Master Shell Scripting: From Basics to Advanced Techniques

Shell Programming

1. Introduction

Shell is a C‑written program that provides a command language and a scripting environment for accessing operating‑system kernel services. A Shell script is a program written for the shell; "shell programming" usually refers to writing these scripts rather than developing the shell itself. Linux systems typically include several shell interpreters; you can list them with cat /etc/shells. Bash is the most common because it is free, easy to use, and the default on most distributions.

2. Quick Start

2.1 Write a script

Create a file /export/hello.sh with the following content:

#!/bin/bash

echo 'hello world'

The #! line tells the system which interpreter to use.

2.2 Execute the script

Method 1 : run the interpreter explicitly

/bin/sh hello.sh
# or
/bin/bash hello.sh

Both produce hello world. In this context sh is a shortcut to bash.

Method 2 : invoke the script directly after giving it execute permission

chmod 755 hello.sh
./hello.sh

3. Shell Variables

3.1 Definition

Assign a value without a leading $ and without spaces around the equals sign:

your_name="runoob.com"

Variable names may contain letters, numbers, and underscores, but cannot start with a number or contain punctuation or Bash reserved keywords.

Valid: RUNOOB, LD_LIBRARY_PATH, _var, var2 Invalid: ?var=123,

user*name=runoob

3.2 Using variables

Reference a variable with $ or ${} for clarity:

your_name="zhangsan"

echo $your_name

echo ${your_name}

3.3 Deleting variables

Use unset to remove a variable (cannot delete read‑only variables):

unset myUrl

3.4 Read‑only variables

Mark a variable as read‑only with readonly. Subsequent assignments generate an error:

myUrl="http://www.google.com"
readonly myUrl
myUrl="http://www.runoob.com"   # error

4. Strings

4.1 Single quotes

skill='java'
str='I am good at $skill'
echo $str   # prints: I am good at $skill

Variables are not expanded inside single quotes.

4.2 Double quotes

skill='java'
str="I am good at $skill"
echo $str   # prints: I am good at java

Double quotes allow variable expansion and escape sequences.

4.3 String length

skill='java'
echo ${#skill}   # 4
expr length "iamlilei"   # 8

4.4 Sub‑string extraction

str="I am good at java"
# from character 2 to the end
echo ${str:2}          # am good at java
# from character 2, length 2
echo ${str:2:2}        # am

4.5 Finding a sub‑string

str="I am good at java"
expr index "$str" am   # 3 (position of first "am")

5. Passing Parameters

When a script is invoked, arguments are accessed as $1, $2, …, $0 (script name). Special variables: $# – number of arguments $* – all arguments as a single string $@ – all arguments as separate quoted strings $$ – PID of the current shell $? – exit status of the last command

Example param.sh:

#!/bin/bash

echo "Shell parameter example!"
echo "Script name: $0"
echo "First param: $1"
echo "Second param: $2"
echo "Third param: $3"

Run with chmod 755 param.sh && ./param.sh 1 2 3 to see the values.

6. Arithmetic Operators

Bash supports arithmetic via expr, $(( )), $[ ], etc. Operators must be spaced. + – addition (e.g., expr $a + $b) - – subtraction * – multiplication (escape * when using expr) / – division % – modulo = – assignment == – numeric equality test != – numeric inequality test

Example:

#!/bin/bash

a=4
b=20
val=`expr $a + $b`
echo $val   # 24
val=`expr $a - $b`
echo $val   # -16
val=`expr $a \* $b`
echo $val   # 80
val=`expr $b / $a`
echo $val   # 5
((c=a+b))
echo $c   # 24

7. Flow Control

7.1 if / else

#!/bin/bash
a=20
if [ $a -gt 10 ]; then
    echo "a is greater than 10"
fi

Extended forms with elif and else allow multiple branches.

7.2 for loops

# iterate 1 to 5
for i in 1 2 3 4 5; do
    echo $i
done

# iterate 1 to 100
for i in {1..100}; do echo $i; done

# iterate odd numbers 1 to 100
for i in {1..100..2}; do echo $i; done

# iterate files in root directory
for f in `ls /`; do echo $f; done

7.3 while loops

#!/bin/bash
sum=0
i=1
while [ $i -le 100 ]; do
    sum=$[ sum + i ]
    i=$[ i + 1 ]
 done
echo $sum   # 5050

7.4 Infinite loop with break

#!/bin/bash
i=0
while true; do
    echo "$i $(date +"%Y-%m-%d %H:%M:%S")"
    i=$[ i + 1 ]
    if [ $i -eq 10 ]; then
        break
    fi
done

7.5 continue

#!/bin/bash
for i in {1..30}; do
    if [ $[ i % 3 ] -eq 0 ]; then
        continue   # skip multiples of 3
    fi
    echo $i
done

7.6 case (switch)

#!/bin/bash
echo 'Enter a number between 1 and 4:'
read aNum
case $aNum in
    1) echo 'You chose 1' ;;
    2) echo 'You chose 2' ;;
    3) echo 'You chose 3' ;;
    4) echo 'You chose 4' ;;
    *) echo 'Invalid input' ;;
esac

8. Functions

8.1 Definition

# simple function
myfunc() {
    echo "This is my first shell function"
}

# call the function
myfunc

8.2 Parameters to functions

#!/bin/bash
funWithParam() {
    echo "First param: $1"
    echo "Second param: $2"
    echo "Tenth param (using braces): ${10}"
    echo "Total number of params: $#"
    echo "All params as a string: $*"
}

funWithParam 1 2 3 4 5 6 7 8 9 34 73

9. Arrays

9.1 Defining arrays

#!/bin/bash
my_array=(A B "C" D)

9.2 Accessing elements

echo "First element: ${my_array[0]}"
echo "Second element: ${my_array[1]}"

9.3 Getting all elements

echo "All elements: ${my_array[*]}"

9.4 Array length

echo "Number of elements: ${#my_array[*]}"

9.5 Traversing arrays

# method 1 – iterate over values
for val in "${my_array[@]}"; do echo $val; done

# method 2 – index based
len=${#my_array[@]}
for ((i=0;i<len;i++)); do echo ${my_array[$i]}; done

10. Sourcing Other Files

External scripts can be included with . filename or source filename, enabling code reuse.

# test1.sh
my_arr=(AA BB CC)

# test2.sh
source ./test1.sh
for var in "${my_arr[@]}"; do echo $var; done

Conclusion

This overview covers the essential concepts of shell scripting, providing a foundation for automating tasks, managing system resources, and building more complex command‑line tools.