Sefism.Linux, terminal and Git: Windows

Tauseef Fayyaz

Chapter 02

Shell basics

Moving around, working with files, and how the shell expands patterns, connects commands and finds programs.

Absolute and relative paths

Also called absolute path, relative path, pwd, cd, ls, ~

What: An absolute path starts at the root /; a relative path starts from the current folder. ~ is your home, . is here and .. is the parent. pwd prints where you are, cd moves and ls -la lists everything.
Why: Every command runs relative to the current folder, so knowing where you are prevents most early accidents.

Shell

pwd
ls -la
cd ~/code/app
cd ..
cd -          # back to the previous folder

Watch out

Running a destructive command without checking pwd first.

File commands

Also called mkdir, cp, mv, rm, touch

What: mkdir -p creates folders (and parents), touch creates an empty file, cp -r copies, mv moves or renames, rm deletes and rm -r deletes folders.
Why: They are the basis of every script and every server task.

Shell

mkdir -p app/{src,tests,docs}
touch app/README.md
cp -r app app-backup
mv app/README.md app/readme.md
ls app-backup/*.md   # preview before deleting
rm -r app-backup

Watch out

There is no recycle bin: rm -rf with a typo or an empty variable can delete far more than intended.

Globbing and brace expansion

Also called globbing, glob, wildcards, *, ?

What: The shell expands * (any characters), ? (one character) and [abc] (one of a set) into matching file names before the command runs. Braces like {a,b} build lists.
Why: You can act on groups of files in one command, and it explains why a command received a list of names rather than a pattern.
Example: rm *.tmp is expanded by the shell to rm a.tmp b.tmp before rm ever runs.

Watch out

A pattern that matches nothing is passed literally in Bash, so the command sees a real *.tmp and fails confusingly.

Quoting

Also called single quotes, double quotes, escaping

What: Single quotes keep text exactly as written. Double quotes keep spaces together but still expand $VAR and $(cmd). A backslash escapes one character.
Why: File names with spaces and variables with unexpected content break commands unless they are quoted.

Shell

name="My Notes"
echo '$name'        # $name
echo "$name"        # My Notes
touch "$name.txt"   # one file, not two
ls My\ Notes.txt

Watch out

Leaving variables unquoted in scripts, so an empty or spaced value changes the command.

Pipes

Also called |, pipeline

What: A pipe | sends the standard output of one command into the standard input of the next.
Why: Small single-purpose tools combine into powerful one-liners without temporary files.

Shell

history | grep git | tail -5
ls *.md | wc -l
ps aux | grep node

Watch out

Writing the whole pipeline at once; build it one step at a time and check each output.

Redirection and the three streams

Also called standard input, standard output, standard error, stdin, stdout, stderr, >>, 2>&1

What: Programs read standard input and write standard output and standard error. > writes output to a file, >> appends, < reads from a file, 2> redirects errors and 2>&1 merges errors into output.
Why: You can save results, keep logs, silence noise with /dev/null and still see errors on screen.

Shell

ls > files.txt
date >> log.txt
npm test > out.log 2> err.log
./backup.sh >> backup.log 2>&1
cmd > /dev/null 2>&1

Watch out

Using > where you meant >>, which empties the file first.

Environment variables

Also called export, env

What: Named values inherited by every program the shell starts, such as HOME, USER, EDITOR and PATH. export makes a variable visible to child programs.
Why: Tools read configuration and secrets from them without hard-coding values into code.

Shell

echo "$HOME"
env | sort | less
export API_URL="http://localhost:3000"
NODE_ENV=test npm test    # for one command only
unset API_URL

Watch out

Expecting a variable set in one tab to exist in another, or after closing the terminal.

PATH

Also called PATH variable, command not found, which, type

What: A colon-separated list of folders the shell searches, in order, for commands. The first match wins.
Why: "Command not found" and "wrong version runs" are almost always PATH problems.

Shell

echo "$PATH" | tr ':' '\n'
which python3
type -a node
export PATH="$HOME/.local/bin:$PATH"

Watch out

Adding the same PATH line to several startup files until nobody knows which one applies.

Shell startup files and aliases

Also called shell startup file, startup file, shell profile, .bashrc, .zshrc, .profile, alias

What: Files the shell runs when it starts: ~/.bashrc for interactive Bash, ~/.zshrc for zsh, and login files such as ~/.profile or ~/.zprofile. Aliases and small functions live there too.
Why: PATH changes, exports and shortcuts you add there apply to every new terminal.

Shell

# in ~/.bashrc or ~/.zshrc
export EDITOR="nano"
alias gs="git status"
alias ll="ls -la"
mkcd() { mkdir -p "$1" && cd "$1"; }

source ~/.bashrc   # reload without a new tab

Watch out

Editing .bashrc while your shell is zsh, then wondering why nothing changed.

nano and Vim

Also called Vim, nano, vi, vimtutor, terminal editor

What: Terminal editors. nano shows its shortcuts: Ctrl+O saves, Ctrl+X exits. Vim is modal: i to insert, Esc for normal mode, :wq to save and quit, :q! to quit without saving.
Why: Servers often have no graphical editor, and Git opens an editor for merges and some commits.
Example: Run vimtutor for a guided 30-minute Vim lesson.

Watch out

Getting stuck in Vim; press Esc, then type :q! and Enter.

Opening files and the clipboard

Also called File Explorer, open, xdg-open, explorer.exe, pbcopy

What: Commands that hand a file to your desktop or move text to and from the clipboard. They differ by operating system.
Why: Opening a folder or copying a command's output without the mouse keeps you in the terminal.

Shell

explorer.exe .          # open folder in File Explorer
cat key.pub | clip.exe  # copy to Windows clipboard
powershell.exe Get-Clipboard  # paste
wslview report.html     # if wslu is installed

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