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11.12. Functions

This section introduces functions so that you can use them interactively or store them in your initialization files. Later, when discussing scripts, functions will be covered in more depth. Functions can be used when an alias is not enough, that is, for passing arguments. Functions are often defined in the user's initialization file, .profile. They are like mini-scripts, but unlike scripts, functions run in the current environment; that is, the shell does not fork a child process to execute a function. All variables are shared with the shell that invoked the function. Often functions are used to improve the modularity of a script. Once defined, they can be used repeatedly and even stored in another directory.

Functions must be defined before they are invoked; there are two formats used to define them. One format came from the Bourne shell and the other is new with the Korn shell. Functions can be exported from one invocation of the shell to the next. The typeset function and unset command can be used to list and unset functions. See Table 11.17.

Table 11.17. Commands to List and Set Functions

Command

Function

typeset –f

Lists functions and their definitions; functions is an alias for typeset –f

typeset +f

Lists only function names

unset –f name

Unsets a function


11.12.1 Defining Functions

There are two acceptable formats for defining functions: the Bourne shell format (still allowed for upward compatibility) and the new Korn shell format. A function must be defined before it can be used.[5]

[5] The POSIX standard defines functions with the Bourne shell syntax, but variables and traps cannot be local in scope, as with the new Korn shell definition.

FORMAT


(Bourne Shell)

functionname() { commands ; commands; }



(Korn Shell)

function functionname { commands; commands; }

Example 11.59. 

1   $ function fun { pwd; ls; date; }



2   $ fun

    /home/jody/ellie/prac

    abc      abc123   file1.bak  none      nothing  tmp

    abc1     abc2     file2      nonsense  nowhere  touch

    abc122   file1    file2.bak  noone     one

    Mon Feb 9 11:15:48 PST 2004



3   $ function greet { print "Hi $1 and $2"; }



4   $ greet tom joe             # Here $1 is tom and $2 is joe

    Hi tom and joe



5   $ set jane nina lizzy

6   $ print $*

    jane nina lizzy



7   $ greet tom joe

    Hi tom and joe



8   $ print $1 $2

    jane nina

EXPLANATION

  1. The function fun is named and defined. The name is followed by a list of commands enclosed in curly braces. Each command is separated by a semicolon. There must be a space after the first curly brace or you will get a syntax error such as ksh: syntax error: `}' unexpected. A function must be defined before it can be used.

  2. The function behaves like a script or an alias when invoked. Each of the commands in the function definition is executed in turn.

  3. There are two positional parameters used in the function greet. When arguments are given to the function, the positional parameters are assigned those values.

  4. The arguments to the function tom and joe are assigned to $1 and $2, respectively. The positional parameters in a function are private to the function and will not interfere with any used outside the function.

  5. The positional parameters are set at the command line. These variables have nothing to do with the ones set in the function.

  6. $* displays the values of the currently set positional parameters.

  7. The function greet is called. The values assigned to the positional parameters $1 and $2 are tom and joe, respectively.

  8. The positional variables assigned at the command line are unaffected by those set in the function.

11.12.2 Functions and Aliases

When processing the command line, the shell looks for aliases before special built-in commands and for special built-ins before functions. If a function has the same name as a built-in, the built-in will take priority over the function. An alias for a special built-in can be defined, and then the function name can be given the name of the alias to override the order of processing.

Example 11.60. 

(The ENV File)

1   alias cd=_cd

2   function _cd {

3   \cd $1

4   print $(basename $PWD)

5   }



(The Command Line)

$ cd /

/

$ cd $HOME/bin

bin

$ cd ..

ellie

EXPLANATION

  1. The alias for cd is assigned _cd.

  2. The function _cd is defined. The opening curly brace marks the start of the function definition.

  3. If an alias is preceded by a backslash, alias substitution is not performed. The backslash precedes cd to execute the built-in cd command, not the alias. Without the backslash, the function would be recursive and the shell would display an error message: cd_: recursion too deep. $1 is the argument (name of a directory) passed to cd.

  4. The name of the directory (not the full pathname) is printed.

  5. The closing curly brace marks the end of the function definition.

11.12.3 Listing Functions

To list functions and their definitions, use the typeset command.

Example 11.61. 

(The Command Line)

1   $ typeset –f

    function fun

    {

    pwd; ls; date; }

    function greet

    {

    print "hi $1 and $2"; }

2   $ typeset +f

    fun

    greet

EXPLANATION

  1. The typeset command, with the –f option, lists the function and its definition.

  2. The typeset command, with the +f option, lists only the names of defined functions.

11.12.4 Unsetting Functions

When a function is unset, it will be removed from the shell's memory.

Example 11.62. 

(The Command Line)

1   $ typeset –f

    function fun

    {

        pwd; ls; date; }

    function greet

    {

    print "hi $1 and $2"; }



2   $ unset -f fun

3   $ typeset –f

    function greet

    {

    print "hi $1 and $2"; }

EXPLANATION

  1. The typeset –f command displays the function and its definition. Two functions, fun and greet, are displayed.

  2. The built-in command unset, with the –f option, undefines the fun function, removing it from the shell's memory.

  3. The fun function is no longer shown as one of the functions defined when the typeset –f command is executed.

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