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Problems understanding example code

 
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# 1  
Old 07-29-2009
Problems understanding example code
I am really new to UNIX and programming in general and so apologies if this thread is a bit simple.

I have searched and found a piece of sample code for a training program I am currently undertaking, but seeing as I am relatively new, I dont completely understand how it works.

Here is the code:

Code:
file=$1
set -- $(ls -ld $file)

perms=$1
owner=$3

[[ "$perms" = ?r???????? ]] && owner_read=YES
[[ "$perms" = ??w??????? ]] && owner_write=YES
[[ "$perms" = ???x?????? ]] && owner_exec=YES

[[ "$perms" = ????r????? ]] && group_read=YES
[[ "$perms" = ?????w???? ]] && group_write=YES
[[ "$perms" = ??????x??? ]] && group_exec=YES

[[ "$perms" = ???????r?? ]] && world_read=YES
[[ "$perms" = ????????w? ]] && world_write=YES
[[ "$perms" = ?????????x ]] && world_exec=YES

echo "perms: $perms"
echo "OWNER $owner ${owner_read:-NO} ${owner_write:-NO} ${owner_exec:-NO}"
echo "GROUP ${group_read:-NO} ${group_write:-NO} ${group_exec:-NO}"
echo "WORLD ${world_read:-NO} ${world_write:-NO} ${world_exec:-NO}"

Basically I understand how the script works generally, but I am just wondering whether the [[ ]] is equal to a shorthand if statement?

Also is ${owner_read:-NO} checking if the result from the above statement is null and then if so printing "NO"?

Sorry if this is a bit simple.

Thanks for any help.
Last edited by DukeNuke2; 07-29-2009 at 06:51 AM..
# 2  
Old 07-29-2009
Hi.

Yes [[ ]] is essentially a short-hand for an if statement in this case.

${owner_read:-NO} will return NO if owner_read is not set or is empty.
# 3  
Old 07-29-2009
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# 4  
Old 07-29-2009
Just a couple more quick questions.

Code:
set -- $(ls -ld $file)

What is the use of the double --?

I have read up and it states that it doesnt allow change of any of the flag, useful in setting $1 to "-".

Im not 100% sure what it means, I have tested with one - and two and found that you could not change the file name whilst using one as an error occured.
# 5  
Old 07-29-2009
Hi.

(I'm not sure if this is the technically accurate meaning, but...) set -- is used to clear or set positional arguments.

i.e.
Code:
> cat Test
set -- $(ls -l Test)
echo "\"$1\""
echo "\"$9\""
echo

set --
echo "\"$1\""
echo "\"$9\""


> ./Test a b c
"-rwx------"
"Test"

""
""

set -- will remove or overwrite args that you pass to the script (i.e. a b c in this case)

-- is commonly used by programs to signal that there are no more arguments to follow.
# 6  
Old 07-29-2009
Thank you. Was just what I needed to know!
# 7  
Old 07-29-2009
Quote:
Originally Posted by Makaer
Just a couple more quick questions.

Code:
set -- $(ls -ld $file)

What is the use of the double --?

It indicates the end of option arguments; any further arguments will not be parsed as options. This is true of -- with any command.
Quote:
I have read up and it states that it doesnt allow change of any of the flag, useful in setting $1 to "-".

Im not 100% sure what it means, I have tested with one - and two and found that you could not change the file name whilst using one as an error occured.
 
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