shell scripting while if string length != 0

Rob listone at
Fri Aug 1 02:31:08 PDT 2003

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

  cat file_o_commands | while read CMD ; do
    eval $CMD

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 and for some info on these.

----- Original Message -----
From: "Dave [Hawk-Systems]" <dave at>
Subject: shell scripting while if string length != 0

> for reasons best left unsaid, we need to pull in a file full of
> 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
> of script...
> done
> Anyone care to enlighten me a bit?
> Dave
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