BASH Variables with multiple commands and reentrant

Question

I have a bash script that sources contents from another file. The contents of the other file are commands I would like to execute and compare the return value. Some of the commands are have multiple commands separated by either a semicolon (;) or by ampersands (&&) and I can't seem to make this work. To work on this, I created some test scripts as shown:

test.conf is the file being sourced by test

Example-1 (this works), My output is 2 seconds in difference

test.conf

    CMD[1]="date"

test.sh

    . test.conf
    i=2
    echo "$(${CMD[$i]})"
    sleep 2
    echo "$(${CMD[$i]})"

Example-2 (this does not work) test.conf (same script as above)

    CMD[1]="date;date"

Example-3 (tried this, it does not work either) test.conf (same script as above)

    CMD[1]="date && date"

I don't want my variable, CMD, to be inside tick marks because then, the commands would be executed at time of invocation of the source and I see no way of re-evaluating the variable.

This script essentially calls CMD on pass-1 to check something, if on pass-1 I get a false reading, I do some work in the script to correct the false reading and re-execute & re-evaluate the output of CMD; pass-2.

Here is an example. Here I'm checking to see if SSHD is running. If it's not running when I evaluate CMD[1] on pass-1, I will start it and re-evaluate CMD[1] again.

test.conf

    CMD[1]=`pgrep -u root -d , sshd 1>/dev/null; echo $?`

So if I modify this for my test script, then test.conf becomes: NOTE: Tick marks are not showing up but it's the key below the ~ mark on my keyboard.

    CMD[1]=`date;date` or `date && date`

My script looks like this (to handle the tick marks)

    . test.conf
    i=2
    echo "${CMD[$i]}"
    sleep 2
    echo "${CMD[$i]}"

I get the same date/time printed twice despite the 2 second delay. As such, CMD is not getting re-evaluate.

Your question isn't clear - actually, I don't see the question. What exactly is are you trying to do, and where does it go wrong? Your "example", "test.conf CMD[1]=pgrep -u"... makes precious little sense - where is the "source file"? How are you parsing arguments - you're missing quotes as I see it? — Barry Kelly
– Barry Kelly, Commented Jul 13, 2009 at 18:28
This is a test script, not the actual. Hence the reason the $i is still in there (cut -n- past) and this is lacking in clarity. The CMD[x] where x = 0 ... 10 are test conditions that return either 1 or 0; mostly is the process running or not and similar to the pgrep example given above. Along with CMD, I have a corresponding RESTART[x] that I call and after I call it, I re-evaluate the CMD[x] to see if the process restarted. In other examples I am checking for presence of a log file. I didn't want to bore the forum with these details but it would probably have been helpful. Thank you! — Eric
– Eric, Commented Jul 14, 2009 at 12:16

Dennis Williamson · Accepted Answer · 2009-07-13 20:30:58Z

2

First of all, you should never use backticks unless you need to be compatible with an old shell that doesn't support $() - and only then.

Secondly, I don't understand why you're setting CMD[1] but then calling CMD[$i] with i set to 2.

Anyway, this is one way (and it's similar to part of Barry's answer):

CMD[1]='$(date;date)'    # no backticks (remember - they carry Lime disease)
eval echo "${CMD[1]}"    # or $i instead of 1

answered Jul 13, 2009 at 20:30

Dennis Williamson

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3 Comments

Eric Over a year ago

This also works. I hadn't thought of this method. Excellent, Thank You!

MattTT Over a year ago

Do I understand right, that CMD[1] then holds already the outputs of the commands and not the commands itself?

Dennis Williamson Over a year ago

@MattTT: No, it contains the commands because the assignment uses single quotes. The eval executes the commands. This question and answer pertain to a specific set of circumstances and aren't intended to be generally applicable. If you came here trying to solve a problem, there's probably a better way to do it.

Barry Kelly · Accepted Answer · 2009-07-13 18:33:53Z

1

From the couple of lines of your question, I would have expected some approach like this:

#!/bin/bash

while read -r line; do
    # munge $line
    if eval "$line"; then
        # success
    else
        # fail
    fi
done

Where you have backticks in the source, you'll have to escape them to avoid evaluating them too early. Also, backticks aren't the only way to evaluate code - there is eval, as shown above. Maybe it's eval that you were looking for?

For example, this line:

CMD[1]=`pgrep -u root -d , sshd 1>/dev/null; echo $?`

Ought probably look more like this:

CMD[1]='`pgrep -u root -d , sshd 1>/dev/null; echo $?`'

answered Jul 13, 2009 at 18:33

Barry Kelly

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1 Comment

Eric Over a year ago

This works! However, per my and your earlier comments (we agree) I don't want back ticks in there. This script will not live on any other environments outside of Linux, I have no control over BASH and agree with your portability concern!

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