Chapter 23. Functions
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. Wherever there is repetitive code, when a task repeats with only slight variations, then consider using a function.
function function_name {
command...
}
function_name () {
command...
}
This second form will cheer the hearts of C programmers (and is more portable).
As in C, the function's opening bracket may optionally appear on the second line.
function_name ()
{
command...
}
Functions are called, triggered, simply by invoking their names.
Example 23-1. Simple functions
1 #!/bin/bash
2
3 JUST_A_SECOND=1
4
5 funky ()
6 { # This is about as simple as functions get.
7 echo "This is a funky function."
8 echo "Now exiting funky function."
9 } # Function declaration must precede call.
10
11
12 fun ()
13 { # A somewhat more complex function.
14 i=0
15 REPEATS=30
16
17 echo
18 echo "And now the fun really begins."
19 echo
20
21 sleep $JUST_A_SECOND # Hey, wait a second!
22 while [ $i -lt $REPEATS ]
23 do
24 echo "----------FUNCTIONS---------->"
25 echo "<------------ARE-------------"
26 echo "<------------FUN------------>"
27 echo
28 let "i+=1"
29 done
30 }
31
32 # Now, call the functions.
33
34 funky
35 fun
36
37 exit 0 |
The function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C.
1 f1
2 # Will give an error message, since function "f1" not yet defined.
3
4 declare -f f1 # This doesn't help either.
5 f1 # Still an error message.
6
7 # However...
8
9
10 f1 ()
11 {
12 echo "Calling function \"f2\" from within function \"f1\"."
13 f2
14 }
15
16 f2 ()
17 {
18 echo "Function \"f2\"."
19 }
20
21 f1 # Function "f2" is not actually called until this point,
22 #+ although it is referenced before its definition.
23 # This is permissable.
24
25 # Thanks, S.C. |
It is even possible to nest a function within another function, although this is not very useful.
1 f1 ()
2 {
3
4 f2 () # nested
5 {
6 echo "Function \"f2\", inside \"f1\"."
7 }
8
9 }
10
11 f2 # Gives an error message.
12 # Even a preceding "declare -f f2" wouldn't help.
13
14 echo
15
16 f1 # Does nothing, since calling "f1" does not automatically call "f2".
17 f2 # Now, it's all right to call "f2",
18 #+ since its definition has been made visible by calling "f1".
19
20 # Thanks, S.C. |
Function declarations can appear in unlikely places, even where a command would otherwise go.
1 ls -l | foo() { echo "foo"; } # Permissable, but useless.
2
3
4
5 if [ "$USER" = bozo ]
6 then
7 bozo_greet () # Function definition embedded in an if/then construct.
8 {
9 echo "Hello, Bozo."
10 }
11 fi
12
13 bozo_greet # Works only for Bozo, and other users get an error.
14
15
16
17 # Something like this might be useful in some contexts.
18 NO_EXIT=1 # Will enable function definition below.
19
20 [[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-list".
21 # If $NO_EXIT is 1, declares "exit ()".
22 # This disables the "exit" builtin by aliasing it to "true".
23
24 exit # Invokes "exit ()" function, not "exit" builtin.
25
26 # Thanks, S.C. |
23.1. Complex Functions and Function Complexities
Functions may process arguments passed to them and return an exit status 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 23-2. Function Taking Parameters
1 #!/bin/bash
2 # Functions and parameters
3
4 DEFAULT=default # Default param value.
5
6 func2 () {
7 if [ -z "$1" ] # Is parameter #1 zero length?
8 then
9 echo "-Parameter #1 is zero length.-" # Or no parameter passed.
10 else
11 echo "-Param #1 is \"$1\".-"
12 fi
13
14 variable=${1-$DEFAULT} # What does
15 echo "variable = $variable" #+ parameter substitution show?
16 # ---------------------------
17 # It distinguishes between
18 #+ no param and a null param.
19
20 if [ "$2" ]
21 then
22 echo "-Parameter #2 is \"$2\".-"
23 fi
24
25 return 0
26 }
27
28 echo
29
30 echo "Nothing passed."
31 func2 # Called with no params
32 echo
33
34
35 echo "Zero-length parameter passed."
36 func2 "" # Called with zero-length param
37 echo
38
39 echo "Null parameter passed."
40 func2 "$uninitialized_param" # Called with uninitialized param
41 echo
42
43 echo "One parameter passed."
44 func2 first # Called with one param
45 echo
46
47 echo "Two parameters passed."
48 func2 first second # Called with two params
49 echo
50
51 echo "\"\" \"second\" passed."
52 func2 "" second # Called with zero-length first parameter
53 echo # and ASCII string as a second one.
54
55 exit 0 |
 | The shift command works on arguments passed to functions (see Example 34-12). |
But, what about command-line arguments passed to the script? Does a function see them? Well, let's clear up the confusion.
Example 23-3. Functions and command-line args passed to the script
1 #!/bin/bash
2 # func-cmdlinearg.sh
3 # Call this script with a command-line argument,
4 #+ something like $0 arg1.
5
6
7 func ()
8
9 {
10 echo "$1"
11 }
12
13 echo "First call to function: no arg passed."
14 echo "See if command-line arg is seen."
15 func
16 # No! Command-line arg not seen.
17
18 echo "============================================================"
19 echo
20 echo "Second call to function: command-line arg passed explicitly."
21 func $1
22 # Now it's seen!
23
24 exit 0 |
In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals. Functions interpret their arguments literally.
Indirect variable references (see Example 35-2) provide a clumsy sort of mechanism for passing variable pointers to functions.
Example 23-4. Passing an indirect reference to a function
1 #!/bin/bash
2 # ind-func.sh: Passing an indirect reference to a function.
3
4 echo_var ()
5 {
6 echo "$1"
7 }
8
9 message=Hello
10 Hello=Goodbye
11
12 echo_var "$message" # Hello
13 # Now, let's pass an indirect reference to the function.
14 echo_var "${!message}" # Goodbye
15
16 echo "-------------"
17
18 # What happens if we change the contents of "hello" variable?
19 Hello="Hello, again!"
20 echo_var "$message" # Hello
21 echo_var "${!message}" # Hello, again!
22
23 exit 0 |
The next logical question is whether parameters can be dereferenced after being passed to a function.
Example 23-5. Dereferencing a parameter passed to a function
1 #!/bin/bash
2 # dereference.sh
3 # Dereferencing parameter passed to a function.
4 # Script by Bruce W. Clare.
5
6 dereference ()
7 {
8 y=\$"$1" # Name of variable.
9 echo $y # $Junk
10
11 x=`eval "expr \"$y\" "`
12 echo $1=$x
13 eval "$1=\"Some Different Text \"" # Assign new value.
14 }
15
16 Junk="Some Text"
17 echo $Junk "before" # Some Text before
18
19 dereference Junk
20 echo $Junk "after" # Some Different Text after
21
22 exit 0 |
Example 23-6. Again, dereferencing a parameter passed to a function
1 #!/bin/bash
2 # ref-params.sh: Dereferencing a parameter passed to a function.
3 # (Complex Example)
4
5 ITERATIONS=3 # How many times to get input.
6 icount=1
7
8 my_read () {
9 # Called with my_read varname,
10 #+ outputs the previous value between brackets as the default value,
11 #+ then asks for a new value.
12
13 local local_var
14
15 echo -n "Enter a value "
16 eval 'echo -n "[$'$1'] "' # Previous value.
17 # eval echo -n "[\$$1] " # Easier to understand,
18 #+ but loses trailing space in user prompt.
19 read local_var
20 [ -n "$local_var" ] && eval $1=\$local_var
21
22 # "And-list": if "local_var" then set "$1" to its value.
23 }
24
25 echo
26
27 while [ "$icount" -le "$ITERATIONS" ]
28 do
29 my_read var
30 echo "Entry #$icount = $var"
31 let "icount += 1"
32 echo
33 done
34
35
36 # Thanks to Stephane Chazelas for providing this instructive example.
37
38 exit 0 |
- 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 [1] 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 23-7. Maximum of two numbers
1 #!/bin/bash 2 # max.sh: Maximum of two integers. 3 4 E_PARAM_ERR=-198 # If less than 2 params passed to function. 5 EQUAL=-199 # Return value if both params equal. 6 7 max2 () # Returns larger of two numbers. 8 { # Note: numbers compared must be less than 257. 9 if [ -z "$2" ] 10 then 11 return $E_PARAM_ERR 12 fi 13 14 if [ "$1" -eq "$2" ] 15 then 16 return $EQUAL 17 else 18 if [ "$1" -gt "$2" ] 19 then 20 return $1 21 else 22 return $2 23 fi 24 fi 25 } 26 27 max2 33 34 28 return_val=$? 29 30 if [ "$return_val" -eq $E_PARAM_ERR ] 31 then 32 echo "Need to pass two parameters to the function." 33 elif [ "$return_val" -eq $EQUAL ] 34 then 35 echo "The two numbers are equal." 36 else 37 echo "The larger of the two numbers is $return_val." 38 fi 39 40 41 exit 0 42 43 # Exercise (easy): 44 # --------------- 45 # Convert this to an interactive script, 46 #+ that is, have the script ask for input (two numbers).
For a function to return a string or array, use a dedicated variable.
1 count_lines_in_etc_passwd() 2 { 3 [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd)) 4 # If /etc/passwd is readable, set REPLY to line count. 5 # Returns both a parameter value and status information. 6 # The 'echo' seems unnecessary, but . . . 7 #+ it removes excess whitespace from the output. 8 } 9 10 if count_lines_in_etc_passwd 11 then 12 echo "There are $REPLY lines in /etc/passwd." 13 else 14 echo "Cannot count lines in /etc/passwd." 15 fi 16 17 # Thanks, S.C.Example 23-8. 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 is left as an exercise. 8 9 # Usage: roman number-to-convert 10 11 LIMIT=200 12 E_ARG_ERR=65 13 E_OUT_OF_RANGE=66 14 15 if [ -z "$1" ] 16 then 17 echo "Usage: `basename $0` number-to-convert" 18 exit $E_ARG_ERR 19 fi 20 21 num=$1 22 if [ "$num" -gt $LIMIT ] 23 then 24 echo "Out of range!" 25 exit $E_OUT_OF_RANGE 26 fi 27 28 to_roman () # Must declare function before first call to it. 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 # Exercise: 43 # -------- 44 # Explain how this function works. 45 # Hint: division by successive subtraction. 46 } 47 48 49 to_roman $num 100 C 50 num=$? 51 to_roman $num 90 LXXXX 52 num=$? 53 to_roman $num 50 L 54 num=$? 55 to_roman $num 40 XL 56 num=$? 57 to_roman $num 10 X 58 num=$? 59 to_roman $num 9 IX 60 num=$? 61 to_roman $num 5 V 62 num=$? 63 to_roman $num 4 IV 64 num=$? 65 to_roman $num 1 I 66 67 echo 68 69 exit 0See also Example 10-28.
The largest positive integer a function can return is 255. The return command is closely tied to the concept of exit status, which accounts for this particular limitation. Fortunately, there are various workarounds for those situations requiring a large integer return value from a function.
Example 23-9. Testing large return values in a function
1 #!/bin/bash 2 # return-test.sh 3 4 # The largest positive value a function can return is 255. 5 6 return_test () # Returns whatever passed to it. 7 { 8 return $1 9 } 10 11 return_test 27 # o.k. 12 echo $? # Returns 27. 13 14 return_test 255 # Still o.k. 15 echo $? # Returns 255. 16 17 return_test 257 # Error! 18 echo $? # Returns 1 (return code for miscellaneous error). 19 20 # ====================================================== 21 return_test -151896 # Do large negative numbers work? 22 echo $? # Will this return -151896? 23 # No! It returns 168. 24 # Version of Bash before 2.05b permitted 25 #+ large negative integer return values. 26 # Newer versions of Bash plug this loophole. 27 # This may break older scripts. 28 # Caution! 29 # ====================================================== 30 31 exit 0A workaround for obtaining large integer "return values" is to simply assign the "return value" to a global variable.
1 Return_Val= # Global variable to hold oversize return value of function. 2 3 alt_return_test () 4 { 5 fvar=$1 6 Return_Val=$fvar 7 return # Returns 0 (success). 8 } 9 10 alt_return_test 1 11 echo $? # 0 12 echo "return value = $Return_Val" # 1 13 14 alt_return_test 256 15 echo "return value = $Return_Val" # 256 16 17 alt_return_test 257 18 echo "return value = $Return_Val" # 257 19 20 alt_return_test 25701 21 echo "return value = $Return_Val" #25701An even more elegant method is to simply have the function echo its "return value to stdout," and to then capture it by command substitution. See the discussion of this in Section 34.7.
Example 23-10. Comparing two large integers
1 #!/bin/bash 2 # max2.sh: Maximum of two LARGE integers. 3 4 # This is the previous "max.sh" example, 5 #+ modified to permit comparing large integers. 6 7 EQUAL=0 # Return value if both params equal. 8 E_PARAM_ERR=-99999 # Not enough params passed to function. 9 10 max2 () # "Returns" larger of two numbers. 11 { 12 if [ -z "$2" ] 13 then 14 echo $E_PARAM_ERR 15 return 16 fi 17 18 if [ "$1" -eq "$2" ] 19 then 20 echo $EQUAL 21 return 22 else 23 if [ "$1" -gt "$2" ] 24 then 25 retval=$1 26 else 27 retval=$2 28 fi 29 fi 30 31 echo $retval # Echoes (to stdout), rather than returning value. 32 33 } 34 35 36 return_val=$(max2 33001 33997) 37 # This is actually a form of command substitution: 38 #+ treating a function as if it were a command, 39 #+ and assigning the stdout of the function to the variable 'return_val.' 40 41 42 # ========================= OUTPUT ======================== 43 if [ "$return_val" -eq "$E_PARAM_ERR" ] 44 then 45 echo "Error in parameters passed to comparison function!" 46 elif [ "$return_val" -eq "$EQUAL" ] 47 then 48 echo "The two numbers are equal." 49 else 50 echo "The larger of the two numbers is $return_val." 51 fi 52 # ========================================================= 53 54 exit 0 55 56 # Exercises: 57 # --------- 58 # 1) Find a more elegant way of testing 59 #+ the parameters passed to the function. 60 # 2) Simplify the if/then structure at "OUTPUT." 61 # 3) Rewrite the script to take input from command-line parameters.See also Example A-7.
Exercise: Using what we have just learned, extend the previous Roman numerals example to accept arbitrarily large input.
- Redirecting the stdin of a function
A function is essentially a code block, which means its stdin can be redirected (as in Example 3-1).
Example 23-11. Real name from username
1 #!/bin/bash 2 # realname.sh 3 # 4 # From username, gets "real name" from /etc/passwd. 5 6 7 ARGCOUNT=1 # Expect one arg. 8 E_WRONGARGS=65 9 10 file=/etc/passwd 11 pattern=$1 12 13 if [ $# -ne "$ARGCOUNT" ] 14 then 15 echo "Usage: `basename $0` USERNAME" 16 exit $E_WRONGARGS 17 fi 18 19 file_excerpt () # Scan file for pattern, then print relevant portion of line. 20 { 21 while read line # "while" does not necessarily need "[ condition ]" 22 do 23 echo "$line" | grep $1 | awk -F":" '{ print $5 }' # Have awk use ":" delimiter. 24 done 25 } <$file # Redirect into function's stdin. 26 27 file_excerpt $pattern 28 29 # Yes, this entire script could be reduced to 30 # grep PATTERN /etc/passwd | awk -F":" '{ print $5 }' 31 # or 32 # awk -F: '/PATTERN/ {print $5}' 33 # or 34 # awk -F: '($1 == "username") { print $5 }' # real name from username 35 # However, it might not be as instructive. 36 37 exit 0There is an alternative, and perhaps less confusing method of redirecting a function's stdin. This involves redirecting the stdin to an embedded bracketed code block within the function.
1 # Instead of: 2 Function () 3 { 4 ... 5 } < file 6 7 # Try this: 8 Function () 9 { 10 { 11 ... 12 } < file 13 } 14 15 # Similarly, 16 17 Function () # This works. 18 { 19 { 20 echo $* 21 } | tr a b 22 } 23 24 Function () # This doesn't work. 25 { 26 echo $* 27 } | tr a b # A nested code block is mandatory here. 28 29 30 # Thanks, S.C.