Shell Programming and Scripting

Hi there,

I'm just starting out with shell scripting and I'm trying to make a test where the script will basically run yum check-update to find out if the server has any available and if it does it e-mails me but if not it will just stop there.

I have it working if there are actually updates to be applied it works well. However, when I'm checking to see if the file is empty it seems to be sending out the e-mail anyway. The script I'm using is:

Code:

#!/bin/bash
/usr/bin/yum check-update | tail -n +3 > newUpdates.txt

SUBJECT="$HOSTNAME needs to be updated"
EMAIL="someemail@domain.com"
EMAILMSG="/root/newUpdates.txt"

if test "$EMAILMSG" = null
then
        rm -rf /root/newUpdates.txt
        exit
else
        /bin/mail -s "$SUBJECT" "$EMAIL" < $EMAILMSG
        rm -rf /root/newUpdates.txt
fi

So I was just running the command and then trying to strip out the first few lines that I'm sending to my temp file because it will have stuff like "Connecting to RedHat Network, etc, etc" and then if it's blank I just want to exit, but if not it should just list the updates.

Can anyone give me some guidance here??

Replace

Code:

if test "$EMAILMSG" = null

with

Code:

if [[ ! -s $EMAILMSG ]]

.

okay, that worked well. I'll have to test it on another box that has some updates because I updated my test machine to test the 2nd half.

Can you explain the syntax there for me? Originally I was trying something kind of like that within a single set of [] but what exactly is that one doing?

[[ ]] or [ ] act like "words" in shell. The word they almost exactly represent is "test".

So if you look up test documentation you find a lot of stuff you can test for.

Code:

 [  -s  /path/to/somefile ]

is the equivalent to

Code:

 test -s /path/to/somefile

and -s means 'Does the file exist and is the file size greater than zero (not empty)?'
! in front of the -s reverses the meaning because ! is the "not" operator.

Check out the other -[options] when you get a chance.

fantastic! thanks.

---------- Post updated at 02:08 PM ---------- Previous update was at 11:38 AM ----------

One last question...

what about if I wanted to look for a specific line of text in the output file. For example, if it said "No updates available" then exit but if not, run the mail command. Is that possible with something like grep -a ?

You can do something like below:-

Code:

if [ `grep -i "no updates available" <output file> | wc -l` -eq 0 ]
then
      exit
else
      # Send Mail
fi

One could leave out test, backticks, pipe and count:

Code:

if grep -q "No updates available" outfile
then
  exit
else
  echo "send mail"
fi