Master Shell Scripting: From Basics to Advanced Control Structures

Java developers often work on Linux servers, packaging projects as JAR or WAR files, uploading them via XFTP, and starting them with a service script; now they can learn how to write their own shell scripts.

Shell Script

A shell script is a program file that contains a series of commands interpreted by a command‑line interpreter (the shell). When commands are placed in a file instead of typed interactively, the file becomes a shell script.

Shell scripts can include assignments, calculations, loops, and other operations.

1. View the current default shell

<code>echo $SHELL</code>

Output:

/bin/bash

2. View all shells supported by the system

<code>cat /etc/shells</code>

Output:

/bin/sh /bin/bash /usr/bin/sh /usr/bin/bash

How to Write a Shell Script

Create a directory and a script file, then edit it:

<code>mkdir /usr/local/shelltest</code>

<code>touch test.sh</code>

<code>vim test.sh</code>

Example script (saved as

test.sh

)

<code>#!/bin/bash

echo "Hello World Shell"
</code>

Run the script:

<code>bash test.sh</code>

Result:

Hello World Shell

The line

#!/bin/bash

(shebang) tells the system to use

/bin/bash

as the interpreter.

To execute a script directly you must grant execute permission:

<code>chmod +x test.sh</code>

Then run it with

./test.sh

.

Shell Script Variables

Define a variable:

<code>name=zhiyikeji</code>

Use the variable with

$

:

<code>echo $name</code>

<code>echo ${name}</code>

Both produce

zhiyikeji

. Braces are optional but help avoid ambiguity.

Delete a variable:

<code>unset name</code>

After unsetting,

echo $name

outputs nothing.

Make a variable read‑only:

<code>readonly name</code>

Attempting to reassign a read‑only variable results in an error.

Shell Script Flow Control

If

<code>#!/bin/bash
if [ $1 -gt 2 ]; then
    echo "Value greater than 2"
else
    echo "Value less than or equal to 2"
    exit
fi
</code>

Run with

sh test2.sh 1

→

Value less than or equal to 2

;

sh test2.sh 3

→

Value greater than 2

.

Case

<code>case $1 in
    start)
        echo "start already begun"
        ;;
    stop)
        echo "stop command executed"
        ;;
esac
exit
</code>

Running

sh service.sh start

prints

start already begun

;

sh service.sh stop

prints

stop command executed

.

For Loop

<code>j=$1
for ((i=1;i<=j;i++))
do
    echo $i
done
</code>

Example:

sh fortest.sh 6

outputs numbers 1 through 6.

While Loop

<code>a=1
b=1
while ((a <= 9))
do
    while ((b <= a))
    do
        let "c=a*b"
        echo -n "$a*$b=$c "
        let b++
    done
    let a++
    let b=1
    echo ""
 done
</code>

Produces the classic 9×9 multiplication table.

Shell scripting is a practical skill for automating tasks on Linux servers; mastering these basics enables you to write simple automation scripts and lay the groundwork for more complex DevOps workflows.