Date: Fri, 1 Aug 2003 19:01:00 +0930 From: "Rob" <listone@deathbeforedecaf.net> To: "Dave [Hawk-Systems]" <dave@hawk-systems.com>, "FreeBSD Questions" <freebsd-questions@FreeBSD.ORG> Subject: Re: shell scripting while if string length != 0 Message-ID: <00a001c3580f$a00dd280$a4b826cb@goo> References: <DBEIKNMKGOBGNDHAAKGNMEGJCPAC.dave@hawk-systems.com>
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I'm not a shell guru, but pipelines don't necessarily run in sequence.
In line 5 of your script, the part that says
sed '1d' > /path/to/file_o_commands
will destroy all contents of the original file. This may or may not
happen before
cat /path/to/file_o_commands
has finished reading it.
If you just want to execute the lines of a file in order, use something
like
cat file_o_commands | while read CMD ; do
eval $CMD
done
On the other hand, if you want the script to hang around at the end of
the file and wait for new commands, you may need a named pipe (FIFO).
This is a file that one process writes to and another one reads from,
not necessarily at the same time.
See http://www.linuxjournal.com/article.php?sid=2156 and
http://tldp.org/LDP/lpg/node15.html for some info on these.
----- Original Message -----
From: "Dave [Hawk-Systems]" <dave@hawk-systems.com>
Subject: shell scripting while if string length != 0
> for reasons best left unsaid, we need to pull in a file full of
partial
> commands, and run them via a shell script on occasion, removing each
command as
> we run it. Have managed to hack togetherthe following shell script,
but and
> stumped on something simple because of my lack of shell knowledge;
>
> the file that holds out commands
> <file_o_commands>
> Server1 df -k
> Server2 df -k
> Server3 top | grep myprog
> Server4 who
>
> add new commands to the end of the file with
> echo "Server2 who" >> /path/to/file_o_commands
>
> then when we need to, run through the commands
> <file_to_run_stuff>
> #!/bin/sh
> # get top command
> DOCOMMAND=`head -n 1 /path/to/file_o_commands`
> # remove that command
> cat /path/to/file_o_commands | sed '1d' > /path/to/file_o_commands
> # run that command
> ssh ${DOCOMMAND}
>
> this works as intended with 1 exception, we need to add a while in
there to loop
> through the file and stop processing an exit when `head -n 1
> /path/to/file_o_commands` does not return a line.
>
> I almost want to borrow -n from if
>
> while [ -n (DOCOMMAND=`head -n 1 /path/to/file_o_commands`) ] do
> ...rest of script...
> done
>
> Anyone care to enlighten me a bit?
>
> Dave
>
>
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