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Chapter 11. Functions

In this chapter, we will discuss

  • What functions are

  • Creation and displaying of functions from the command line

  • Functions in scripts

  • Passing arguments to functions

  • When to use functions

11.1. Introduction

11.1.1. What are functions?

Shell functions are a way to group commands for later execution, using a single name for this group, or routine. The name of the routine must be unique within the shell or script. All the commands that make up a function are executed like regular commands. When calling on a function as a simple command name, the list of commands associated with that function name is executed. A function is executed within the shell in which it has been declared: no new process is created to interpret the commands.

Special built-in commands are found before shell functions during command lookup. The special built-ins are: break, :, ., continue, eval, exec, exit, export, readonly, return, set, shift, trap and unset.

11.1.2. Function syntax

Functions either use the syntax

function FUNCTION { COMMANDS; }

or

FUNCTION () { COMMANDS; }

Both define a shell function FUNCTION. The use of the built-in command function is optional; however, if it is not used, parentheses are needed.

The commands listed between curly braces make up the body of the function. These commands are executed whenever FUNCTION is specified as the name of a command. The exit status is the exit status of the last command executed in the body.

NoteCommon mistakes
 

The curly braces must be separated from the body by spaces, otherwise they are interpreted in the wrong way.

The body of a function should end in a semicolon or a newline.

11.1.3. Positional parameters in functions

Functions are like mini-scripts: they can accept parameters, they can use variables only known within the function (using the local shell built-in) and they can return values to the calling shell.

A function also has a system for interpreting positional parameters. However, the positional parameters passed to a function are not the same as the ones passed to a command or script.

When a function is executed, the arguments to the function become the positional parameters during its execution. The special parameter # that expands to the number of positional parameters is updated to reflect the change. Positional parameter 0 is unchanged. The Bash variable FUNCNAME is set to the name of the function, while it is executing.

If the return built-in is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter # are restored to the values they had prior to the function's execution. If a numeric argument is given to return, that status is returned.

11.1.4. Displaying functions

All functions known by the current shell can be displayed using the set built-in without options. Functions are retained after they are used, unless they are unset after use. The which command also displays functions:


[lydia@cointreau ~] which zless
zless is a function
zless ()
{
    zcat "$@" | "$PAGER"
}

[lydia@cointreau ~] echo $PAGER
less

This is the sort of function that is typically configured in the user's shell resource configuration files. Functions are more flexible than aliases and provide a simple and easy way of adapting the user environment.

Here's one for DOS users:


dir ()
{
    ls -F --color=auto -lF --color=always "$@" | less -r
}

11.1.5. Examples of functions in scripts

11.1.5.1. Recycling

There are plenty of scripts on your system that use functions as a structured way of handling series of commands. On some Linux systems, for instance, you will find the /etc/rc.d/init.d/functions definition file, which is sourced in all init scripts. Using this method, common tasks such as checking if a process runs, starting or stopping a daemon and so on, only have to be written once, in a general way. If the same task is needed again, the code is recycled. From this functions file the checkpid function:


# Check if $pid (could be plural) are running
checkpid() {
        local i

        for i in $* ; do
                [ -d "/proc/$i" ] && return 0
        done
        return 1
}

This function is reused in the same script in other functions, which are reused in other scripts. The daemon function, for instance, is used in the majority of the startup scripts for starting a server process (on machines that use this system).

11.1.5.2. Setting the path

This section might be found in your /etc/profile file. The function pathmunge is defined and then used to set the path for the root and other users:


pathmunge () {
        if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then
           if [ "$2" = "after" ] ; then
              PATH=$PATH:$1
           else
              PATH=$1:$PATH
           fi
        fi
}

# Path manipulation
if [ `id -u` = 0 ]; then
        pathmunge /sbin
        pathmunge /usr/sbin
        pathmunge /usr/local/sbin
fi

pathmunge /usr/X11R6/bin after

unset pathmunge

The function takes its first argument to be a path name. If this path name is not yet in the current path, it is added. The second argument to the function defines if the path will be added in front or after the current PATH definition.

Normal users only get /usr/X11R6/bin added to their paths, while root gets a couple of extra directories containing system commands. After being used, the function is unset so that it is not retained.

11.1.5.3. Remote backups

The following example uses SSH keys for enabling the remote connection. It uses two functions, buplinux and bupbash, that each make a .tar file, which is then compressed and sent to a remote server. After that, the local copy is cleaned up.

On Sunday, only bupbash is executed.


#/bin/bash

LOGFILE="/nethome/lydia/log/backupscript.log"
echo "Starting backups for `date`" >> "$LOGFILE"

buplinux()
{
DIR="/nethome/lydia/xml/db/linux-basics/"
TAR="Linux.tar"
BZIP="$TAR.bz2"
SERVER="rincewind"
RDIR="/var/www/intra/lydia/html/training/"

cd "$DIR"
tar cf "$TAR" src/*.xml src/images/*.png src/images/*.eps
echo "Compressing $TAR..." >> "$LOGFILE"
bzip2 "$TAR"
echo "...done." >> "$LOGFILE"
echo "Copying to $SERVER..." >> "$LOGFILE"
scp "$BZIP" "$SERVER:$RDIR" > /dev/null 2>&1
echo "...done." >> "$LOGFILE"
echo -e "Done backing up Linux course:\nSource files, PNG and EPS images.\nRubbish removed." >> "$LOGFILE"
rm "$BZIP"
}

bupbash()
{
DIR="/nethome/lydia/xml/db/"
TAR="Bash.tar"
BZIP="$TAR.bz2"
FILES="bash-programming/"
SERVER="rincewind"
RDIR="/var/www/intra/lydia/html/training/"

cd "$DIR"
tar cf "$TAR" "$FILES"
echo "Compressing $TAR..." >> "$LOGFILE"
bzip2 "$TAR"
echo "...done." >> "$LOGFILE"
echo "Copying to $SERVER..." >> "$LOGFILE"
scp "$BZIP" "$SERVER:$RDIR" > /dev/null 2>&1
echo "...done." >> "$LOGFILE"

echo -e "Done backing up Bash course:\n$FILES\nRubbish removed." >> "$LOGFILE"
rm "$BZIP"
}

DAY=`date +%w`

if [ "$DAY" -lt "2" ]; then
  echo "It is `date +%A`, only backing up Bash course." >> "$LOGFILE"
  bupbash
else
  buplinux
  bupbash
fi


echo -e "Remote backup `date` SUCCESS\n----------" >> "$LOGFILE"

This script runs from cron, meaning without user interaction, so we redirect standard error from the scp command to /dev/null.

It might be argued that all the separate steps can be combined in a command such as

tar c dir_to_backup/ | bzip2 | ssh server "cat > backup.tar.bz2"

However, if you are interested in intermediate results, which might be recovered upon failure of the script, this is not what you want.

The expression

command &> file

is equivalent to

command > file 2>&1


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