Grep $value `grep $value2 ``` - Nested grep?

Question

I'm a complete noob at awk/sed so forgive me if I'm missing something obvious here.

Basically I'm trying to do a nested grep, i.e. something akin to:

grep $value `exim -Mvh $(`exim -bpru | grep $eximID | more`)`

Breakdown:

grep $value IN COMMAND 
--> exim -Mvh (print exim mail headers) FROM RESULTS OF 
    ---> exim -bpru | grep $eximID | more

• $value is the string I'm looking for
• $eximID is the string I'm looking for within exim -bpru (list all exim thingies)

No idea if what I'm trying to accomplish would be easier with awk/sed hence the question really.

I tried to make that as legible as possible but nested nesting is hard yo

Edit Tada! My script is now workings thanks to you guys! Here it is, unfinished, but working:

#!/usr/bin/bash

    echo "Enter the email address you want to search for + compare sender info via exim IDs."
    read searchTarget
    echo "Enter the target domain the email is coming from."
    read searchDomain

    #domanList is array for list of exim IDs needed
    domainList=($(exim -bpru | grep "$searchDomain" | awk '{ print $3 }'))


    for i in "${domainList[@]}"
            do
                    echo "$(exim -Mvh  $i | grep $searchTarget)"
                    #echo "$(grep $searchTarget $(exim -Mvh $i))"
            done

Answer 1

grep $value `exim -Mvh $(`exim -bpru | grep $eximID | more`)`

This isn't right. The backticks (`command`) and $(command) do the same thing, it's just an alternative syntax. The advantage of using $() is that it's better nestable, so it's a good habit to always use that.

So, let's fix this, we now end up with:

grep "$value" "$(exim -Mvh "$(exim -bpru | grep "$eximID")")" | more

I relocated the more command, for what I think will be obvious reasons. more just paginates data for the user, feeding the output of more to something else almost never makes sense.

I've also quoted the variables, this is also a good habit, because otherwise things will break when there are certain characters in your variable (most common is the a space).

I can't test if this gives you the output you want, if it doesn't, then update your answer with a few lines of example data, and the expected output.

Answer 2

If you're going to do it with back-quotes (not recommended; it is hard work), then you have to write:

grep $value `exim -Mvh $(\`exim -bpru | grep $eximID\`)`

(where I've removed the more since when used like that it behaves like cat and there's no point in using cat at the end of the commands like that either).

It would be more sane to use the $(…) notation throughout:

grep $value $(exim -Mvh $( $(exim -bpru | grep $eximID)))

And it seems more plausible that you don't need quite that many sets of indirection and this is what you're really after:

grep $value $(exim -Mvh $(exim -bpru | grep $eximID))

You should look at:

• Why didn't back quotes in a shell script help me cd to a directory?
• What is the benefit of using $(…) instead of back ticks in shell scripts?
• Why does \$ reduce to $ inside backquotes [though not inside $(…)]?

and no doubt there are other related questions too.