Helpful Resources

• Official GNU reference: https://www.gnu.org/software/bash/manual/html_node/index.html
• Bash Guide for Beginners: https://www.tldp.org/LDP/Bash-Beginners-Guide/html/
• Bash Cheatsheet: https://mywiki.wooledge.org/BashSheet

Table of Contents

• Helpful Resources
• Table of Contents
• Running Scripts from Anywhere
  • Bin Executable
  • Symbolic Link
  • Aliases
  • Extend Path
• Script Configurations
  • Bash safe mode
  • Bash configurations
• Brace Expansion
• Variable Expansion
  • String interpolation
  • Default values
  • Pattern matching
  • Regex group pattern matching
• Arithmetic Expressions
• Compound Commands
  • Grouping Statements
  • Sub Shells
  • Command Groups
• Networks
  • Ports
• Nifty Terminal Commands
  • Generate Timestamps
  • Set Operations
• Miscellaneous
  • Config Files
  • Colors
  • Debugging Scripts

Running Scripts from Anywhere

First you will need executable permission to run a script.

# Check to see what permissions files have.
ls -la

# Grant user executable permission.
sudo chmod u+x script.sh

# Then run the script.
./script.sh

Bin Executable

Add the script to your local bin directory with executable permission.

# Usually bin executables do not have file extensions.
sudo chmod u+x script

# Copy file to user's local binaries folder.
sudo cp script /usr/local/bin/

Symbolic Link

https://stackoverflow.com/questions/1951742/how-to-symlink-a-file-in-linux

sudo chmod u+x <file>

# Create a new symbolic link (will fail if already exist).
sudo ln -s <file> /usr/local/bin/<symlink-name>

# Create or update symbolic link.
sudo ln -sf <file> /usr/local/bin/<symlink-name>

# Example
sudo ln -s /opt/android-studio/bin/studio.sh /usr/local/bin/android-studio

# Example with cwd
sudo ln -s "$(cwd)/sherlock.py" /usr/local/bin/sherlock

Aliases

You can create an alias in your .bashrc

alias <alias-name>='<script-path>'

Extend Path

You can also add any directory to be part of the bash executable path: $PATH

# Add the following line in your `.bash_profile`.
export PATH="<my-directory>":$PATH

# Example
export PATH="~/Workspace/dotfiles/bin":$PATH

Script Configurations

Use set -o to turn on a feature and set +o to turn it off.

Bash safe mode

# Enable bash safe mode.
set -euo pipefail

# Exit script if you try to use an uninitialized variable.
set -o nounset

# Exit script if a statement returns a non-true return value.
set -o errexit

# Use the error of the first failure rather than last error in pipe.
set -o pipefail

Bash configurations

# Export any variables that are defined.
set -o allexport

# Enable bash history.
set -o history

Brace Expansion

• https://unix.stackexchange.com/questions/6035/when-do-you-use-brace-expansion

# Rename file (no spaces between commas).
mv myfile.{js,jsx}

# Create files with different extension.
touch myfile.{html,css,js,test.js,stories.js}

# Show output of the cartesian product.
echo {a,b,c}/{1,2,3} | xargs -n1
echo {1..10}/{1..10} | xargs -n1

# Create cartesian product of directories.
mkdir -p ./{ComponentA,ComponentB}/{test,stories}
mkdir -p ./{1..5}/{1..5}

Variable Expansion

• https://mywiki.wooledge.org/BashFAQ/100
• http://tldp.org/LDP/abs/html/variable-expansion.html
• https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

String interpolation

# String interpolation with variables.
echo "I want to eat $fruit"
> I want to eat orange

Default values

# Initialize variable with default value.
fruit=${1:-'orange'}
echo "I want to eat ${fruit}s"
> I want to eat oranges

Pattern matching

# Extracting user and repo using pattern matching from beginning (#).
url='https://github.com/codenameyau/intro-to-bash'
echo "${url#*//*/}"
> codenameyau/intro-to-bash

# Extracting repo using greedy pattern matching from beginning (##).
url='https://github.com/codenameyau/intro-to-bash'
echo "${url##*/}"
> intro-to-bash

# Extracting user profile using pattern matching from end (%).
url='https://github.com/codenameyau/intro-to-bash'
echo "${url%/*}"
> https://github.com/codenameyau

# Extracting origin using pattern matching from end (%).
url='https://github.com/codenameyau/intro-to-bash'
echo "${url%/*/*}"
> https://github.com

# Extracting protocol using greedy pattern matching from end (%%).
url='https://github.com/codenameyau/intro-to-bash'
echo "${url%%://*}"
> https

Regex group pattern matching

# Capturing substring in regex group match.
string="origin  https://github.com/USERNAME/REPOSITORY.git (fetch)"

# Capture matched substring USERNAME
echo $(expr "$string" : .*github.com/'\(.*\)'/)
> USERNAME

# Capture matched substring REPOSITORY
echo $(expr "$string" : .*/'\(.*\).git')
> REPOSITORY

Arithmetic Expressions

• Tutorial: https://mywiki.wooledge.org/ArithmeticExpression
• Full reference: http://wiki.bash-hackers.org/syntax/arith_expr

# Wrap expression with double parens to specify math context.
echo $((1 + 1))
> 2

echo $((3 < 2))
> 0

echo $((1 < 2))
> 1

# Using for loop. Spaces are allowed for assignment.
for ((i = 0; i < 10; i++)); do echo "$1"; done

Compound Commands

• https://mywiki.wooledge.org/BashGuide/CompoundCommands
• https://mywiki.wooledge.org/BashGuide/TestsAndConditionals#Grouping_Statements

Grouping Statements

You can use short circuiting with guard && and default || statements for basic command grouping.

# Will exit the shell if cd exists.
cd /tmp && exit 1

# Will exit the shell if cd does not exist.
cd /tmp || exit 1

Sub Shells

This will launch the commands inside the parenthesis in subshell where variables have their own scope and will disappear once the command is complete or exits.

# The current directory $CWD variable will not be affected.
(cd /tmp || exit 1; date > timestamp)

Command Groups

You can group commands with {} to run a series of commands whose final output could be used. This shares the same variables as the current session.

{command_1; command_2; command_3} | command_4

Networks

Ports

Run this command kill a process running on specific port.

lsof -t -i :8000

kill $(lsof -t -i :8000)

Run this command to see a list of open ports on a system.

# Show tcp, udp, listening ports, pid, numeric hosts, ipv4
sudo netstat -tulpn4

Nifty Terminal Commands

Generate Timestamps

https://stackoverflow.com/questions/1204669/how-can-i-generate-unix-timestamps

# Timestamp: Thu Dec 28 10:44:45 EST 2017
date

# Timestamp: 1514475851 (seconds)
date +%s

# Timestamp: 1514476017560 (milliseconds)
date +%s%3N

Set Operations

These operations must have files sorted first.

# Get lines common to both files.
comm -12 file1 file2

# Read from std-in, sort, get lines in common, and copy to clipboard.
cat | sort | comm -13 - s1 | pbcopy

# Get lines only found in file1.
comm -23 file1 file2

# Sort files as well.
comm -13 <(sort file1) <(sort file2)
comm -23 <(sort file1) <(sort file2)

Rerunning commands

# Rerun the previous command with sudo.
sudo !!

# Rurun the previous command with substitutions.
^before^after^

Miscellaneous

Config Files

Your program's configuration files are stored here. They are not removed by apt purge and may often contain unnecessary application data.

cd ~/./config

Colors

https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux

Black        0;30     Dark Gray     1;30
Red          0;31     Light Red     1;31
Green        0;32     Light Green   1;32
Brown/Orange 0;33     Yellow        1;33
Blue         0;34     Light Blue    1;34
Purple       0;35     Light Purple  1;35
Cyan         0;36     Light Cyan    1;36
Light Gray   0;37     White         1;37

# Reset colors
Color_Off='\033[0m'       # Text Reset

# Regular colors
Black='\033[0;30m'        # Black
Red='\033[0;31m'          # Red
Green='\033[0;32m'        # Green
Yellow='\033[0;33m'       # Yellow
Blue='\033[0;34m'         # Blue
Purple='\033[0;35m'       # Purple
Cyan='\033[0;36m'         # Cyan
White='\033[0;37m'        # White

# Bold
BBlack='\033[1;30m'       # Black
BRed='\033[1;31m'         # Red
BGreen='\033[1;32m'       # Green
BYellow='\033[1;33m'      # Yellow
BBlue='\033[1;34m'        # Blue
BPurple='\033[1;35m'      # Purple
BCyan='\033[1;36m'        # Cyan
BWhite='\033[1;37m'       # White

# Underline
UBlack='\033[4;30m'       # Black
URed='\033[4;31m'         # Red
UGreen='\033[4;32m'       # Green
UYellow='\033[4;33m'      # Yellow
UBlue='\033[4;34m'        # Blue
UPurple='\033[4;35m'      # Purple
UCyan='\033[4;36m'        # Cyan
UWhite='\033[4;37m'       # White

Debugging Scripts

• Guide to bash debugging
• Always quote variables

Method 1: Run your script with the -x flag.

$ bash -x myscript.sh

Method 2: Modify the script's interpreter directive to include the -x flag.

#!/bin/bash -x

Method 3: Selectively debug sections of code.

#!/bin/bash

# Turn on debugging.
set -x

# Turn off debugging.
set +x