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.
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
#!/bin/bash
# Functions and parameters
DEFAULT=default # Default param value.
func2 () {
if [ -z "$1" ] # Is parameter #1 zero length?
then
echo "-Parameter #1 is zero length.-" # Or no parameter passed.
else
echo "-Param #1 is \"$1\".-"
fi
variable=${1-$DEFAULT} # What does
echo "variable = $variable" #+ parameter substitution show?
# ---------------------------
# It distinguishes between
#+ no param and a null param.
if [ "$2" ]
then
echo "-Parameter #2 is \"$2\".-"
fi
return 0
}
echo
echo "Nothing passed."
func2 # Called with no params
echo
echo "Zero-length parameter passed."
func2 "" # Called with zero-length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param" # Called with uninitialized param
echo
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 34-12). |
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 23-3. Passing an indirect reference to a function
#!/bin/bash
# ind-func.sh: Passing an indirect reference to a function.
echo_var ()
{
echo "$1"
}
message=Hello
Hello=Goodbye
echo_var "$message" # Hello
# Now, let's pass an indirect reference to the function.
echo_var "${!message}" # Goodbye
echo "-------------"
# What happens if we change the contents of "hello" variable?
Hello="Hello, again!"
echo_var "$message" # Hello
echo_var "${!message}" # Hello, again!
exit 0 |
The next logical question is whether parameters can be dereferenced after being passed to a function.
Example 23-4. Dereferencing a parameter passed to a function
#!/bin/bash
# dereference.sh
# Dereferencing parameter passed to a function.
# Script by Bruce W. Clare.
dereference ()
{
y=\$"$1" # Name of variable.
echo $y # $Junk
x=`eval "expr \"$y\" "`
echo $1=$x
eval "$1=\"Some Different Text \"" # Assign new value.
}
Junk="Some Text"
echo $Junk "before" # Some Text before
dereference Junk
echo $Junk "after" # Some Different Text after
exit 0 |
Example 23-5. Again, dereferencing a parameter passed to a function
#!/bin/bash
# ref-params.sh: Dereferencing a parameter passed to a function.
# (Complex Example)
ITERATIONS=3 # How many times to get input.
icount=1
my_read () {
# Called with my_read varname,
#+ outputs the previous value between brackets as the default value,
#+ then asks for a new value.
local local_var
echo -n "Enter a value "
eval 'echo -n "[$'$1'] "' # Previous value.
# eval echo -n "[\$$1] " # Easier to understand,
#+ but loses trailing space in user prompt.
read local_var
[ -n "$local_var" ] && eval $1=\$local_var
# "And-list": if "local_var" then set "$1" to its value.
}
echo
while [ "$icount" -le "$ITERATIONS" ]
do
my_read var
echo "Entry #$icount = $var"
let "icount += 1"
echo
done
# Thanks to Stephane Chazelas for providing this instructive example.
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 23-6. Maximum of two numbers
#!/bin/bash # max.sh: Maximum of two integers. E_PARAM_ERR=-198 # If less than 2 params passed to function. EQUAL=-199 # 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 fi if [ "$1" -eq "$2" ] then return $EQUAL else if [ "$1" -gt "$2" ] then return $1 else return $2 fi fi } max2 33 34 return_val=$? if [ "$return_val" -eq $E_PARAM_ERR ] then echo "Need to pass two parameters to the function." elif [ "$return_val" -eq $EQUAL ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi exit 0 # Exercise (easy): # --------------- # Convert this to an interactive script, #+ that is, have the script ask for input (two numbers).
For a function to return a string or array, use a dedicated variable.
count_lines_in_etc_passwd() { [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd)) # If /etc/passwd is readable, set REPLY to line count. # Returns both a parameter value and status information. # The 'echo' seems unnecessary, but . . . #+ it removes excess whitespace from the output. } if count_lines_in_etc_passwd then echo "There are $REPLY lines in /etc/passwd." else echo "Cannot count lines in /etc/passwd." fi # Thanks, S.C.Example 23-7. Converting numbers to Roman numerals
#!/bin/bash # Arabic number to Roman numeral conversion # Range: 0 - 200 # It's crude, but it works. # Extending the range and otherwise improving the script is left as an exercise. # Usage: roman number-to-convert LIMIT=200 E_ARG_ERR=65 E_OUT_OF_RANGE=66 if [ -z "$1" ] then echo "Usage: `basename $0` number-to-convert" exit $E_ARG_ERR fi num=$1 if [ "$num" -gt $LIMIT ] then echo "Out of range!" exit $E_OUT_OF_RANGE fi to_roman () # Must declare function before first call to it. { number=$1 factor=$2 rchar=$3 let "remainder = number - factor" while [ "$remainder" -ge 0 ] do echo -n $rchar let "number -= factor" let "remainder = number - factor" done return $number # Exercise: # -------- # Explain how this function works. # Hint: division by successive subtraction. } to_roman $num 100 C num=$? to_roman $num 90 LXXXX num=$? to_roman $num 50 L num=$? to_roman $num 40 XL num=$? to_roman $num 10 X num=$? to_roman $num 9 IX num=$? to_roman $num 5 V num=$? to_roman $num 4 IV num=$? to_roman $num 1 I echo 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 23-8. Testing large return values in a function
#!/bin/bash # return-test.sh # The largest positive value a function can return is 255. return_test () # Returns whatever passed to it. { return $1 } return_test 27 # o.k. echo $? # Returns 27. return_test 255 # Still o.k. echo $? # Returns 255. return_test 257 # Error! echo $? # Returns 1 (return code for miscellaneous error). # ====================================================== return_test -151896 # Do large negative numbers work? echo $? # Will this return -151896? # No! It returns 168. # Version of Bash before 2.05b permitted #+ large negative integer return values. # Newer versions of Bash plug this loophole. # This may break older scripts. # Caution! # ====================================================== exit 0A workaround for obtaining large integer "return values" is to simply assign the "return value" to a global variable.
Return_Val= # Global variable to hold oversize return value of function. alt_return_test () { fvar=$1 Return_Val=$fvar return # Returns 0 (success). } alt_return_test 1 echo $? # 0 echo "return value = $Return_Val" # 1 alt_return_test 256 echo "return value = $Return_Val" # 256 alt_return_test 257 echo "return value = $Return_Val" # 257 alt_return_test 25701 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 23-9. Comparing two large integers
#!/bin/bash # max2.sh: Maximum of two LARGE integers. # This is the previous "max.sh" example, #+ modified to permit comparing large integers. EQUAL=0 # Return value if both params equal. E_PARAM_ERR=-99999 # Not enough params passed to function. max2 () # "Returns" larger of two numbers. { if [ -z "$2" ] then echo $E_PARAM_ERR return fi if [ "$1" -eq "$2" ] then echo $EQUAL return else if [ "$1" -gt "$2" ] then retval=$1 else retval=$2 fi fi echo $retval # Echoes (to stdout), rather than returning value. } return_val=$(max2 33001 33997) # This is actually a form of command substitution: #+ treating a function as if it were a command, #+ and assigning the stdout of the function to the variable 'return_val.' # ========================= OUTPUT ======================== if [ "$return_val" -eq "$E_PARAM_ERR" ] then echo "Error in parameters passed to comparison function!" elif [ "$return_val" -eq "$EQUAL" ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi # ========================================================= exit 0 # Exercises: # --------- # 1) Find a more elegant way of testing #+ the parameters passed to the function. # 2) Simplify the if/then structure at "OUTPUT." # 3) Rewrite the script to take input from command-line parameters.See also Example A-8.
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 23-10. Real name from username
#!/bin/bash # From username, gets "real name" from /etc/passwd. ARGCOUNT=1 # Expect one arg. E_WRONGARGS=65 file=/etc/passwd pattern=$1 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` USERNAME" exit $E_WRONGARGS fi file_excerpt () # Scan file for pattern, the print relevant portion of line. { while read line # while does not necessarily need "[ condition]" do echo "$line" | grep $1 | awk -F":" '{ print $5 }' # Have awk use ":" delimiter. done } <$file # Redirect into function's stdin. file_excerpt $pattern # Yes, this entire script could be reduced to # grep PATTERN /etc/passwd | awk -F":" '{ print $5 }' # or # awk -F: '/PATTERN/ {print $5}' # or # awk -F: '($1 == "username") { print $5 }' # real name from username # However, it might not be as instructive. 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.
# Instead of: Function () { ... } < file # Try this: Function () { { ... } < file } # Similarly, Function () # This works. { { echo $* } | tr a b } Function () # This doesn't work. { echo $* } | tr a b # A nested code block is mandatory here. # Thanks, S.C.