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 echo $SHELL Output: /bin/bash 2. View all shells supported by the system cat /etc/shells 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:

mkdir /usr/local/shelltest

touch test.sh

vim test.sh

Example script (saved as test.sh)

#!/bin/bash

echo "Hello World Shell"

Run the script: bash test.sh 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: chmod +x test.sh Then run it with ./test.sh.

Shell Script Variables

Define a variable: name=zhiyikeji Use the variable with $:

echo $name

echo ${name}

Both produce zhiyikeji. Braces are optional but help avoid ambiguity.

Delete a variable: unset name After unsetting, echo $name outputs nothing.

Make a variable read‑only: readonly name Attempting to reassign a read‑only variable results in an error.

Shell Script Flow Control

If

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

Run with sh test2.sh 1 → Value less than or equal to 2; sh test2.sh 3 → Value greater than 2.

Case

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

Running sh service.sh start prints start already begun; sh service.sh stop prints stop command executed.

For Loop

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

Example: sh fortest.sh 6 outputs numbers 1 through 6.

While Loop

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

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.