Master Shell Scripting: From Basics to Advanced Control Structures

What is a script file?

A script is a plain‑text file interpreted by a command processor. When executed, the commands inside run sequentially. Frequently used Linux commands can be stored in a file (e.g., run.sh) and executed as a script.

#!/bin/bash
ls
pwd
cd ..
touch hello.c

Writing shell scripts

Basic rules

File extension should be .sh.

The first line must be #!/bin/bash to specify the Bash interpreter.

Lines beginning with # are comments.

Use echo to output text.

Use cat to view file contents.

Shell variables

All variables are treated as strings, even numeric values.

Creating variables

#!/bin/bash
a=10
b=" 10"
readonly c=3   # read‑only variable

Referencing variables

#!/bin/bash
a=10
echo $a
echo ${a}
echo "${a}"

The script prints three lines, each containing 10.

Deleting variables

#!/bin/bash
a=10
unset a
echo $a   # nothing is printed because the variable was removed

Reading variables from the keyboard

#!/bin/bash
echo "please input the first number:"
read a
echo "This number is: $a"

Variable types

Special variables

Example a.sh prints common positional and status variables:

#!/bin/bash
echo $1   # first argument
echo $2   # second argument
echo $3   # third argument
echo $0   # script name
echo $#   # number of arguments
echo $@   # all arguments as separate words
echo $*   # all arguments as a single word
echo $?   # exit status of last command
echo $$   # PID of the script
echo "finally"

System / environment variables

Predefined variables such as HOME, PWD, PATH, USER can be listed with env. To make a variable available to child processes, export it:

export VAR_NAME

For persistence, add the export line to ~/.bashrc and reload with source ~/.bashrc.

Redirection operators

When the target file does not exist, redirection creates it.

#!/bin/bash
echo hello > 1.c          # write to 1.c
cat < 1.c > 2.c            # copy content to 2.c
cat 2.c                    # display 2.c

Pipe ( | )

The pipe connects the standard output of the left command to the standard input of the right command.

Single vs double quotes

Single quotes suppress all special meaning; double quotes allow variable expansion ( $) and command substitution.

#!/bin/bash
a=10
echo ${a}          # 10
echo "${a}"        # 10
echo '${a}'          # $a (no expansion)

grep search options

-i : ignore case

-r : recursive search

-l : print only matching file names

-n : print line numbers

-v : invert match (show non‑matching lines)

-w : match whole words only

-c : print count of matching lines

Example file 1.txt:

"hello world"
"this is a test"
12

Command:

grep "hello" 1.txt

Test operators

#!/bin/bash
VAR=2
test $VAR -gt 1
echo $?   # 0 if true, 1 if false

VAR1=3
[ $VAR1 -gt 1 ]   # spaces required around brackets
echo $?

Arrays

Definition methods:

a=(1 2 3 4 5)

a[0]=1; a[1]=2; a[2]=3

a=([1]=1 [2]=2)

Example:

#!/bin/bash
a=(2 5 7 10)
echo ${a[2]}        # element at index 2
echo ${#a[*]}       # array length
echo ${a[@]:2}      # slice from index 2 to end
echo ${a[@]:1:2}   # two elements starting at index 1

If statements

Place if and then on separate lines, or terminate the condition with ; when they share a line.

#!/bin/bash
# style 1
if [ $USER == "self" ]
then
  echo $USER
fi

# style 2 (single line)
if [ $PWD == "/home/self/" ]; then echo $PWD; fi

# style 3 with elif / else
if [ $PWD == "/home/self/" ]; then
  echo "HOME $PWD"
elif [ $PWD == "/mnt/hgfs/share/5.shell/3" ]; then
  echo "SHARE $PWD"
else
  echo "else"
fi

Case statements

#!/bin/bash
case $1 in
  "y") echo inputed y ;;
  "n") echo inputed n ;;
  *)   echo "inputed *" ;;
esac

For loops

#!/bin/bash
# iterate over a list
for i in 1 2 3 4 5; do
  echo $i
done

# C‑style loop
for ((i=0;i<5;i++)); do
  echo $i
done

# iterate over files in a directory
for i in /etc/*; do
  echo $i
done

While loops

#!/bin/bash
var=0
while [ $var -ne 10 ]; do
  echo $var
  var=$((var+1))
done

Until loops

#!/bin/bash
myvar=0
until [ $myvar -eq 10 ]; do
  echo $myvar
  myvar=$((myvar+1))
done

Shell functions

#!/bin/bash
func() {
  echo "hello world"
  echo $0   # script name
  echo $1   # first argument
  return 255
}

func 12 33
exit 0
echo $?

Writing an Expect script for automated interaction

Install the Expect interpreter (Debian/Ubuntu):

sudo apt-get install expect

Example test.exp automates an scp transfer by providing the password and handling the host‑key prompt.

#!/usr/bin/expect
# Set variables
set user "HwHiAiUser"
set host "192.168.21.8"
set password "Mind@123"

# Start scp
spawn scp test $user@$host:~

# Handle prompts
expect {
  "password:" {
    send "${password}\r"
  }
  "yes/no" {
    send "yes\r"
    expect "password:" { send "${password}\r" }
  }
}
# Wait for scp to finish
expect eof