Access variables using an external string.[SHELL script]

Question

I currently have code as

if [[ "$FIRSTFLAG" == 1 ]] ; then
   all_comp+=("FIRST")
fi
if [[ "$SECONDFLAG" == 1 ]] ; then
   all_comp+=("SECOND")
fi
if [[ "$THIRDFLAG" == 1 ]] ; then
    all_comp+=("THIRD")
fi

all_comp is just an array

So, im working on a solution to reduce the repetitive code
I know that we can use case here.
I wonder if there is a solution that can be done using array and for loop \
For example(I know its syntactically wrong)

names=("FIRST" "SECOND" "THIRD")
for i in $names[@]; do
   if [[ ${i}FLAG == 1 ]]; then     <- This line is the issue
      all_comp+=("$i")
   fi
done

So please tell me if there is a solution for such code example

Stephen Quan · Accepted Answer · 2022-03-11 10:12:55Z

1

You need to use indirect expansion by saving the constructed variable name, e.g. iflag=${i}FLAG, then you can use access the indirect expansion with ${!iflag}, e.g.

FIRSTFLAG=1
SECONDFLAG=0
THIRDFLAG=1
all_comp=()
names=("FIRST" "SECOND" "THIRD")
for i in ${names[@]}; do
   iflag=${i}FLAG
   if [[ ${!iflag} == 1 ]]; then
      all_comp+=("$i")
   fi
done
echo ${all_comp[@]} # Outputs: FIRST THIRD

Oh another answer, you can make use of the arithmetic expansion operator (( )) i.e.

FIRSTFLAG=1
SECONDFLAG=0
THIRDFLAG=1
all_comp=()
names=("FIRST" "SECOND" "THIRD")
for i in ${names[@]}; do
   if (( ${i}FLAG == 1 )); then
      all_comp+=("$i")
      (( ${i}FLAG = 99 ))
   fi
done
echo ${all_comp[@]} # FIRST THIRD
echo $FIRSTFLAG # 99
echo $SECONDFLAG # 0
echo $THIRDFLAG # 99

Reference:

https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html#Shell-Parameter-Expansion

edited Mar 11, 2022 at 10:12

answered Mar 11, 2022 at 9:53

Stephen Quan

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4 Comments

sai akhil Over a year ago

what if I want to assign the flag value to ${i}FLAG inside the for loop for suppose ```${i}FLAG=`cat flags.properties |grep -i FIRST | cut -d "=" -f2````

Stephen Quan Over a year ago

I think that followup question should be asked as a new StackOverflow question, but, you could probably make use of eval to solve that. e.g. eval "${i}FLAG=1" would force FIRSTFLAG, SECONDFLAG or THIRDFLAG to become equal to 1.

Stephen Quan Over a year ago

Actually, it appears you can just go (( ${i}FLAG=1 )).

sai akhil Over a year ago

Yeah, I just realised that. Thank you.

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