Regard input string as defined array name in BASH

Question

I am currently working on a script that receives the input string and then use it as the defined array name. But I stuck with this, thanks in advance

a=(1 2 3)
b=(4 5 6)
c=(7 8 9)

if input from user is "a", I want the result_array will be like (1 2 3)?

a=(1 2 3); b=(4 5 6); input="a"; result_array=("${${input}[@]}"); echo ${result_array[@]}

bash: ${${input}[@]}: bad substitution

All I want is: result_array=("${${input}[@]}") => result_array=("${a[@]}") = (1 2 3)

Note: I dont want to use IF statements like "if input = a or b or c, result_array= a or b or c", because I have to do this step many times.

Anubis · Accepted Answer · 2017-08-15 09:49:49Z

1

a=(1 2 3); 
b=(4 5 6);
# ...
input="a"; # get the user input
result_array=${input}[@];
echo \(${!result_array}\)

will print:

(1 2 3)

edited Aug 15, 2017 at 9:49

Anubis

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answered Aug 15, 2017 at 9:26

tso

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1 Comment

Phuc Nguyen Over a year ago

great! that's what I need. Thank you

lw0v0wl · Accepted Answer · 2017-08-15 10:31:03Z

1

Case is more simple than IF in this case. Also faster.

#!/bin/bash

case "$1" in
    a) result_array=(1 2 3);;
    b) result_array=(4 5 6);;
    c) result_array=(7 8 9);;
esac

echo ${result_array[@]}

I also made a script calculate the array according to char ascii positions compered to a, since you said you don't like to use IF.

#!/bin/bash

# a ascii value
a_num=$(echo a|od -t d1 | awk '{printf "%s" ,$2}')

# input ascii value
ascii=$(echo $1|od -t d1 | awk '{printf "%s" ,$2}')

multiplier=$((($ascii - $a_num) * 3))

result_array=( $((1 + $multiplier)) $((2 + $multiplier)) $((3 + $multiplier)))

echo ${result_array[@]}

Ouputs:

./ascii.sh a
1 2 3
./ascii.sh b
4 5 6
./ascii.sh c
7 8 9
./ascii.sh w
67 68 69
./ascii.sh x
70 71 72

edited Aug 15, 2017 at 10:31

answered Aug 15, 2017 at 9:28

lw0v0wl

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1 Comment

Phuc Nguyen Over a year ago

although it is not what I want at the beginning, I like your idea (using "case" to handle this situation). Thanks alot

glenn jackman · Accepted Answer · 2017-08-15 11:40:32Z

0

there are 2 ways to do this. Starting from

a=(1 2 3)
b=(4 5 6)
# ...
input="a"

use a nameref (bash v4.4)
```
declare -n input                 # set the "nameref" attribute for the variable
result_array=( "${input[@]}")
printf "%s\n" "${result_array[@]}"
```
```
1
2
3
```
To "reuse" the input variable to point to b, input=b is insufficient. That sets input[0] (and hence a[0]) to "b". You need use declare -n input=b

use indirect expansion (as @tso did in his answer)

tmp="${input}[@]"           # a string that looks like array expansion
result_array=( "${!tmp}" )
printf "%s\n" "${result_array[@]}"

1
2
3

edited Aug 15, 2017 at 11:40

answered Aug 15, 2017 at 11:33

glenn jackman

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