Line from bash command output stored in variable as string

Question

I'm trying to find a solution to a problem analog to this one:

#command_A

A_output_Line_1
A_output_Line_2
A_output_Line_3

#command_B

B_output_Line_1
B_output_Line_2

Now I need to compare A_output_Line_2 and B_output_Line_1 and echo "Correct" if they are equal and "Not Correct" otherwise.

I guess the easiest way to do this is to copy a line of output in some variable and then after executing the two commands, simply compare the variables and echo something. This I need to implement in a bash script and any information on how to get certain line of output stored in a variable would help me put the pieces together. Also, it would be cool if anyone can tell me not only how to copy/store a line, but probably just a word or sequence like : line 1, bytes 4-12, stored like string in a variable.

I am not a complete beginner but also not anywhere near advanced linux bash user. Thanks to any help in advance and sorry for bad english!

While registering on SO, there ought to be a question: How did you come to know about us? — devnull
– devnull, Commented Jul 5, 2013 at 11:53
grep, sed, awk and maybe cut are commands that are helpful for doing what you want. The man pages should help and a lot of examples for nearly everything are shared across the internet. :) — David
– David, Commented Jun 13, 2016 at 11:14

Daren Thomas · Accepted Answer · 2013-07-05 12:13:46Z

An easier way might be to use diff, no?

Something like:

command_A > command_A.output
command_B > command_B.output
diff command_A.output command_B.output

This will work for comparing multiple strings.

But, since you want to know about single lines (and words in the lines) here are some pointers:

# first line of output of command_A
command_A | head -n 1

The -n 1 option says only to use the first line (default is 10 I think)

# second line of output of command_A
command_A | head -n 2 | tail -n 1

that will take the first two lines of the output of command_A and then the last of those two lines. Happy times :)

You can now store this information in a variable:

export output_A=`command_A | head -n 2 | tail -n 1`
export output_B=`command_B | head -n 1`

And then compare it:

if [ "$output_A" == "$output_B" ]; then echo 'Correct'; else echo 'Not Correct'; fi

To just get parts of a string, try looking into cut or (for more powerful stuff) sed and awk.

Also, just learing a good general purpose scripting language like python or ruby (even perl) can go a long way with this kind of problem.

Could be simplified using process substitution: diff <(command_A) <(command_B).

slips · Accepted Answer · 2013-07-05 12:08:22Z

0

Use the IFS (internal field separator) to separate on newlines and store the outputs in an array.

#!/bin/bash

IFS='
'
array_a=( $(./a.sh) )
array_b=( $(./b.sh) )

if [ "${array_a[1]}" = "${array_b[0]}" ]; then
    echo "CORRECT"
else
    echo "INCORRECT"
fi

answered Jul 5, 2013 at 12:08

slips

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