi,

-d file - True if file exists and is a directory.

x$dm = x => This checks if $dm is empty (or not). You see this convention (prepending a character) sometimes.

$dm = "" => should also work.

Hope this helps.

Beaten by colucix (again...)

cooky_jun · 05-21-2010, 05:40 AM

Thanks guys

PMP · 05-21-2010, 05:42 AM

Please mark this thread as solved !!

Thank You

colucix · 05-21-2010, 07:27 AM

Quote:

Originally Posted by druuna

$dm = "" => should also work.

Actually this works only by means of double brackets or by embedding $dm in double quotes:

Code:

$ [[ $dm = "" ]] && echo ok
ok
$ [ "$dm" = "" ] && echo ok
ok
$ [ $dm = "" ] && echo ok
bash: [: =: unary operator expected
$

Another option is:

Code:

$ [ -z $dm ] && echo ok
ok

but I suspect that in this case (without quotes) the test reduces to testing a string [ -z ], that is always true.

Quote:

Beaten by colucix (again...)

Just for a bunch of seconds...