Bash Crash Course

TLDR:

• devhints.io/bash (my prefered cheat sheet)
• shell check extention for vs code
• shellcheck
• shellharden
• more program examples

What is an OS

• Communicate with hardware.
• Run programs.

Programs are the real work horse, they can do anything.

What is bash?

• Text based OS interface.
  • With heavy focus on the filesystem for hardware communication.
• Start programs.
• Read and write files.
• REPL (Read Evaluate Print Loop).

Why do we use it?

Your server/CICD has it. 🤖

Programs can compose in interesting ways. 💥

Starting programs

Write the name of the program.

# Print your current Working Directory
pwd

# List directory contents
ls

# Print environment
env

# Manual
man

# Read as much as you can see from a file
less

Optionally pass some arguments, almost like a function call in a programming language.

ls -l
man ls
less --help

The PATH and other "magic" places

Where programs are found.

# Show you mine
echo $PATH

# So, where is my program? _which_ program?
man which

which pwd
which ls
which env
/usr/bin/env

You are special. 🏠

echo $HOME
ls $HOME
ls ~

Others just require special help, from you. 🤷‍♂️

echo $JAVA_HOME
echo $GOPATH

Pipes - input, output and "errors"

Inputs and outputs, all mixed up.

cat

Files, files and more files. From every (re)direction.

cat < readme.md
cat > output_cat.txt

cat readme.md

Markdown

npm install -g marked
which marked
marked -h
marked

# H1
And a paragraph

Common convention, word "flags" have double --, short "flags" (single letter) have one -. (Common, but anything goes)

marked --input readme.md
marked --input readme.md --output output_space.html
marked --input=readme.md --output=output_equal.html
marked -i readme.md -o output_short.html

marked < readme.md
marked   < readme.md   > output_redirected.html

cat readme.md | marked > output_redirected2.html

Wait, it was supposed to be files, not the internet?

# Download from github
curl https://api.github.com/users/pirfalt

# Save to file
curl https://api.github.com/users/pirfalt > users_pirfalt.json

# Was that an error?
curl https://api.github.com/users/pirfalt 2> users_pirfalt_error.json

# Yes, you can multi redirect
curl https://api.github.com/users/pirfalt > users_pirfalt.json 2> users_pirfalt_error.json

# And long lines can be broken
curl https://api.github.com/users/pirfalt \
  > users_pirfalt.json \
  2> users_pirfalt_error.json

The appstore programstore:s

Get the programs you don't have.

Mac

brew
brew list

Linux (ubuntu)

docker run -it --rm ubuntu:20.04
echo $SHELL
env
curl

apt --help
apt update
apt install curl
curl

exit

Also on "linux" yum (redhat, centos), apk (alpine) ...

Windows

• https://docs.microsoft.com/en-us/windows/wsl/ 👌
  • Just like linux.
• https://chocolatey.org/
• ¯\_(ツ)\_/¯

Programs running other programs

Bash starting more bash

bash -c 'cd ~ ; ls'
#             ^   Thats new

Python

python -c 'import os; print(os.environ)'

NodeJS

node -e 'console.log(process.env)'

jq

cat users_pirfalt.json | jq '.'

cat users_pirfalt.json | jq '. | { login, location, company }'

cat users_pirfalt.json | jq '.repos_url'

man jq
cat users_pirfalt.json | jq -r '.repos_url'
curl $() # ??

cat readme.md | grep 'echo'

Syntax

I still have to look it up, very often. devhints.io/bash.

Variables

variable_declaration_and_assignment="first"
echo "$variable_declaration_and_assignment"

single_quotes='includes: $variable_declaration_and_assignment'
echo "$single_quotes"

double_quotes="includes: $variable_declaration_and_assignment"
echo "$double_quotes"

no_quotes=includes:\ $variable_declaration_and_assignment
echo "$no_quotes"

# My own personal rule of thumb:
#  Always use single quotes. Until you need double quotes.
#  No quotes are problems waiting to happen.

# And use shellcheck/shellharden.

Functions

curl -H 'Accept: application/vnd.github.v3+json' https://api.github.com/users/pirfalt

# Call github with the correct headers
call_github() {
  local args=$*
  curl -H 'Accept: application/vnd.github.v3+json' $args
}

call_github https://api.github.com/users/pirfalt

# Start crawling
find_repos_for_user() {
  local user="$1"

  local user_url="https://api.github.com/users/$user"
  local repo_url="$(call_github "$user_url" | jq -r '.repos_url')"
  call_github "$repo_url" | jq '.[] | [ .name, .html_url ]'
}

find_repos_for_user pirfalt

# Late input
find_repos_for_user_v2() {
  echo -n 'Please enter a username: '
  read user
  local user_url="https://api.github.com/users/$user"
  local repo_url="$(call_github "$user_url" | jq -r '.repos_url')"
  call_github "$repo_url" | jq '.[] | [ .name, .html_url ]'
}

find_repos_for_user_v2

Branches

Please don't. bash can do if-then-else logic. But it's messy. 🤷‍♂️

Loops

ls | while read file_name; do echo "Found: $file_name in this directory"; done

for f in $(ls); do echo "Found: $f in this directory"; done

for c in 1 2 3 4; do echo "space separated $c"; done

while true; do ls; sleep 2; done

Scripts

echo '#!/bin/bash

echo "My script which lists files... and location"
echo ""
ls -al
echo ""
pwd
' > script.sh

./script.sh

chmod +x ./script.sh