Bash Cheatsheet

# ── Navigation & files ──────────────────
pwd; cd dir; cd ..; cd -          # - = previous dir
ls -la                            # all files, details
cp src dst; cp -r dir1 dir2
mv old new                        # move/rename
rm file; rm -rf dir               # careful with -rf
mkdir -p a/b/c
touch file; cat file; less file
head -20 f; tail -f log           # -f = follow live

# ── Finding things ──────────────────────
grep -rn "pattern" src/           # recursive + line numbers
grep -i case-insensitive; grep -v invert
find . -name "*.js" -not -path "*/node_modules/*"
which python3; history | grep ssh

# ── Pipes & redirection ─────────────────
cmd > out.txt 2>&1                # both streams to file
cmd >> append.txt
cmd1 | cmd2                       # pipe
cmd1 && cmd2                      # 2 only if 1 succeeded
cmd1 || echo "failed"
wc -l; sort | uniq -c | sort -rn  # count occurrences
cut -d, -f2 file.csv              # 2nd CSV column
xargs: find . -name "*.tmp" | xargs rm

#!/usr/bin/env bash
set -euo pipefail          # strict mode: die on errors

# ── Variables & quoting ─────────────────
name="Ada"                 # no spaces around =
echo "$name"               # ALWAYS quote expansions
result=$(date +%F)         # command substitution
n=$(( 2 + 3 ))             # arithmetic

# ── Arguments ───────────────────────────
# $0 script  $1..$9 args  $# count  "$@" all args
# $? last exit code       $$ this PID

# ── Conditions ──────────────────────────
if [[ -f "$file" ]]; then echo exists; fi
# -f file  -d dir  -z empty-string  -n non-empty
# == != for strings | -eq -ne -lt -gt for numbers
[[ "$a" == "yes" && $n -gt 3 ]]

# ── Loops & functions ───────────────────
for f in *.txt; do echo "$f"; done
for i in {1..5}; do echo "$i"; done
while read -r line; do echo "$line"; done < file

greet() {
    local who="$1"         # local scope
    echo "Hello, $who"
    return 0
}
greet "Ada"

case "$1" in
    start) run ;;
    stop)  halt ;;
    *)     echo "usage: $0 start|stop"; exit 1 ;;
esac