Text and streams

cat — print a whole file

The cat command prints a file's contents to the terminal and joins several files into one stream.

less — page through a long file with search

The less pager shows a long file or command output screen by screen — scroll, search by word, follow the end.

head — the first lines of a file

The head command prints the beginning of a file or of command output — 10 lines by default, -n sets how many.

tail — the last lines and following a log

The tail command prints the end of a file, and with -f it keeps showing new lines as they are appended to a log.

Basic file commands

Navigation and files — ls, cd, cp, mv, rm, cat, less, head, tail, terminal copy-paste.

Bash scripts: running, variables, arguments

A script is a text file of shell commands: the #! line picks the interpreter, arguments arrive in $1, $2 and "$@".

tee — output to a file and the screen

Splits a stream: writes to a file AND shows it on screen; sudo tee to write into protected files.

echo — printing text

Printing strings and variables; -n and -e, escaping, when to prefer printf.

grep — searching text

The grep command finds lines matching a pattern in files and command output: regex, recursion, context.

wc — count lines, words and bytes

The wc command counts lines (-l), words (-w), bytes (-c) and characters (-m) in a file or in command output.

sort — sort lines

The sort command orders lines of a file or command output: as text, as numbers (-n), by field (-k), reversed (-r).

uniq — remove or count repeated lines

The uniq command collapses adjacent identical lines and counts them with -c; sort almost always comes before it.

cut — cut fields out of lines

The cut command takes fields out of every line by a delimiter (-d and -f) or characters by position (-c).

Glob patterns and brace expansion

Shell substitutions — * ? [] {} — and how they differ from regular expressions.

Output redirection and pipelines

The shell sends a command's input and output to a file or another command — >, >>, 2>&1, /dev/null, the | pipe.

sed and awk — editing and fields in a text stream

sed edits and deletes lines by pattern, awk splits lines into fields and computes over them, line by line.

Regular expressions — patterns for grep, sed and awk

A regular expression is a pattern that many strings match; grep searches by it, sed replaces by it, awk selects lines.

xargs — building arguments from a stream

Turns stdin lines into a command's arguments; without -0, file names with spaces and newlines break the parsing.

sort, uniq, cut, wc, tr — text stream processing

Small pipeline utilities; uniq only removes adjacent duplicates, so it almost always needs sort before it.

diff and patch — comparing and applying changes

Comparing files and applying patches: diff two configs, see what changed, apply a patch.

jq — processing JSON on the command line

Parsing and filtering JSON on the command line — fields, arrays, filters, formatting.

Environment variables and PATH

Environment variables: env, export, $PATH, ~/.bashrc and profiles; where "command not found" comes from.

printf and echo — printing text in scripts

Formatted output in the shell — more reliable than echo, formats and special characters.

vim — editing files in the terminal

The bare minimum for editing configs; Esc always returns to Normal mode, where letters are commands, not text.

nano — a simple console editor

A friendly console editor — save, quit, search, for editing config files.

date — date and time in the shell

The date command: current time, output formats, timestamps for files and logs, date arithmetic.

Locale and encoding

Language, encoding and formats — LANG/LC_*, generating locales, UTF-8 problems.