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Bash Scripting Fundamentals

#bash #cmd #coding #terminal

1. Introduction to Bash

1.1 What is Bash?

Bash (Bourne Again Shell) is a command processor that typically runs in a text window where the user types commands that cause actions.

Image description
Bash can also read and execute commands from a file, called a script.

1.2 Creating and Running a Script

  • Creating a script: Use a text editor to create a file with the .sh extension. 
  nano script.sh
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  • Making the script executable: 
  chmod +x script.sh
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  • Running the script: 
  ./script.sh
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2. Basic Commands

2.1 Echo

  • Description: Prints text to the terminal. 
  echo "Hello, World!"
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2.2 Variables

  • Description: Storing and using data. 
  NAME="John"
  echo $NAME
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2.3 Comments

  • Description: Adding comments to a script for readability. 
  # This is a comment
  echo "This is a script"
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3. Input and Output

3.1 Reading User Input

  • Description: Taking input from the user. 
  read "Enter your name: " NAME
  echo "Hello, $NAME"
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3.2 Redirecting Output

  • Description: Redirecting command output to a file. 
  echo "This is a test" > file.txt
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3.3 Appending Output

  • Description: Appending command output to a file. 
  echo "This is a test" >> file.txt
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3.4 Redirecting Input

  • Description: Using a file as input to a command. 
  wc -l < file.txt
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3.5 Piping

  • Description: Using the output of one command as input to another. 
  cat file.txt | grep "test"
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4. Control Structures

4.1 Conditional Statements

  • Description: Making decisions in scripts. 
  if [ "$NAME" == "John" ]; then
    echo "Hello, John!"
  else
    echo "You are not John."
  fi
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4.2 Loops

  • Description: Repeating a set of commands.

4.2.1 For Loop 

  for i in {1..5}; do
    echo "Welcome $i"
  done
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4.2.2 While Loop 

  COUNTER=0
  while [ $COUNTER -lt 5 ]; do
    echo "Counter: $COUNTER"
    COUNTER=$((COUNTER + 1))
  done
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4.2.3 Until Loop 

  COUNTER=0
  until [ $COUNTER -ge 5 ]; do
    echo "Counter: $COUNTER"
    COUNTER=$((COUNTER + 1))
  done
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5. Functions

5.1 Defining and Calling Functions

  • Description: Grouping commands into a reusable block. 
  my_function() {
    echo "Hello from my_function"
  }
  my_function
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6. Data Structures

6.1 Arrays

  • Description: Using arrays to store multiple values. 
  NAMES=("John" "Jane" "Doe")
  echo "First Name: ${NAMES[0]}"
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6.2 Tuples (Simulated with Arrays)

  • Description: Using arrays to simulate tuples. 
  TUPLE=("Alice" 25 "Developer")
  echo "Name: ${TUPLE[0]}, Age: ${TUPLE[1]}, Job: ${TUPLE[2]}"
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6.3 Sets (Simulated with Arrays and Associative Arrays)

  • Description: Using arrays and associative arrays to simulate sets. 
  declare -A SET
  SET["Apple"]=1
  SET["Banana"]=1
  if [[ ${SET["Apple"]} ]]; then
    echo "Apple is in the set."
  fi
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7. Operators

7.1 Mathematical Operators

  • Description: Performing arithmetic operations. 
  A=5
  B=3
  SUM=$((A + B))
  echo "Sum: $SUM"
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7.2 Logical Operators

  • Description: Using logical operators in conditions. 
  if [ $A -eq 5 ] && [ $B -eq 3 ]; then
    echo "Both conditions are true."
  fi
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7.3 Semantic Operators

  • Description: Using comparison operators. 
  if [ "$A" -eq 5 ]; then
    echo "A is 5"
  fi
  if [ "$A" -ne 5 ]; then
    echo "A is not 5"
  fi
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8. Type Casting

  • Description: Type casting in Bash is generally about ensuring variable content is treated correctly. 
  VAR="123"
  NUM=$((VAR + 0))  # Cast string to number for arithmetic
  echo $NUM
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9. API Calls

  • Description: Making HTTP requests using tools like curl. 
  RESPONSE=$(curl -s -X GET "https://api.example.com/data")
  echo "Response: $RESPONSE"
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10. Automating Tasks

10.1 Cron Jobs

  • Description: Scheduling scripts to run automatically. 
  # Edit the crontab file
  crontab -e
  # Add a job to run every day at midnight
  0 0 * * * /path/to/script.sh
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10.2 Using at

  • Description: Scheduling a one-time task. 
  echo "/path/to/script.sh" | at now + 5 minutes
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10.3 Background Jobs

  • Description: Running scripts in the background. 
  ./script.sh &
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11. Advanced Topics

11.1 Using sed and awk

  • Description: Text processing with sed and awk.

11.1.1 sed 

  sed 's/old/new/g' file.txt
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11.1.2 awk 

  awk '{print $1}' file.txt
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11.2 Traps

  • Description: Capturing signals. 
  trap "echo 'Script interrupted'; exit" SIGINT
  while true; do
    echo "Running..."
    sleep 1
  done
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12. Debugging

12.1 Enabling Debugging

  • Description: Running a script in debug mode. 
  bash -x script.sh
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12.2 Using set

  • Description: Setting options for debugging. 
  set -x  # Enable debugging
  set +x  # Disable debugging
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12.3 Error Handling

  • Description: Handling errors in scripts. 
  command || { echo "Command failed"; exit 1; }
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12.4 Using trap for Cleanup

  • Description: Cleaning up resources on script exit. 
  cleanup() {
    echo "Cleaning up..."
    # perform cleanup here
  }
  trap cleanup EXIT
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12.5 Logging

  • Description: Creating log files for your scripts. 
  LOG_FILE="/var/log/script.log"
  echo "Script started" >> $LOG_FILE
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13. More on Arrays

13.1 Array Operations

  • Description: Array length and iteration. 
  ARR=("one" "two" "three")
  echo "Array length: ${#ARR[@]}"
  for ITEM in "${ARR[@]}"; do
    echo $ITEM
  done
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13.2 Associative Arrays

  • Description: Using associative arrays. 
  declare -A ASSOC_ARRAY
  ASSOC_ARRAY["name"]="John"
  ASSOC_ARRAY["age"]="30"
  echo "Name: ${ASSOC_ARRAY["name"]}"
  echo "Age: ${ASSOC_ARRAY["age"]}"
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13.3 Multidimensional Arrays

  • Description: Simulating multidimensional arrays. 
  ARRAY_2D[0]="1 2 3"
  ARRAY_2D[1]="4 5 6"
  ARRAY_2D[2]="

7 8 9"
  for ROW in "${ARRAY_2D[@]}"; do
    for ITEM in $ROW; do
      echo -n "$ITEM "
    done
    echo
  done
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14. String Operations

14.1 Substring Extraction

  • Description: Extracting a substring from a string. 
  STRING="Hello World"
  echo ${STRING:6:5}  # Outputs "World"
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14.2 String Length

  • Description: Getting the length of a string. 
  echo ${#STRING}  # Outputs "11"
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14.3 String Replacement

  • Description: Replacing part of a string. 
  echo ${STRING/World/Bash}  # Outputs "Hello Bash"
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15. File Operations

15.1 File Tests

  • Description: Testing properties of files. 
  if [ -f "file.txt" ]; then
    echo "file.txt is a regular file."
  fi
  if [ -d "directory" ]; then
    echo "directory is a directory."
  fi
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15.2 Reading Files

  • Description: Reading a file line by line. 
  while IFS= read -r line; do
    echo "$line"
  done < file.txt
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15.3 Writing to Files

  • Description: Writing to a file. 
  echo "Hello, World!" > file.txt
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15.4 Appending to Files

  • Description: Appending to a file. 
  echo "This is a new line" >> file.txt
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16. Network Operations

16.1 Downloading Files

  • Description: Using wget or curl to download files. 
  wget http://example.com/file.zip
  curl -O http://example.com/file.zip
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16.2 Uploading Files

  • Description: Using curl to upload files. 
  curl -F "file=@/path/to/file" http://example.com/upload
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16.3 Checking Internet Connection

  • Description: Pinging a server to check for internet connectivity. 
  ping -c 4 google.com
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17. Handling Large Data

17.1 Sorting and Uniqueness

  • Description: Sorting data and removing duplicates. 
  sort file.txt | uniq > sorted.txt
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17.2 Finding Patterns

  • Description: Using grep to find patterns. 
  grep "pattern" file.txt
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17.3 Splitting Files

  • Description: Splitting large files into smaller parts. 
  split -l 1000 largefile.txt smallfile_
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18. Process Management

18.1 Checking Running Processes

  • Description: Using ps to list running processes. 
  ps aux
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18.2 Killing Processes

  • Description: Using kill to terminate processes. 
  kill -9 PID
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18.3 Monitoring System Resources

  • Description: Using top or htop to monitor system resources. 
  top
  htop  # Requires installation
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19. Packaging and Distribution

19.1 Creating Tar Archives

  • Description: Archiving files using tar. 
  tar -cvf archive.tar file1 file2 directory/
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19.2 Extracting Tar Archives

  • Description: Extracting files from a tar archive. 
  tar -xvf archive.tar
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19.3 Compressing Files

  • Description: Compressing files using gzip. 
  gzip file.txt
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19.4 Decompressing Files

  • Description: Decompressing files using gunzip. 
  gunzip file.txt.gz
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This extensive guide covers fundamental and advanced topics in Bash scripting

Top comments (7)

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moopet profile image
Ben Sinclair •

In 4.1, you use a double-equals for equality:

if [ "$NAME" == "John" ]; then
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and while this works in bash, it's not POSIX and won't work in most other shells. I'd recommend sticking to a single equals sign for equality:

if [ "$NAME" = "John" ]; then
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In 18.2 you mention "Using kill to terminate processes" by using kill -9. That's might be slightly confusing since SIGKILL is 9 and SIGTERM is 15.

I'd steer clear of part 13 and similar altogether. If you're at the point where you're using "advanced arrays" in the shell, you almost certainly want to be using a dedicated scripting language instead.

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bobbyiliev profile image
Bobby Iliev •

Great post 👏 Well done!

In case that anyone wants to learn more, here is a free eBook:

GitHub logo bobbyiliev / introduction-to-bash-scripting

Free Introduction to Bash Scripting eBook

💡 Introduction to Bash Scripting

This is an open-source introduction to Bash scripting guide/ebook that will help you learn the basics of Bash scripting and start writing awesome Bash scripts that will help you automate your daily SysOps, DevOps, and Dev tasks. No matter if you are a DevOps/SysOps engineer, developer, or just a Linux enthusiast, you can use Bash scripts to combine different Linux commands and automate boring and repetitive daily tasks, so that you can focus on more productive and fun things.

The guide is suitable for anyone working as a developer, system administrator, or a DevOps engineer and wants to learn the basics of Bash scripting.

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The first 13 chapters would be purely focused on getting some solid Bash scripting foundations then the rest of…

View on GitHub
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litlyx profile image
Antonio | CEO at Litlyx.com •

Great article! Your guide on Bash scripting fundamentals is an excellent resource for beginners. This tutorial is a great starting point.
Keep up the good work.

Sharing love from Italy.

Antonio, CEO at Litlyx

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elsyng profile image
Ellis • • Edited

Very nice summary, easy reading, without clutter👍

"Debugging": I think "tracing" would be a better term.

"help", "man", "type" (all 3 together) might be nice to add, so one can go deeper into any command. ("type" is very useful, to tell a file, command, builtin, function etc.)

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ankurloriya profile image
Ankur Loriya •

First of all thanks for creating this page this is really helpful handbook.

Update section
5.1: (Function)
How to pass arguments ?
How to function handle arguments?

Create IF condition complete handbook in which contains

  • File exist check
  • File extension check
  • Boolean check
  • String compare with number
  • Float/fraction number check
  • AND and OR operator use
  • Convert to not operation
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ddebajyati profile image
Debajyati Dey •

Informative

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