CODEWITHSUNDEEP
bash
Vivek
Vivek

Reputation: 13248

How to expand variables using previously defined variables inside a file

I have a property file having multiple key value pairs. Some of the values uses previously defined keys. Following is the sample 

  xpath=abc/temp.txt
  fullpath=$HOME/$xpath
  ...

I want to parse this file line by line and print the lines along with resolving env variables like $HOME as well as earlier defined variables like $xpath

The expected output is

  xpath=abc/temp.txt
  fullpath=tempuser/abc/temp.txt
  ...

How do I expand the variables in this way in a bash script

Upvotes: 1

Views: 74

Answers (2)

bishop
bishop

Reputation: 39414

Since the assignments are valid bash, source settings.env suffices to evaluate them. However, you also want to print the assignments, so more trickery required:

PS4="\000" source <(echo 'set -x'; cat settings.env; echo '{ set +x; } 2>/dev/null')

This trick uses the bash debugging facility of set -x to print the output of each assignment as it's performed. In words:

  • PS4="\000" removes the debugging prompt, which by default is +
  • <() creates a new file, by the pipeline contained within the parentheses
  • set -x enables debugging
  • cat settings.env inserts your settings
  • { set +x; } 2>dev/null disables debugging, without outputting it doing so

Upvotes: 0

chengpohi
chengpohi

Reputation: 14217

If you want to parse line by line and interpret the variables, You maybe want to use the eval for evaluating the String, like:

  while IFS='=' read -r key value
  do
    key=$(echo $key | tr '.' '_')
    if [[ ! -z $key ]]
    then
        v=`eval "echo ${value}"`
        eval "${key}='${v}'"
        echo "${key}=${v}"
    fi
  done < "my.properties"

In the above code snippet, use the eval with echo to interpret the variables.

Upvotes: 1

Related Questions