Passing arguments by reference

by

It’s 2018, and this question deserves an update. At least in Bash, as of Bash 4.3-alpha, you can use namerefs to pass function arguments by reference:

function boo() 
{
    local -n ref=$1
    ref="new" 
}

SOME_VAR='old'
echo $SOME_VAR # -> old
boo SOME_VAR
echo $SOME_VAR # -> new

The critical pieces here are:

  • Passing the variable’s name to boo, not its value: boo SOME_VAR, not boo $SOME_VAR.

  • Inside the function, using local -n ref=$1 to declare a nameref to the variable named by $1, meaning it’s not a reference to $1 itself, but rather to a variable whose name $1 holds, i.e. SOME_VAR in our case. The value on the right-hand side should just be a string naming an existing variable: it doesn’t matter how you get the string, so things like local -n ref="my_var" or local -n ref=$(get_var_name) would work too. declare can also replace local in contexts that allow/require that. See chapter on Shell Parameters in Bash Reference Manual for more information.

The advantage of this approach is (arguably) better readability and, most importantly, avoiding eval, whose security pitfalls are many and well-documented.

More Related Contents:

  1. How do I split a string on a delimiter in Bash?
  2. Correct Bash and shell script variable capitalization
  3. How to urlencode data for curl command?
  4. Why equal to operator does not work if it is surrounded by space?
  5. sudo echo “something” >> /etc/privilegedFile doesn’t work [duplicate]
  6. How can I declare and use Boolean variables in a shell script?
  7. Test for non-zero length string in Bash: [ -n “$var” ] or [ “$var” ]
  8. How can I suppress all output from a command using Bash?
  9. Random number from a range in a Bash Script
  10. How to get a password from a shell script without echoing
  11. How to get the part of a file after the first line that matches a regular expression
  12. Copy folder recursively, excluding some folders
  13. Get just the filename from a path in a Bash script [duplicate]
  14. How to resolve symbolic links in a shell script
  15. Why equal to operator does not work if it is not surrounded by space?
  16. Extract version number from a string
  17. How to tell if a string is not defined in a Bash shell script
  18. Is there a way to avoid positional arguments in bash?
  19. What does `kill -0 $pid` in a shell script do?
  20. While loop stops reading after the first line in Bash
  21. Why does “local” sweep the return code of a command?
  22. Recursively rename files using find and sed
  23. Batch renaming files with Bash
  24. How to generate random number in Bash?
  25. In bash, how do I bind a function key to a command?
  26. “Invalid Arithmetic Operator” when doing floating-point math in bash
  27. Using sed to mass rename files
  28. Execute a shell function with timeout
  29. Convert Unix timestamp to a date string
  30. How to update one file in a zip archive

Leave a Comment