Bash

Bash is the command language of most Linux and macOS terminals: you type commands to navigate folders, manage files, and automate tasks. You run it interactively or save commands in a .sh script.

Where am I, what's here?

pwd prints the current folder; ls lists its contents.

pwd                  # => /home/ada/projects
ls                   # names only
ls -la               # long format, including hidden files

Move around

cd changes directory. . is here, .. is the parent, ~ is your home folder.

cd projects/app      # go into a subfolder
cd ..                # up one level
cd ~                 # back to home
cd -                 # back to the previous folder

Create files and folders

mkdir makes folders, touch makes an empty file.

mkdir notes              # one folder
mkdir -p src/app/utils   # create nested folders in one go
touch todo.txt           # empty file (or update its timestamp)

Copy, move, rename, delete

cp copies, mv moves or renames, rm deletes. There is no undo — be careful.

cp file.txt backup.txt       # copy
cp -r src/ dist/             # copy a folder and its contents
mv old.txt new.txt           # rename
mv report.txt ~/Documents/   # move
rm file.txt                  # delete a file
rm -r folder/                # delete a folder and everything in it

Look inside a file

cat dumps the whole file; head/tail show the start or end; less scrolls page by page.

cat notes.txt           # print the whole file
head -n 5 log.txt       # first 5 lines
tail -n 20 log.txt      # last 20 lines
tail -f log.txt         # follow new lines as they're written (Ctrl-C to stop)
less big.txt            # scroll (q to quit)

Find files and search text

find locates files by name; grep searches for text inside files.

find . -name "*.js"           # all .js files under the current folder
grep "error" log.txt          # lines containing "error"
grep -ri "todo" src/          # recursive, case-insensitive search in src/
grep -n "fix" notes.txt       # show line numbers

Pipes and redirection

A pipe | feeds one command's output into the next; > writes output to a file, >> appends.

ls -la | grep ".txt"          # list, keep only .txt lines
cat log.txt | wc -l           # count lines in the file
echo "hello" > greeting.txt   # write (overwrites the file)
echo "again" >> greeting.txt  # append a line

Variables

Assign with no spaces around =; read the value back with a $ prefix.

name="Ada"
echo "Hello, $name"     # => Hello, Ada
greeting="Hi ${name}!"  # braces help when text follows the name
echo "$greeting"        # => Hi Ada!

Command substitution

Wrap a command in $( ) to capture its output into a variable.

today=$(date +%Y-%m-%d)
echo "Today is $today"        # => Today is 2026-06-24
files=$(ls | wc -l)
echo "$files files here"

Conditionals

if runs a block when a test passes. Use [ ... ] for the test, with spaces inside the brackets.

if [ -f config.json ]; then
  echo "config exists"
else
  echo "no config"
fi

Common test flags: -f file exists, -d folder exists, -z string is empty.

if [ -z "$name" ]; then
  echo "name is empty"
fi

Compare values

Numbers use -eq/-lt/-gt; strings use = and !=.

count=5
if [ "$count" -gt 3 ]; then
  echo "more than three"   # => more than three
fi
if [ "$name" = "Ada" ]; then
  echo "hi Ada"
fi

Loops

for repeats over a list; while repeats while a condition holds.

for file in *.txt; do
  echo "Found $file"
done

count=1
while [ "$count" -le 3 ]; do
  echo "count is $count"
  count=$((count + 1))
done

Functions

Group commands under a name; refer to arguments as $1, $2, and so on.

greet() {
  echo "Hello, $1!"
}
greet "Ada"     # => Hello, Ada!

Write a script

Start the file with a shebang line so the system knows to run it with Bash.

#!/usr/bin/env bash
echo "Backing up..."
cp -r src/ backup/
echo "Done."

Make it executable once, then run it by path.

chmod +x backup.sh      # mark as runnable
./backup.sh             # run it

File permissions

chmod sets who can read/write/execute; ls -l shows the current permissions.

ls -l script.sh         # => -rw-r--r-- 1 ada ada ...
chmod +x script.sh      # add execute permission
chmod 644 notes.txt     # owner read/write, others read-only

Environment and PATH

Environment variables configure your shell; PATH lists folders searched for commands.

echo "$HOME"            # => /home/ada
echo "$PATH"            # colon-separated folders
export API_KEY="abc123" # set a variable for this session and child processes

Handy shortcuts

These save time at the prompt every day.

history                 # list recent commands
!!                      # run the previous command again
clear                   # clear the screen (or Ctrl-L)
which node              # show the full path of a command
man ls                  # open the manual for a command (q to quit)

Chaining commands

&& runs the next command only if the previous one succeeded; ; runs regardless.

mkdir build && cd build       # cd only if mkdir worked
npm install ; npm test        # run both, even if install fails
command || echo "it failed"   # run the fallback only on failure