5. Shell functions

Functions can be declared in one of these two forms, which are equivalent:

function name {
    commands
    return
}

name () {
    commands
    return
}

A simple example of a function is in the script fun.sh:

fun.sh

#!/bin/bash

# Shell function demo

function step2() {
    echo "Step 2"
    return
}

# Main program starts here

echo "Step 1"
step2
echo "Step 3"

vim fun.sh

:q!

./fun.sh

Inside a function we can use local variables:

local-vars.sh

#!/bin/bash

# Script to demonstrate local variables

foo=0    # global variable foo

funct_1 () {
    local foo    # variable foo local to func_1

    foo=1
    echo "funct_1: foo = $foo"
}

funct_2 () {
    local foo    # variable foo local to func_2

    foo=2
    echo "funct_2: foo = $foo"
}


echo "global:  foo = $foo"
funct_1
echo "global:  foo = $foo"
funct_2
echo "global:  foo = $foo"

vim local-vars.sh

:q!

./local-vars.sh

Let's display some additional info on the report page, using functions. We would like to display info about:

System uptime and load.
Disk space.
Home space.

Let's define a function for each of these:

vim sys_info.sh

/cat

Press capital O and input:

report_uptime () {
    return
}

report_disk_space () {
    return
}

report_home_space () {
    return
}

Press ESC and :w

Let's call these functions inside the html body:

/TIMESTAMP

Press lowercase o and type:

$(report_uptime)
$(report_disk_space)
$(report_home_space)

Press ESC and :wq

The script should look like this:

!/bin/bash

# Program to output a system information page.

declare -r TITLE="System Information Report for $HOSTNAME"
CURRENT_TIME=$(date +"%x %r %Z")
TIMESTAMP="Generated on $CURRENT_TIME, by $USER"

report_uptime() {
    return
}

report_disk_space() {
    return
}

report_home_space() {
    return
}

cat << _EOF_
<html>
    <head>
        <title>$TITLE</title>
    </head>
    <body>
        <h1>$TITLE</h1>
        <p>$TIMESTAMP</p>
$(report_uptime)
$(report_disk_space)
$(report_home_space)
    </body>
</html>
_EOF_

./sys_info.sh

The output should look like this:

<html>
    <head>
        <title>System Information Report for linuxmint</title>
    </head>
    <body>
        <h1>System Information Report for linuxmint</h1>
        <p>Generated on 09/28/23 03:01:28 PM UTC, by dashamir</p>



    </body>
</html>

We see that each function is replaced by an empty line and we don't know what is going on. Let's display some feedback from each function:

vim sys_info.sh

/^report_uptime

Press o and insert this line:

echo "Function report_uptime executed."

Press ESC and then search:

/^report_disk_space

Press o and insert this line:

echo "Function report_disk_space executed."

Press ESC and then search:

/^report_home_space

Press o and insert this line:

echo "Function report_home_space executed."

Press ESC and :wq

The functions should look like this:

report_uptime() {
    echo "Function report_uptime executed."
    return
}

report_disk_space() {
    echo "Function report_disk_space executed."
    return
}

report_home_space() {
    echo "Function report_home_space executed."
    return
}

./sys_info.sh

The output should look like this:

<html>
    <head>
        <title>System Information Report for linuxmint</title>
    </head>
    <body>
        <h1>System Information Report for linuxmint</h1>
        <p>Generated on 09/28/23 03:13:29 PM UTC, by dashamir</p>
Function report_uptime executed.
Function report_disk_space executed.
Function report_home_space executed.
    </body>
</html>

Now let's provide the real data:

uptime

df -h .

du -hs $HOME

vim sys_info.sh

/^report_uptime

Type: j dd O.

cat <<- _EOF_
    <h2>System Uptime</h2>
    <pre>$(uptime)</pre>
    _EOF_

attenzione

Use TAB to indent the lines below cat.

Press ESC.

/^report_disk_space

Type: j dd O.

cat <<- _EOF_
    <h2>Disk Space Utilization</h2>
    <pre>$(df -h .)</pre>
    _EOF_

Press ESC.

/^report_home_space

Type: j dd O.

cat <<- _EOF_
    <h2>Home Space Utilization</h2>
    <pre>$(du -hs $HOME)</pre>
    _EOF_

Press ESC and :wq

The functions should look like this:

report_uptime() {
    cat <<- _EOF_
        <h2>System Uptime</h2>
        <pre>$(uptime)</pre>
        _EOF_
    return
}

report_disk_space() {
    cat <<- _EOF_
        <h2>Disk Space Utilization</h2>
        <pre>$(df -h .)</pre>
        _EOF_
    return
}

report_home_space() {
    cat <<- _EOF_
        <h2>Home Space Utilization</h2>
        <pre>$(du -hs $HOME)</pre>
        _EOF_
    return
}

./sys_info.sh

Check it in browser:

./sys_info.sh > sys_info.html

lynx sys_info.html

Quit with qy.

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