Modify a shell variable inside awk block of code

Question

Is there any way to modify a shell variable inside awk block of code?

--------- [shell_awk.sh]---------------------
 #!/bin/bash

shell_variable_1=<value A>
shell_variable_2=<value B>
shell_variable_3=<value C>

awk 'function A(X)
{ return X+1 }
{ a=A('$shell_variable_1')
  b=A('$shell_variable_2')
  c=A('$shell_variable_3')
  shell_variable_1=a
  shell_variable_2=b
  shell_variable_3=c
}' FILE.TXT
--------- [shell_awk.sh]---------------------

This is a very simple example, the real script load a file and make some changes using functions, I need to keep each value before change into a specific variable, so then I can register into MySQL the before and after value.

The after value is received from parameters ($1, $2 and so on).

The value before I already know how to get it from the file.

All is done well, except the shell_variable been set by awk variable. Outside from awk block code is easy to set, but inside, is it possible?

Answer 1

No program -- in awk, shell, or any other language -- can directly modify a parent process's memory. That includes variables. However, of course, your awk can write contents to stdout, and the parent shell can read that content and modify its own variables itself.

---

Here's an example of awk that writes key/value pairs out to be read by bash. It's not perfect -- read the caveats below.

#!/bin/bash

shell_variable_1=10
shell_variable_2=20
shell_variable_3=30

run_awk() {
        awk -v shell_variable_1="$shell_variable_1" \
            -v shell_variable_2="$shell_variable_2" \
            -v shell_variable_3="$shell_variable_3" '
        function A(X) { return X+1 }
        { a=A(shell_variable_1)
          b=A(shell_variable_2)
          c=A(shell_variable_3) }
        END {
          print "shell_variable_1=" a
          print "shell_variable_2=" b
          print "shell_variable_3=" c
        }' <<<""
}

while IFS="=" read -r key value; do
        printf -v "$key" '%s' "$value"
done < <(run_awk)

for var in shell_variable_{1,2,3}; do
  printf 'New value for %s is %s\n' "$var" "${!var}"
done

---

Advantages

• Doesn't use eval. Content such as $(rm -rf ~) in the output from awk won't be executed by your shell.

Disadvantages

• Can't handle variable contents with newlines. (You could fix this by NUL-delimiting output from your awk script, and adding -d '' to the read command).
• A hostile awk script could modify PATH, LD_LIBRARY_PATH, or other security-sensitive variables. (You could fix this by reading variables into an associative array, rather than the global namespace, or by enforcing a prefix on their names).
• The code above uses several ksh extensions also available in bash; however, it will not run with POSIX sh. Thus, be sure not to run this via sh scriptname (which only guarantees POSIX functionality).