Assign value to a variable in unix dynamically - variable indirection

Question

HOST_NAME_="localhost:8080" 
HOST_NAME_stg="stg.com:8080"
HOST_NAME_qa="qa.com:8080"
HOST_NAME=${!"HOST_NAME_$1"}

echo -n ${HOST_NAME}

I get the error bad substitution.But I want stg.com:8080 to printed if the argument passed is stg.How do I do that

Answer 1

Note: Given your use of ${!...} to refer to a variable indirectly, I assume that you're using bash - POSIX-features-only shells and many others do NOT support this feature (at least with this syntax).

Try the following:

HOST_NAME_="localhost:8080" 
HOST_NAME_stg="stg.com:8080"
HOST_NAME_qa="qa.com:8080"

HOST_NAME_INDIRECT=HOST_NAME_$1

HOST_NAME=${!HOST_NAME_INDIRECT}

echo -n "${HOST_NAME}"

When referencing a variable indirectly with ${!...}, the argument containing the target variable name must itself be a variable, not a string literal, hence the intermediate variable $HOST_NAME_INDIRECT.

Also consider avoiding all-uppercase variable names to avoid potential conflicts with standard environment variables (such as $HOSTNAME) and special shell variables (such as $SECONDS).

---

Here's how various popular shells do variable indirection:

Assume the following two variables:

a='value of a'
nameOfA='a'

• bash

   echo "${!nameOfA}" # -> 'value of a'
  
   # bash 4.3+
   declare -n aliasOfA='a'  # in functions: `local -n`
   echo "$aliasOfA"         # same as `echo "$a"` -> 'value of a'
  

• zsh

  echo "${(P)nameOfA}"  # -> 'value of a'
  

• ksh 93+:

   nameref aliasOfA='a'  # same as `typeset -n`
   echo "$aliasOfA"      # same as `echo "$a"` -> 'value of a'
  

• sh (POSIX-features-only shells)

   eval echo \"'$'$nameOfA\"  # -> 'value of a'
  

Caveat: Using eval is potentially unsafe. If $nameOfA contained something like 'a; $(rm *)', the rm command would execute too.

Answer 2

Thanks for the reply.Yeah using a temporary variable does solve the problem.But I figured out if to do this it serves the purpose but it is slightly complicated

eval "HOST_NAME=\$HOST_NAME_$2"