Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "Zero−length parameter passed."
func2 ""
# Called with zero−length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param"
echo
# Called with uninitialized param
echo "One parameter passed."
func2 first
# Called with one param
echo
echo "Two parameters passed."
func2 first second # Called with two params
echo
echo "\"\" \"second\" passed."
func2 "" second # Called with zero−length first parameter
echo
# and ASCII string as a second one.
exit 0
The shift command works on arguments passed to functions (see Example 34−10).
In contrast to certain other programming languages, shell scripts normally pass only value parameters to
functions. [51] Variable names (which are actually pointers), if passed as parameters to functions, will be
treated as string literals and cannot be dereferenced. Functions interpret their arguments literally.
Exit and Return
exit status
Functions return a value, called an exit status. The exit status may be explicitly specified by a return
statement, otherwise it is the exit status of the last command in the function (0 if successful, and a
non−zero error code if not). This exit status may be used in the script by referencing it as $?. This
mechanism effectively permits script functions to have a "return value" similar to C functions.
return
Terminates a function. A return command [52] optionally takes an integer argument, which is
returned to the calling script as the "exit status" of the function, and this exit status is assigned to the
variable $?.
Example 23−3. Maximum of two numbers
#!/bin/bash
# max.sh: Maximum of two integers.
E_PARAM_ERR=−198
EQUAL=−199
# If less than 2 params passed to function.
# Return value if both params equal.
max2 ()
# Returns larger of two numbers.
{ # Note: numbers compared must be less than 257.
if [ −z "$2" ]
then
return $E_PARAM_ERR
Chapter 23. Functions 277