Bash for loop use variable name as an array

fur · 11-03-2011, 12:53 PM

Writing a script where I have several arrays defined and each array contain a list of servers.

array names,
T1 T2 T3 T4 T5

I want to have a for loop go though another for loop containing the array names, then perform actions based on the contects of each one.

Tried this, but doesn't work.

Code:

for ARRAY in T1 T2 T3 T4 T5; do
        for SERV in ${$ARRAY[@]}; do
                doing stuff...
                doing stuff...
        done
done

I would expect using ${$ARRAY[@]} would cause the "for SERV" loop to go through each arrays values.

gives an error like this,

line 117: ${$ARRAY[@]}: bad substitution

Can you use a variable name when referencing an array is my question.

NevemTeve · 11-03-2011, 01:52 PM

Untested: ${${ARRAY}[0]}

catkin · 11-03-2011, 02:39 PM

Untested:

Code:

for ARRAY in T1 T2 T3 T4 T5; do
        for SERV in $( eval '"${'$ARRAY'[@]}"' ); do
                doing stuff...
                doing stuff...
        done
done

eval is dangerous when the following string contents are uncontrolled but in this case ...

David the H. · 11-03-2011, 02:43 PM

No, that won't work. Not without using eval, which is NOT recommended.

http://mywiki.wooledge.org/BashFAQ/048

This is what's known as indirect referencing.

http://mywiki.wooledge.org/BashFAQ/006

There are probably several ways to do what you want, with associative arrays being the best bet.

Edit: simple example:

Code:

#!/bin/bash

declare -A T1 T2 T3 T4 T5

T1=( [s1]=server1 [s2]=server2 [s3]=server3 )
T2=( [s1]=server1 [s2]=server2 [s3]=server3 )
T3=( [s1]=server1 [s2]=server2 [s3]=server3 )
T4=( [s1]=server1 [s2]=server2 [s3]=server3 )
T5=( [s1]=server1 [s2]=server2 [s3]=server3 )

for i in s1 s2 s3; do

     echo "T1: ${T1[$i]} T2: ${T2[$i]} T3: ${T3[$i]} T4: ${T4[$i]} T5: ${T5[$i]}"

done

Juako · 11-03-2011, 02:51 PM

See my answer here, in the function (named "swap") that takes an array name as a parameter.

In this situation you seem to control the input for eval, so:

Code:

#!/bin/bash
T1=( server1 server2 server3 )
T2=( server4 server5 server6 )
T3=( server7 server8 server9 )
T4=( server10 server11 server10 )
T5=( server13 server14 server15 )

for ARRAYNAME in T1 T2 T3 T4 T5; do
    eval "THE_ARRAY=\${$ARRAYNAME[@]}"
    for SERV in ${THE_ARRAY[@]}; do
        echo -n "$SERV "
    done
    echo
done

But beware when you use eval, test it very thorougly. It's very easy for it to go out of control, even worst: it's very easy for you to *think* it won't go out of control. So again *think carefully* and if at all possible use another solution.