LinuxQuestions.org - [SOLVED] Friday BASH Script question

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- - Friday BASH Script question - Check output during for loop (https://www.linuxquestions.org/questions/programming-9/friday-bash-script-question-check-output-during-for-loop-4175459715/)

Friday BASH Script question - Check output during for loop

Hey folks,

It's been a long friday and was a late night last night so I'm pretty sure I'm just not thinking about this correctly but basically what I would like to do is check for a set of output while doing a for loop, break out of the loop if it contains a certain string of text.

So lets say I have a list of duplicates in a file /tmp/dupes.txt

Format:

Code:

client1

client2

client3

client4

Now lets say I do a

Code:



for dupe in $(/bin/cat /tmp/dupes.txt)

do

database query that provides text output

done

The query is a bit long and have ommitted as its not necssary but the output of the query comes in the following format:

Code:



                        name: client name;

            scheduled backup: Enabled;

Basically I want to only pull out the client names where the string for scheduled backup is == "Enabled" and discard output if it does not.

Is this easily achievable with my for loop setup or would I need to look at re-writing this a bit more robustly. Tried to nest an if statement in the for loop but wasnt able to get it to work.

Well I just went the lazy way here, logged all output to another tmp file, then do a grep on that for enabled. Not the most efficient but gets the job done. Will leave "unsolved" for a little while longer incase anyone has suggestions.

Something like (not tested)

Code:

for dupe in $(/bin/cat /tmp/dupes.txt)

do

  buf=$(database query using $dupe that provides text output)

  case $buf in

      *'scheduled backup: Enabled'* )

          do stuff

  esac  

done

You could use a regex instead of the case statement, something like [[ $buf =~ scheduled backup: Enabled ]] but it's a relatively recent feature and some regexes upset bash so the safe way is to put the regex in a variable ...

Code:

regex='scheduled backup: Enabled'

if [[ $buf =~ $regex ]]

... so it's not much cleaner than the more portable case statement anyway.

You could throw an if statement in there to accomplish what you're looking for. While this isn't the most robust way of doing it, it should work:

Code:

for dupe in $(/bin/cat /tmp/dupes.txt); do

        var1=$(database query that provides text output)

        echo $var1 | grep -q Enabled

        if [[ $? -eq 0 ]]; then

                echo $var1 | grep name

        fi

done

Please, don't ever use for loops with command substitutions, and especially not with cat.

Always use a while+read loop with text file or data stream input.

Otherwise, it's pretty much what Catkin said before.

Code:

#!/bin/bash



re='name: ([^;]+);.*scheduled backup: Enabled;'



while read -r query; do



    output=$( database query with "$query" )



    if [[ $output =~ $re ]]; then

        echo "${BASH_REMATCH[1]}"

    fi



done </tmp/dupes.txt

If you aren't using bash, then you'd have to change the built-in regex test to one or more grep/sed/awk commands or similar. But the basic process should be the same.

Another option, when you are in bash, is to use the mapfile built-in to populate an array with line entries. then you can safely use a for loop.

Code:

#!/bin/bash



re='name: ([^;]+);.*scheduled backup: Enabled;'



mapfile -t queries </tmp/dupes.txt



for query in "${queries[@]}"; do



    output=$( database query with "$query" )



    if [[ $output =~ $re ]]; then

        echo "${BASH_REMATCH[1]}"

    fi



done

David, in all my time with BASH nobody has ever mentioned the for loop reading gotchas that you have on that wiki post. Thank you for that especially. I re-scripted it today and it is a more scalable solution by far. Thanks to everyone for their input, my half-a$$ed script from last week is pretty sloppy.

Glad to have helped out.

Yes, there's a lot of poor coding advice going around. While I've been scripting for almost 10 years now, it wasn't until I discovered the wooledge site a few years ago that I really learned what constitutes good and bad use of its features. I highly suggest reading through the BashGuide, BashFAQ, and BashPitfalls pages. It should help your coding immensely.

BTW, I notice I made a scripting mistake, capturing the query to the variable $output, but stupidly using the variable $text in the follow-up test. I'm updating the above to fix that.