How do I get the grep output 0 or 1 and test it?

DEF. · 04-07-2009, 09:22 AM

I am trying to use a shell script to find a string in a file and do something when found.

For example:

(sudo pppd call gsm > pppdout &)

FOUND=1
while [ $FOUND -eq 1 ]; do
FOUND = $(grep "Script" pppdout)
sleep 1
done

wait

What should happen is pppd will start in a different process and stream it's output to pppdout. pppdout should be created in the current folder. Then the script should periodically check the pppdout file for the string Script (which eventually will appear, some seconds later) and when found exit the script. Ultimately the script will do something useful when the text is found. However, the output from the program is a repeating: 'scriptname.sh: 12: FOUND: not found'

Where scriptname.sh would be the name of your script and 12 refers to the line with 'done'.

Why does grep not find the text, or at least why deos my script not check the grep output correctly?

Thanks in advance.

dwhitney67 · 04-07-2009, 09:41 AM

Would this work for you?

Code:

#!/bin/bash

while [ true ]
do
        grep -q "Script" pppdout

        if [ $? -eq 0 ]
        then
                break
        fi

        sleep 1
done

DEF. · 04-07-2009, 09:56 AM

Yes yours does work, thanks.

Now I understand your version, but can you explain what is wrong with my version please? Can you not use a programs output i.e. $(grep...) as I did?

David1357 · 04-07-2009, 10:06 AM

Quote:

Originally Posted by DEF.

Now I understand your version, but can you explain what is wrong with my version please? Can you not use a programs output i.e. $(grep...) as I did?

If you want the return code from grep, you need to set FOUND to $?. You were storing the string produced by $(grep "Script" pppdout), which was empty. Since FOUND was set to "", it was no longer equal to 1, and the loop exited. You really wanted

Code:

(sudo pppd call gsm > pppdout &)

FOUND=1
while [ $FOUND -eq 1 ]; do
    sleep 1
    grep "Script" pppdout
    # Note the lack of white space around the equal sign.
    # If there is white space after FOUND, the shell interprets
    # it as a command.
    FOUND=$?
done

wait

I put the sleep before the grep and assignment because it makes no sense to sleep after you have updated the loop conditional.

dwhitney67 · 04-07-2009, 10:08 AM

Quote:

Originally Posted by DEF.

Yes yours does work, thanks.

Now I understand your version, but can you explain what is wrong with my version please? Can you not use a programs output i.e. $(grep...) as I did?

Apparently not.

Seriously, I do not know of another way to get the status of the command.

DEF. · 04-07-2009, 10:11 AM

Thanks you guys - I suck at shell programming

David1357 · 04-07-2009, 10:11 AM

Quote:

Originally Posted by dwhitney67

Apparently not.

I think I got him sorted.

The_Kernel · 04-07-2009, 01:16 PM

Quote:

Originally Posted by dwhitney67

Apparently not.

Seriously, I do not know of another way to get the status of the command.

Well if grep did match anything it would have some output, and if it didn't then no output, so you could just as well test if the output of the command is zero length. Also it's unnecessary to have the "if" statement separate from the grep command. You could just do 'if grep -q "Script" pppdout' to the same affect.

Kenhelm · 04-07-2009, 01:32 PM

In loops such as
while [ .. ];do
...
done
the return code of the 'test' command is being used to control the loop.
Other commands can be used instead of 'test'.
So this works too

Code:

until grep -q "Script" pppdout;do
  sleep 1
done

ntubski · 04-07-2009, 08:42 PM

The original script had a syntax error; there must be no spaces around the "=" in assignments.

Code:

FOUND = $(grep "Script" pppdout)

So that code will execute a program called FOUND with 2 arguments: "=" and the output of grep "Script" pppdout.

If you take out the spaces you would then have the problem David1357 mentioned.

ta0kira · 04-07-2009, 09:45 PM

It can be a little easier than that.

Code:

if test "`grep -c "pattern" "file"`" -ge 1; then
  echo found
else
  echo not found
fi

Kevin Barry