Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash | ||
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Like "real" programming languages, bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Whenever there is repetitive code, when a task repeats with only slight variations, then writing a function should be investigated.
function function-name {
command...
}
function-name () {
command...
}
This second form will cheer the hearts of C programmers.
The opening bracket in the function may optionally be placed on the second line, to more nearly resemble C function syntax.
function-name ()
{
command...
}
Functions are called, triggered, simply by invoking their names.
Note that the function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C.
Example 3-80. Simple function
1 #!/bin/bash 2 3 funky () 4 { 5 echo This is a funky function. 6 echo Now exiting funky function. 7 } 8 9 # Note: function must precede call. 10 11 # Now, call the function. 12 13 funky 14 15 exit 0 |
More complex functions may have arguments passed to them and return exit values to the script for further processing.
1 function-name $arg1 $arg2 |
The function refers to the passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.
Example 3-81. Function Taking Parameters
1 #!/bin/bash 2 3 func2 () { 4 if [ -z $1 ] 5 # Checks if any params. 6 then 7 echo "No parameters passed to function." 8 return 0 9 else 10 echo "Param #1 is $1." 11 fi 12 13 if [ $2 ] 14 then 15 echo "Parameter #2 is $2." 16 fi 17 } 18 19 func2 20 # Called with no params 21 echo 22 23 func2 first 24 # Called with one param 25 echo 26 27 func2 first second 28 # Called with two params 29 echo 30 31 exit 0 |
![]() | In contrast to certain other programming languages, shell scripts permit passing only value parameters to functions. 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. |
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 $?.
Terminates a function. The return statement 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 3-82. Converting numbers to Roman numerals
1 #!/bin/bash 2 3 # Arabic number to Roman numeral conversion 4 # Range 0 - 200 5 # It's crude, but it works. 6 7 # Extending the range and otherwise improving the script 8 # is left as an exercise for the reader. 9 10 # Usage: roman number-to-convert 11 12 ARG_ERR=1 13 OUT_OF_RANGE=200 14 15 if [ -z $1 ] 16 then 17 echo "Usage: `basename $0` number-to-convert" 18 exit $ARG_ERR 19 fi 20 21 num=$1 22 if [ $num -gt $OUT_OF_RANGE ] 23 then 24 echo "Out of range!" 25 exit $OUT_OF_RANGE 26 fi 27 28 to_roman () 29 { 30 number=$1 31 factor=$2 32 rchar=$3 33 let "remainder = number - factor" 34 while [ $remainder -ge 0 ] 35 do 36 echo -n $rchar 37 let "number -= factor" 38 let "remainder = number - factor" 39 done 40 41 return $number 42 } 43 44 # Note: must declare function 45 # before first call to it. 46 47 to_roman $num 100 C 48 num=$? 49 to_roman $num 90 LXXXX 50 num=$? 51 to_roman $num 50 L 52 num=$? 53 to_roman $num 40 XL 54 num=$? 55 to_roman $num 10 X 56 num=$? 57 to_roman $num 9 IX 58 num=$? 59 to_roman $num 5 V 60 num=$? 61 to_roman $num 4 IV 62 num=$? 63 to_roman $num 1 I 64 65 echo 66 67 exit 0 |
A variable declared as local is one that is visible only within the block of code in which it appears. In a shell script, this means the variable has meaning only within its own function.
Example 3-83. Local variable visibility
1 #!/bin/bash 2 3 func () 4 { 5 local a=23 6 echo 7 echo "a in function is $a" 8 echo 9 } 10 11 func 12 13 # Now, see if local 'a' 14 # exists outside function. 15 16 echo "a outside function is $a" 17 echo 18 # Nope, 'a' not visible globally. 19 20 exit 0 |
Local variables permit recursion (a recursive function is one that calls itself), but this practice usually involves much computational overhead and is definitely not recommended in a shell script.
Example 3-84. Recursion, using a local variable
1 #!/bin/bash 2 3 # factorial 4 # --------- 5 6 7 # Does bash permit recursion? 8 # Well, yes, but... 9 # You gotta have rocks in your head to try it. 10 11 12 MAX_ARG=5 13 WRONG_ARGS=1 14 RANGE_ERR=2 15 16 17 if [ -z $1 ] 18 then 19 echo "Usage: `basename $0` number" 20 exit $WRONG_ARGS 21 fi 22 23 if [ $1 -gt $MAX_ARG ] 24 then 25 echo "Out of range (5 is maximum)." 26 # Let's get real now... 27 # If you want greater range than this, rewrite it in a real programming language. 28 exit $RANGE_ERR 29 fi 30 31 fact () 32 { 33 local number=$1 34 # Variable "number" must be declared as local otherwise this doesn't work. 35 if [ $number -eq 0 ] 36 then 37 factorial=1 38 else 39 let "decrnum = number - 1" 40 fact $decrnum # Recursive function call. 41 let "factorial = $number * $?" 42 fi 43 44 return $factorial 45 } 46 47 fact $1 48 echo "Factorial of $1 is $?." 49 50 exit 0 |