reading command output by line in Bourne script?

ocicat · 02-13-2010, 05:57 PM

I have a command which generates the following output:

Code:

$ sudo vnconfig -l
vnd0: not in use
vnd1: not in use
vnd2: not in use
vnd3: not in use
$

I would like to find the first vnd device which is not in use.

The following is a nice first try...

Code:

$ echo $(sudo vnconfig -l) | grep 'not in use'
vnd0: not in use vnd1: not in use vnd2: not in use vnd3: not in use

...but all lines are concatenated together.

The following seems reasonable, but the results are far worse:

Code:

$ cat test.sh
#!/bin/sh

for LINE in $(vnconfig -l) ; do
    echo $LINE
done
$ sudo sh test.sh
vnd0:
not
in
use
...
$

Reading the output line-by-line would be ideal, but so far my foo is weak:

Code:

$ cat test.sh
#!/bin/sh

while read LINE ; do
    echo $LINE
done < $(vnconfig -l)
$ sudo sh test.sh
test.sh[5]: cannot open vnd0: not in use
vnd1: not in use
vnd2: not in use
vnd3: not in use: No such file or directory
$

Is there a way to read the command's output line-by-line without resorting to redirecting output into a temporary file?

Thanks.

irmin · 02-13-2010, 07:10 PM

There is a possibility. But this requires to switch from sh to bash:

Code:

#!/bin/bash or /usr/bin/bash or /usr/local/bin/bash

(while read LINE ; do
    echo $LINE
done) < <(vnconfig -l)

tuxdev · 02-13-2010, 07:11 PM

Code:

device="$(vncconfig -l | grep "not in use" | head -n 1)"
device="${device%%:*}"

ghostdog74 · 02-13-2010, 09:49 PM

@OP, you can do it with just one awk statement

Code:

vncconfig | awk -F":" '/not in use/{print $1;exit}'

BenCollver · 02-13-2010, 10:47 PM

Quote:

Code:

while read LINE ; do
    echo $LINE
done < $(vnconfig -l)

That was close. Here is one that will work.

Code:

vnconfig -l | while read LINE
do
    echo $LINE
done

BenCollver · 02-13-2010, 10:51 PM

For your purposes, a pipe would be better than a while loop.

vnconfig -l | grep 'not in use' | head -1 | cut -d : -f 1

"grep" finds lines matching 'not in use'
"head -1" prints the first line
"cut -d : -f 1" prints the name

AnanthaP · 02-14-2010, 02:48 AM

Pseudo code:
$(sudo vnconfig -l) | sed 's/not in use/not in use\n'/ | grep not in use?

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