Variable inside a variable in a loop

Question

I have a question regarding how a variable can be called with another variable name inside a loop.

The following script does not work:

#!/bin/bash
# Comparing test1.txt with test2.txt, test1.ini with test2.ini, test1.conf with test2.conf

FIRSTFILE1=test1.txt;
SECONDFILE1=test2.txt;
FIRSTFILE2=test1.ini;
SECONDFILE2=test2.ini;
FIRSTFILE3=test1.conf;
SECONDFILE3=test2.conf;

for NUM in {1..3};
do
  diff --brief <(sort $FIRSTFILE$NUM) <(sort $SECONDFILE$NUM) > /dev/null
  value=$?
  if [ $value -eq 1 ]
  then
    echo "different"
  else
    echo "identical"
  fi
done

Answer 1

Not an explanation to the question if it is strictly about combining to variables into one but the following would give you a working result of iterating through your files:

for EXT in txt ini conf;
do
  diff --brief <(sort test1.${EXT}) <(sort test2.${EXT}) > /dev/null
  value=$?
  if [ $value -eq 1 ]
  then
    echo "different"
  else
    echo "identical"
  fi
done

Answer 2

You are looking for indirect parameter expansion. You can achieve this in bash using an exclamation point.

#!/bin/bash                                                                        
FIRSTFILE1=test1.txt;
SECONDFILE1=test2.txt;
FIRSTFILE2=test1.ini;
SECONDFILE2=test2.ini;
FIRSTFILE3=test1.conf;
SECONDFILE3=test2.conf;

for NUM in {1..3};
do
    a=FIRSTFILE$NUM
    b=SECONDFILE$NUM
    echo ${!a}
    echo ${!b}
done

You will need some more testing to find a oneliner :). For more information see: http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

Answer 3

Use arrays, because that is what they are for:

#!/bin/bash

first=(  test1.txt test1.ini test1.conf )
second=( test2.txt test2.ini test2.conf )

for (( i = 0; i < ${#first[@]}; ++i )); do
    if cmp -s "${first[i]}" "${second[i]}"; then
        echo 'same'
    else
        echo 'different'
    fi
done

Or, if your filenames are all this predictable,

#!/bin/bash

suffixes=( .txt .ini .conf )

for suffix in "${suffixes[@]}"; do
    if cmp -s "test1$suffix" "test2$suffix"; then
        echo 'same'
    else
        echo 'different'
    fi
done