Bash Basics Cheatsheet Bash Basics Cheatsheet

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

bash -x script.sh          # debug an existing script without editing it
set +e                     # temporarily allow failures
some_flaky_command || true # tolerate a single failing command under set -e

name="karandeep"           # no spaces around =
readonly REGION="ca-central-1"
export PATH="$PATH:/opt/bin"
unset name

echo "$name"               # double quotes expand variables; use by default
echo '$name'               # single quotes: literal, no expansion
echo "${name}_backup"      # braces to delimit the variable name

file="app.tar.gz"
echo "${file%%.*}"     # app          (strip longest from end-matching .*)
echo "${file%.*}"      # app.tar      (strip shortest)
echo "${file#*.}"      # tar.gz
path="/var/log/nginx/access.log"
echo "${path##*/}"     # access.log   (basename)
echo "${path%/*}"      # /var/log/nginx (dirname)
env="${DEPLOY_ENV:?DEPLOY_ENV must be set}"

if [[ -f "$config" && "$env" == "prod" ]]; then
  echo "prod config found"
elif [[ "$env" =~ ^(dev|stage)$ ]]; then
  echo "non-prod"
else
  echo "unknown env" >&2
  exit 1
fi

for f in /etc/*.conf; do echo "$f"; done          # glob loop
for i in {1..5}; do echo "attempt $i"; done       # brace range
for ((i = 0; i < 10; i++)); do echo "$i"; done    # C-style

while read -r line; do                            # read a file safely
  echo "line: $line"
done < servers.txt

until curl -sf http://localhost:8080/health; do   # wait for a service
  sleep 2
done

while true; do date; sleep 5; done                # poll loop
break; continue                                   # work as expected

deploy() {
  local env="$1"                  # local keeps vars scoped
  local version="${2:-latest}"    # default for optional args
  echo "deploying $version to $env"
  return 0                        # numeric status only; echo for data
}

deploy prod v1.2.3
result="$(deploy stage)"          # capture output via command substitution

hosts=("web1" "web2" "db1")
hosts+=("cache1")                 # append
echo "${hosts[0]}"                # first element
echo "${hosts[@]}"                # all elements
echo "${#hosts[@]}"               # length
echo "${hosts[@]:1:2}"            # slice: web2 db1
for h in "${hosts[@]}"; do ssh "$h" uptime; done

declare -A ports=([http]=80 [https]=443 [ssh]=22)
ports[dns]=53
echo "${ports[https]}"            # 443
for k in "${!ports[@]}"; do echo "$k -> ${ports[$k]}"; done

mapfile -t lines < servers.txt    # file into array, one line per element

somecommand
echo "$?"                         # status of last command: 0 = success

grep -q "ERROR" app.log && echo "errors found"
systemctl is-active nginx || systemctl start nginx
command -v jq >/dev/null || { echo "jq required" >&2; exit 127; }

tmpdir="$(mktemp -d)"
cleanup() { rm -rf "$tmpdir"; }
trap cleanup EXIT                          # runs on any exit, even set -e

trap 'echo "failed at line $LINENO" >&2' ERR
trap 'echo "interrupted"; exit 130' INT TERM

cat <<EOF > /etc/myapp/config.yaml
env: $DEPLOY_ENV
region: ca-central-1
EOF

cat <<'EOF'                       # quoted delimiter: NO expansion
literal $HOME stays as-is
EOF

sudo tee /etc/motd <<EOF >/dev/null
Managed by automation. Do not edit.
EOF

grep "prod" <<< "$config_text"    # here-string: feed a variable to stdin

now="$(date -u +%Y-%m-%dT%H:%M:%SZ)"       # prefer $() over backticks
files="$(find /var/log -name '*.gz' | wc -l)"

(( count = files + 1 ))                     # arithmetic context
echo $(( 7 / 2 ))                           # 3 (integer division only)
(( count > 100 )) && echo "log buildup"
((i++)); ((total += batch))
echo $(( RANDOM % 6 + 1 ))                  # dice roll

awk 'BEGIN { printf "%.2f\n", 7/2 }'        # need floats? use awk or bc