Shell Scripting: Expert Recipes for Linux, Bash, and More

Indirection

One particularly useful trick in the bash shell is indirection. You’ll have to be a little bit careful when using it, as it’s easy to get confused as to which variable is which, but it can be a real life-saver.

It is possible to use the value of one variable as the name of anther variable. For example, the following:

for mything in PATH GDMSESSION HOSTNAME
do 
  echo $myvar is ${!myvar}
done

will run like this:

PATH is /usr/bin:/bin:/usr/local/bin:/home/steve/bin:/usr/games
GDMSESSION is gnome
HOSTNAME is declan

Not terribly useful, at first glance. But it means that you can create your own variable names on-the-fly and then access the data within that name:

$ cat empdata.sh
#!/bin/bash
# Employee Data
Dave_Fullname="Dave Smith"
Dave_Country="USA"
Dave_Email=[e-mail]
Jim_Fullname="Jim Jones"
Jim_Country="Germany"
Jim_Email=[e-mail]
Bob_Fullname="Bob Anderson"
Bob_Country="Australia"
Bob_Email=[e-mail]
echo "Select an Employee:"
select Employee in Dave Jim Bob
do
  echo "What do you want to know about ${Employee}?"
  select Data in Fullname Country Email
  do
    echo $Employee               # Jim
    echo $Data                   # Email
    empdata=${Employee}_${Data}  # Jim_Email
    echo "${Employee}'s ${Data} is ${!empdata}"      # [e-mail]
    break
  done 
  break
done

empdata.sh

$ ./empdata.sh Select an Employee: ...