Master Bash: From Basic Scripts to Advanced Automation with Expect

What is a script file?

A script file is a plain‑text file that contains a sequence of commands interpreted by a command parser (e.g., Bash). When executed, the commands run in order, allowing frequently used Linux commands to be stored and reused.

#!/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 interpreter.

Comments start with #.

Use echo to output text.

Use cat to view file contents.

Shell Variables

Creating variables

Variable assignment uses the syntax name=value with no spaces around the equals sign. Values containing spaces must be quoted.

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

Referencing variables

Access a variable with $name. The three forms below are equivalent:

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

Deleting variables

Remove a variable with unset name.

#!/bin/bash
a=10
unset a
echo $a  # no output

Reading input from the keyboard

Prompt the user with read var.

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

Special Variables

#!/bin/bash
echo $1   # first positional argument
echo $2
echo $3
echo $0   # script name
echo $#   # number of arguments
echo $@
echo $*
echo $?
echo $$

Environment Variables

Predefined variables such as HOME, PWD, USER, HOSTNAME, etc., can be listed with env. Export a variable with export VAR to make it available to child processes, and add it to ~/.bashrc for persistence.

Redirection Operators

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

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

Pipe Operator

The pipe | passes the standard output of the command on the left as standard input to the command on the right.

Single vs Double Quotes

Single quotes '...' suppress all special meanings. Double quotes "..." preserve variable expansion ( $) and command substitution.

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

grep Search Options

Typical usage: grep [options] pattern file. Common options: -i – ignore case -r – recursive search -l – list matching file names -n – show line numbers -v – invert match -w – match whole words -c – count matching lines

Example file 1.txt:

"hello world"
"this is a test"
12

Command grep "hello" 1.txt prints the line containing "hello".

Test Operators

#!/bin/bash
VAR=2
test $VAR -gt 1
echo $?
[ $VAR -gt 1 ]   # spaces required around brackets
echo $?

Arrays

Defining arrays:

a=(1 2 3 4 5)

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

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

Accessing arrays:

#!/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

Control Structures

if Statement

Place if and then on separate lines, or terminate the condition with a semicolon when on the same 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/share/" ]; then
  echo "SHARE $PWD"
else
  echo "else"
fi

case Statement

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

for Loop

#!/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
for f in /etc/*; do
  echo $f
done

while Loop

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

until Loop

#!/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 $?

Automating Interaction with expect

Install the Expect interpreter: sudo apt-get install expect The following Expect script 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