BASH Tutorial

ECHO - Text back to you

VIM is a good text and file editor for Linux

    vim (file name) enter

    :w writes the file, q to quit

    use :wq to write file and quit at the same time

    To quit without making changes use :q! while in the file

    To read your file, use cat (filename)

To exit edit mode after writing your file while in VIM, hit the esc key to use console to write and exit file

CHMOD - modifies permissions for a file

    Type chmod u+x (file name) to give the owner permissions

LESS: gives you one page at a time when opening a file

FIND: Will help you find something is a directory

    Example: find / -name "filename*"

AWK: Used for precise seeking in a file and as output filtering

    awk '{print $1}' file.txt

  awk -F, '{print $2}' data.csv

SED: A command line tool to modify values in a text file using regular expressions

    sed 's/foo/bar/' file.txt

    sed 's/foo/bar/g' file.txt

TMUX: Creates another session

Example:

    tmux new -s name create session

    tmux ls list sessions

    tmux attach -t name attach session

    Ctrl+b d detach

    Ctrl+b c new window

    Ctrl+b % vertical split

    Ctrl+b " horizontal split

    Ctrl+b arrow move pane

    Ctrl+b n next window

SSH: A tool to access your linux machine remotely.

    Type: username@machineIP then accept user fingerprints and then enter your password

TOP/HTOP: A list of the processes running on your machine

    TOP lists all process

HTOP puts it in a more organized way

Other Useful Commands:

command './(filename)' runs the file

command 'ls' is a list command to see files and directories

command 'grep' filters out everything except the word that follows grep

Using "" can feed a string into a command

FLOW CONTROL

if/elif/else statements:

    Basic Concept:

     if condition1

     run block

     else if condition2

     run block

     else

     fallback

Example:

    if [ $score -ge 90 ]; then

    echo "Grade A"

    elif [ $score -ge 80 ]; then

    echo "Grade B"

    else

    echo "Grade C"

    fi

VARIABLES

Fundamental for passing information between commands and making scripts more dynamic.

No spaces around '='

Variable expansion:

    variable=value

Example:

    name="Alice"

    echo $name

    Output = Alice

COMMMAND SUBSTITUTION

Command substitution runs a command and replaces the command itself with its output.

$(command)

Example:

    echo "Today is $(date)"

    Output: Today is Tue Mar 10 12:30:00 PDT

    current_dir=$(pwd)

    echo "You are in $current_dir"

*Command substitution allows Bash scripts to treat the command output as data.

10 essential Bash scripts where command substitution is essential:

1. Timestamped Backup File

backup="database_$(date +%Y-%m-%d).sql"

cp database.sql "$backup"

2. Count Files in a Directory

file_count=$(ls | wc -l)

echo "There are $file_count files in this directory."

3. Detect Current User

current_user=$(whoami)

echo "Running script as $current_user"

4. Get System Uptime

uptime_info=$(uptime)

echo "System status: $uptime_info"

5. Find the Largest File

largest=$(ls -S | head -n 1)

echo "Largest file: $largest"

6. Count Errors in a Log File

errors=$(grep -c "ERROR" app.log)

echo "Error count: $errors"

7. Create a Dynamic Directory Name

project_dir="project_$(date +%s)"

mkdir "$project_dir"

8. Get Current Git Branch

branch=$(git branch --show-current)

echo "Current branch: $branch"

9. Get System IP Address

ip=$(hostname -I)

echo "Server IP: $ip"

10. Check Disk Usage

usage=$(df / | tail -1 | awk '{print $5}')

echo "Disk usage: $usage"

REDIRECTION

Redirection in Bash lets you control where command input comes from and where output goes. You can send output to files, read from files, or connect commands together by using redirection features in Bash.

Using '>' symbol moves a command or echo into a file

ex: Hello World! > hello.txt moves the echo into the hello.txt file

Using one '>' will overwrite what was previously written in that file. Use '>>' to ADD contents to the file

command > file # stdout to file (overwrite)

command >> file # stdout to file (append)

command < file # file to stdin

command > file # stderr to file

command > file 2>&1 # stdout + stderr to file

command > /dev/null # discard output

command | command # pipe output to next command

EXIT CODES

Exit codes (also called exit statuses or return codes) tell the shell whether a command succeeded or failed. Every command in Bash finishes with a numeric code that scripts can check to decide what to do next.

When a command finishes, it returns a number between 0 and 255

0 = success

non-zero = failure

5 Common Exit Codes:

0 success

1 general error

2 misuse of shell command

126 command cannot execute

127 command not found

STRING MANIPULATION

Strings in Bash are just variables containing text.

Quick String Manipulation Cheat Sheet:

${#var} # string length

${var:0:5} # substring

${var#pattern} # remove prefix (short)

${var##pattern} # remove prefix (long)

${var%pattern} # remove suffix (short)

${var%%pattern} # remove suffix (long)

${var/pat/repl} # replace first match

${var//pat/repl} # replace all matches

${var^^} # uppercase

${var,,} # lowercase

${var:-default} # default value

${var:=default} # assign default

Example:

path="/home/user/report.txt"

filename=${path##*/}

echo "$filename"

Output: report.txt

DIRECTORY TREE STRUCTURE

Directory tree structure is essential when working in Bash because the shell navigates and manipulates files based on the system's hierachical filesystem and can be easy to get lost when trying to navigate in a CLI.

Conceptually looks like this:

/

├── bin

├── etc

├── home

│ ├── alice

│ └── bob

├── usr

└── var

Root directory: /

Example:

cd /

ls

This shows the top-level directories of the system.

To check the current directory you are in: pwd

The command 'cd' is used to move to a directory

Example: cd /home/user

cd Documents - Moves you into that directory

cd .. - moves you to parent directory

Quick Directory List:

• pwd # show current directory
• ls # list directory contents
• cd directory # change directory
• cd .. # move to parent
• cd ~ # go to home directory
• mkdir dir # create directory
• mkdir -p a/b/c # create nested directories
• rm -r dir # remove directory recursively
• ls -R # recursive listing
• ls -a # show hidden files

RELATIVE PATHS

Relative paths in Bash are ways to refer to files or directories based on your current location in the filesystem, instead of specifying the full (absolute) path.

• . # current directory
• .. # parent directory
• ../.. # grandparent directory
• folder/file.txt # file inside subfolder
• ../file.txt # file in parent folder
• ./file.txt # file in current folder

FILE SYSTEM HIERARCHY

• /bin - Essential command binaries
• /etc - Configuration files
• /home - User directories
• /usr - Applications and libraries
• /var - Logs and variable data
• /tmp - Temporary files

1. Cursor Movement

Move around the command line quickly.

2. Editing Text

Quickly delete or edit parts of your command.

3. Command History

Reuse previous commands.

Example:

sudo !!

Runs the previous command with sudo.

4. Screen / Process Control

5. Tab Completion (Very Useful)

Example:

cd Doc<Tab>

Auto-completes to documents

Ctrl + R is one of the most powerful shortcuts. You can start typing part of a previous command and Bash will find it instantly.

Example:

Ctrl + R
git

It searches your history for commands containing git.