bash_return

Summary

This Bash extension written in C allows script functions to return numeric or string values.

Status: Experimental

Generally, two methods can be used to return results from functions to the caller of the function. (I) Using command substitution to capture the output may cause poor performance. (II) Storing the result in a global variable is less compact and requires two statements.

With this Bash extension, functions can be used in both ways depending on a command line switch. See Details.

Installation

First Install dependencies

Compile:

compile_C.sh  bashbuiltin_cg_return.c

Usage

The compiled builtins must be loaded once.

enable -f /home/cgille/compiled/bashbuiltin_cg_return.so   init_retval set_retval

The Bash builtin init_retval must be called at the top of the function body. The return value of the function is set with the builtin set_retval.

square(){
    init_retval
    : Do something
    set_retval $(($1*$1))
}

When the function is called with the switch -$, then the returned value is obtained from a variable whose name is stored in the global variable RETVAL. Usage of a global variable is possible because Bash is not multi-threaded. Normally, dollar signs need to be quoted in UNIX shells. Note that the dollar sign does not need to be quoted here because it is followed by a space. This rather unusual command line switch is unlikely to be used already. Note that -$ must be the first parameter. It is shifted out by init_retval and will not be in the argument list any more.

square -$  3
echo "The Square of 3 is ${RETVAL}"

Without the option -$, the result is printed to the standard output.

square 3

Example with Arrays

my_seq(){
    init_retval
    local int n=$1
    local aa=()
    for((i=0;i<n;i++)); do
        aa+=( $((i*i)) )
    done
    set_retval "${aa[@]}"
}
my_seq 10

To capture these numbers in an array variable, call it as follows.

my_seq -$ 10
numbers=("${RETVAL[@]}")
echo "There are ${#numbers[@]} numbers. These are ${numbers[@]}"

Benchmarks

Performing 40000 function calls with the script benchmark_set_retval.sh

classical      51 Seconds
Novel method  1.2 Seconds