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multiple variables assignement (stdout/stderr outputs)

 
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# 1  
Old 08-24-2010
multiple variables assignement (stdout/stderr outputs)
Hi all,

I've been looking around for this for a while and can't seem to find a satifactory way to do what I want:

I would like to assign the output of stdout to a variable and that of stderr to another one, and this without using temporary files/named pipes. In other words be able to assign several variable at a time.

For instance:
Code:
touch a_file
STDOUT=$(ls not_a_file a_file) # but error message is lost unless redirected somewhere
ls: cannot access not_a_file: No such file or directory
echo $STDOUT
a_file

Is it even possible? problems with backticks (``) or $(...) is that it only assigns 1 variable at a time I believe. Anyone's got any idea?

I could do:

Code:
STDOUT=$(ls not_a_file a_file 2>&1)
echo $STDOUT
ls: cannot access not_a_file: No such file or directory a_file

and parse the output but is the order even guaranteed? even if it is, distinguishing the stderr part from the stdout one would be difficult...

Thanks!
Anthony
Last edited by anthalamus; 08-24-2010 at 05:53 PM..
# 2  
Old 08-24-2010
I don't think you can do this without pipes or temporary files. Messages on stderr are usually meant for humans and stdout for parsing by programs anyway, so scripts usually capture stdout for their own use and let operators see via stderr if anything goes wrong. After all, the error message may not be what the program expected.
This User Gave Thanks to Corona688 For This Post:
anthalamus
# 3  
Old 08-25-2010
Quote:
Originally Posted by anthalamus
I would like to assign the output of stdout to a variable and that of stderr to another one, and this without using temporary files/named pipes.
I agree with Corona688, cannot be done without pipes. However, you said 'named pipes' which technically are different beasts so I offer this "mess" for you to consider.

Code:
#!/usr/bin/env ksh 
(ls *.jpg bfoo | sed 's/^/STDOUT /') 2>&1 | awk ' 
        /STDOUT/ { 
                gsub( "STDOUT ", "" ); 
                o1 = o1 $0 "; "; 
                next;
        } 
        {
                o2 = o2 $0 "; ";
        } 
        END{ 
                printf( "%s~%s\n", o1, o2 ); 
        }' |  IFS="~" read  stdout stderr
echo "stdout=$stdout"
echo "stderr=$stderr"

It all can be crammed on one line; I've formatted it to be easier to read. It makes the assumption that the tilde (~) is never found in either the stdout or stderr stream. The records of each output stream are separated with semicolons, so if the output contains them, things will be difficult.

Running the script in my small test environment produced:
Code:
$ t20   
stdout=bar.jpg; foo.jpg; snapshot.jpg; 
stderr=ls: bfoo: not found;

I tested this in Kshell; I don't think it will work under bash as bash has issues executing a command and reading the output into variables. It will also be subject to the limits that Kshell might impose on variable value lengths or that awk might impose on string lengths. If you have a command that produces rather long output/error you could have problems.

Regardless, interesting brain twister.
This User Gave Thanks to agama For This Post:
anthalamus
# 4  
Old 08-25-2010
ha ha thanks agama, I guess i'll keep my temporary files then Smilie, they're not so bad after all!!
It's a pity though, i was hoping some kind of easy redirection like

Code:
ls not_a_file a_file >$OUT 2>$ERR # obviously does not actually work!

It wouldn't seem so hard to provide in shells nowadays, since the same can be achieved with a few commands using files. Anyway, I guess the question of whether it is actually useful or not is worth asking.

Anyway thanks to both of you!

EDIT: well I settled for:
Code:
COMMAND="ls not_a_file a_file"
OUT=`mktemp` && ERR=`mktemp` && eval "$COMMAND" 1>$OUT 2>$ERR; STDOUT=`cat $OUT` && STDERR=`cat $ERR` && rm $OUT $ERR
echo -e "STDOUT=$STDOUT\nSTDERR=$STDERR"

If something thinks of something much better, let me know!!
Last edited by anthalamus; 08-25-2010 at 02:40 PM..
# 5  
Old 08-25-2010
I think that eval is redundant, the command being eval-ed should work without it.
 
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