Using variables as part of variable name in unix

Question

I want to name variable as a_${v}.

for example : v can be 2013 2014

I am now declaring a variable to a_${v}

a_${v}=hI # a_2013 should be Hi

v=2014

so a_${v}=Hello # a_2014 should be Hello

I tried using eval command though it is not throwing error while assigning a value but I am not able to extract the value of variable name

$ v=2013

$ eval a_${v}=Hi


$ v=2014

$ eval a_${v}=Hello

echo ${a_${v}}

is not working.. :(

I am using bash and I don't want to change the variable name i.e dn't want to assign the value to another value

@Pumbaa80 The question (and its answer) you quote is `bash` specific and it can be useful to have a portable, non-bash specific, answer to this question. — Michaël Le Barbier, Sep 07 '14 at 10:01
Simple: remove your last line and instead do these two lines: `varname=$(echo "a_${v}")` followed by `echo ${!varname}`. You're welcome :D I would add this as an answer but it won't let me :/ — CommaToast, Sep 07 '14 at 19:52
This is NOT a duplicate! Need answer for /bin/sh not BASH, and without using eval (if possible). None of the linked answers satisfy this. — user9645, Jun 10 '20 at 12:31

score 11 · Answer 1 · edited Sep 07 '14 at 10:38

11

In bash you can do the below (note the exclamation mark syntax in the last line):

#!/bin/bash

a_2014='hello 2014'
year=2014
varname=a_${year}
echo ${!varname}

edited Sep 07 '14 at 10:38

Farvardin

5,336
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answered Sep 07 '14 at 09:54

bobah

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This small example has saved so much time - thank you !! – somshivam Apr 10 '20 at 16:15

score 2 · Answer 2 · answered Sep 07 '14 at 09:56

Parameter expansion is not recursive, therefore the text ${a_${v}} is really The contents of the variable whose name is 'a_${v}', and the shell complains that this variable name is not valid.

You can achieve recursive expansion with the eval command, as in

eval printf '%s\n' "\${a_${v}}"

To increase the readability and maintainability of your shell scripts, you should limit the use of such constructs and wrap them in appropriate structures. See rc.subr provided on FreeBSD systems for an example.

clt60 · Answer 3 · 2014-09-07T12:21:59.160

In bash 4.3 also:

txt="Value of the variable"

show() { echo "indirect access to $1: ${!1}"; }

a_2014='value of a_2014'
echo "$txt \$a_2014: $a_2014"
show a_2014                    # <-------- this -----------------------+
                               #                                       |
prefix=a                       #                                       |
year=2014                      #                                       |
string="${prefix}_${year}"     #                                       |
echo "\$string: $string"       #                                       |
show "$string"            #$string contains a_2014 e.g. the same as ---+

echo ===4.3====
#declare -n  - only in bash 4.3
#declare -n refvar=${prefix}_${year}
#or
declare -n refvar=${string}

echo "$txt \$refvar: $refvar"
show refvar

echo "==assign to refvar=="
refvar="another hello 2014"
echo "$txt \$refvar: $refvar"
echo "$txt \$a_2014: $a_2014"
show a_2014
show "$string" #same as above
show refvar

prints

Value of the variable $a_2014: value of a_2014
indirect access to a_2014: value of a_2014
$string: a_2014
indirect access to a_2014: value of a_2014
===4.3====
Value of the variable $refvar: value of a_2014
indirect access to refvar: value of a_2014
==assign to refvar==
Value of the variable $refvar: another hello 2014
Value of the variable $a_2014: another hello 2014
indirect access to a_2014: another hello 2014
indirect access to a_2014: another hello 2014
indirect access to refvar: another hello 2014

Using variables as part of variable name in unix

3 Answers3

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