Shell Programming and Scripting

Hi,

I'm trying to write a script that checks gvfs to see if a mount exists so I can run it from network-manager's status hooks. I thought I'd pipe the output of gvfs-mount -l to grep for the particular mounts I care about. When I do this in a bash script:

Code:

cmnd="gvfs-mount -l | grep -i '${mount}'"
dastring=`${cmnd}` #remove me
ret=$?
echo "Output from ${cmnd}" #remove me
echo "$dastring"

$dastring looks like the output from gvfs-mount (although it's detailed as if I gave it the -i option in addition to -l).

If I copy and paste the ${cmnd} (from where I echo it) and run it directly from the terminal, it display the line of text I'm looking for with grep.

Can anyone help me figure out why the pipe works directly in the terminal but does not appear to be doing so in the script?

Thanks in advance.

You can either do this:

Code:

eval dastring=\`${cmnd}\`

or do this for example:

Code:

dastring=$(gvfs-mount -l | grep -i "${mount}")

-or with back ticks:-

Code:

dastring=`gvfs-mount -l | grep -i "${mount}"`

Thanks Scrutinizer - this is what I needed as I want to keep cmnd around for print out later in the script.

I don't understand why backticks in

Code:

dastring=`${cmnd}`

didn't run the command and assign the output to dastring. It must be that dastring equaled that string including the backticks - but it doesn't quite make sense. Could you explain what's going on here?

Thanks a bunch!

When the shell expands the variable holding the command, it recognises that the pipe symbol is special, and quotes it. If you consider this example:

Code:

cmd="echo  foo | awk '{print \"bar \"; print }'"
data=`$cmd`

you would expect that when $cmd is executed the string "bar foo" would be assigned to data. However, the string assigned to data is actually:

Code:

foo | awk '{print "bar "; print }'

If you execute it with -x you'll see in the trace the quotes that the shell adds and that the pipe symbol is being passed to echo (along with everything else past it), and not interpreted by the shell. The output when I ran it was:

Code:

+ cmd=$'echo  foo | awk \'{print "bar "; print }\''
+ echo foo '|' awk $'\'{print' '"bar' '";' print $'}\''
+ data=$'foo | awk \'{print "bar "; print }\''
+ echo $'foo | awk \'{print "bar "; print }\''
foo | awk '{print "bar "; print }'

Notice in the second line the '|' is the clue that the shell has treated the contents of the variable as a single string and has not parsed it as a command.


When you add the eval in front of the expansion, the shell expands the variable and then parses it as a command. The result is that the pipe is then recognised as you intended and both commands are executed as expected.


In your case, the whole string would have been passed to the gvfs-mount command and as that string contained -i, it would have listed details. I suspect it might have also complained about the pipe and/or grep but that would have been to stderr and not captured in your variable.

When things seem strange, I usually put a set -x before the statement that isn't working and set +x ater it to get the details of how the shell is interpreting my code. The output is usually enough to clear up the confusion.


Hope this helps

Thanks agama! Being a bash newb still, I didn't know about the +/-x - that will come in very handy in the future I think. Thanks for that and the detailed explanation regarding pipe!