$devvkit resources --cheatsheet bash-scripting

Bash Scripting

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Bash shell scripting — variables, conditionals, loops, functions, and common patterns.

Bash is the default shell on Linux and macOS. Writing shell scripts automates repetitive tasks, CI/CD pipelines, and local development workflows. This cheatsheet covers scripting fundamentals with a focus on patterns used in real-world scripts.

All scripts should start with #!/bin/bash and be made executable with chmod +x.

Variables & Substitutions

$#!/bin/bash

Shebang — tells the system to use bash to execute the script.Example: #!/bin/bash echo "Hello, World!"

$NAME="value"

Assign a variable. No spaces around =. Reference with $NAME or ${NAME}.Example: APP_DIR="/opt/myapp" echo "Dir: $APP_DIR"

$$1, $2, ... $@

Positional parameters — arguments passed to the script or function.Example: echo "Hello, $1!" # ./script.sh Alice → "Hello, Alice!"

$$(command) or `command`

Command substitution — capture command output into a variable.Example: DATE=$(date +%Y-%m-%d)

$${VAR:-default}

Use default value if VAR is unset or empty.Example: echo "Mode: ${MODE:-production}"

Conditionals

$if [[ condition ]]; then ... fi

Test condition with [[ ]] (bash-specific, more powerful than [ ]).Example: if [[ -f "$FILE" ]]; then echo "exists"; fi

$[[ -f "$FILE" ]]

Test if a file exists and is a regular file.Example: if [[ ! -f "config.json" ]]; then cp config.default.json config.json; fi

$[[ -d "$DIR" ]]

Test if a directory exists.Example: if [[ ! -d "dist" ]]; then mkdir dist; fi

$[[ -z "$VAR" ]]

Test if a variable is empty (zero-length string).Example: if [[ -z "$API_KEY" ]]; then echo "Missing API key"; exit 1; fi

$[[ "$A" == "$B" ]]

String comparison inside [[ ]]. Use != for inequality.Example: if [[ "$NODE_ENV" == "production" ]]; then ...

Loops

$for item in list; do ... done

Iterate over a list of items.Example: for file in *.ts; do echo "Processing $file"; done

$for ((i=0; i<10; i++)); do ... done

C-style for loop with increment.Example: for ((i=0; i<${#ARRAY[@]}; i++)); do echo "${ARRAY[$i]}"; done

$while read line; do ... done < file

Read a file line by line.Example: while IFS= read -r line; do echo "Line: $line"; done < data.csv

Functions

$func_name() { ... }

Define a bash function.Example: log() { echo "[$(date +%T)] $*"; } log "Server started"

$local VAR="value"

Declare a local variable inside a function. Does not leak to global scope.Example: get_name() { local name="$1"; echo "$name"; }

$return n

Return a numeric exit code from a function (0=success, 1-255=error).Example: is_installed() { command -v "$1" >/dev/null 2>&1; return $?; }

File Operations

$cp -r src dest

Copy files or directories recursively.Example: cp -r ./build ./dist-v2

$mkdir -p path/to/dir

Create a directory hierarchy. No error if already exists.Example: mkdir -p src/components/ui

$rm -rf path

Remove a file or directory recursively and forcefully.Example: rm -rf node_modules && npm install

Error Handling & Debugging

$set -e

Exit immediately if any command exits with a non-zero status.Example: set -e # script stops on first error

$set -u

Treat unset variables as an error and exit.Example: set -u # catches typos in variable names

$set -x

Print each command before executing it (debug mode).Example: set -x # see every command as it runs

$trap "cmd" EXIT

Run a command when the script exits, even on error.Example: trap "rm -f /tmp/tempfile" EXIT

$trap "cmd" ERR

Run a command when any command exits with non-zero status.Example: trap "echo Error on line $LINENO" ERR

Bash Scripting

Bash shell scripting — variables, conditionals, loops, functions, and common patterns.

All scripts should start with #!/bin/bash and be made executable with chmod +x.

Variables & Substitutions

#!/bin/bash — Shebang — tells the system to use bash to execute the script.

Example: #!/bin/bash echo "Hello, World!"

NAME="value" — Assign a variable. No spaces around =. Reference with $NAME or ${NAME}.

Example: APP_DIR="/opt/myapp" echo "Dir: $APP_DIR"

$1, $2, ... $@ — Positional parameters — arguments passed to the script or function.

Example: echo "Hello, $1!" # ./script.sh Alice → "Hello, Alice!"

$(command) or `command` — Command substitution — capture command output into a variable.

Example: DATE=$(date +%Y-%m-%d)

${VAR:-default} — Use default value if VAR is unset or empty.

Example: echo "Mode: ${MODE:-production}"

Conditionals

if [[ condition ]]; then ... fi — Test condition with [[ ]] (bash-specific, more powerful than [ ]).

Example: if [[ -f "$FILE" ]]; then echo "exists"; fi

[[ -f "$FILE" ]] — Test if a file exists and is a regular file.

Example: if [[ ! -f "config.json" ]]; then cp config.default.json config.json; fi

[[ -d "$DIR" ]] — Test if a directory exists.

Example: if [[ ! -d "dist" ]]; then mkdir dist; fi

[[ -z "$VAR" ]] — Test if a variable is empty (zero-length string).

Example: if [[ -z "$API_KEY" ]]; then echo "Missing API key"; exit 1; fi

[[ "$A" == "$B" ]] — String comparison inside [[ ]]. Use != for inequality.

Example: if [[ "$NODE_ENV" == "production" ]]; then ...

Loops

for item in list; do ... done — Iterate over a list of items.

Example: for file in *.ts; do echo "Processing $file"; done

for ((i=0; i<10; i++)); do ... done — C-style for loop with increment.

Example: for ((i=0; i<${#ARRAY[@]}; i++)); do echo "${ARRAY[$i]}"; done

while read line; do ... done < file — Read a file line by line.

Example: while IFS= read -r line; do echo "Line: $line"; done < data.csv

Functions

func_name() { ... } — Define a bash function.

Example: log() { echo "[$(date +%T)] $*"; } log "Server started"

local VAR="value" — Declare a local variable inside a function. Does not leak to global scope.

Example: get_name() { local name="$1"; echo "$name"; }

return n — Return a numeric exit code from a function (0=success, 1-255=error).

Example: is_installed() { command -v "$1" >/dev/null 2>&1; return $?; }

File Operations

cp -r src dest — Copy files or directories recursively.

Example: cp -r ./build ./dist-v2

mkdir -p path/to/dir — Create a directory hierarchy. No error if already exists.

Example: mkdir -p src/components/ui

rm -rf path — Remove a file or directory recursively and forcefully.

Example: rm -rf node_modules && npm install

Error Handling & Debugging

set -e — Exit immediately if any command exits with a non-zero status.

Example: set -e # script stops on first error

set -u — Treat unset variables as an error and exit.

Example: set -u # catches typos in variable names

set -x — Print each command before executing it (debug mode).

Example: set -x # see every command as it runs

trap "cmd" EXIT — Run a command when the script exits, even on error.

Example: trap "rm -f /tmp/tempfile" EXIT

trap "cmd" ERR — Run a command when any command exits with non-zero status.

Example: trap "echo Error on line $LINENO" ERR