Complex Functions and Function Complexities

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

then

echo "-Parameter #1 is zero length.-" # Also if no parameter is passed. else

echo "-Param #1 is \"$1\".-"

fi

if [ "$2" ] then

echo "-Parameter #2 is \"$2\".-"

fi

return 0

echo "Null parameter passed."

func2 "$uninitialized_param" # Called with uninitialized param echo

echo "One parameter passed."

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echo "Two parameters passed."

The shift command works on arguments passed to functions (see Example 34-6).

In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. [1] 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 [2] 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.

return $E_PARAM_ERR

fi

if [ "$1" -eq "$2" ] then

return $EQUAL else

if [ "$1" -gt "$2" ] then

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Complex Functions and Function Complexities

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.

}

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 23-4. Converting numbers to Roman numerals

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Complex Functions and Function Complexities

#!/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

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 $num 40 XL num=$?

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Complex Functions and Function Complexities

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 0

See also Example 10-27.

The largest positive integer a function can return is 256. 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 23-5. Testing large return values in a function

#!/bin/bash

#return-test.sh

#The largest positive value a function can return is 256.

exit 0

As we have seen, a function can return a large negative value. This also permits returning large positive integer, using a bit of trickery.

An alternate method of accomplishing this is to simply assign the "return value" to a global variable.

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Example 23-6. 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.

return $E_PARAM_ERR

fi

if [ "$1" -eq "$2" ] then

return $EQUAL else

if [ "$1" -gt "$2" ] then

retval=$1 else

retval=$2

fi

fi

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Complex Functions and Function Complexities

#This is a workaround to enable returning a large integer

#from this function.

echo "Error: Too few parameters." 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

See also Example A-8.

Exercise: Using what we have just learned, extend the previous Roman numerals example to accept arbitrarily large input.

Redirection

Redirecting the stdin of a function

A function is essentially a code block, which means its stdin can be redirected (as in Example 4-1).

Example 23-7. Real name from username

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Complex Functions and Function Complexities

#!/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

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 0

There 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,

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Complex Functions and Function Complexities

# Thanks, S.C.

Notes

[1]Indirect variable references (see Example 35-2) provide a clumsy sort of mechanism for passing variable pointers to functions.

#!/bin/bash

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.

# "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.

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Complex Functions and Function Complexities

exit 0

[2]The return command is a Bash builtin.

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