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March 03, 2025 01:15
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POSIX Shell Scripting Cheatsheet

A quick reference guide to POSIX shell scripting, covering syntax, commands, and best practices for writing portable and robust shell scripts.

Basic Syntax & Structure

Script Structure

#!/bin/sh - Shebang line, specifies the interpreter.

This line should be the first line of the script. It tells the system which interpreter to use to execute the script. Using /bin/sh ensures POSIX compliance.

# Comment - Comments start with a #.

Comments are used to explain the code and are ignored by the interpreter.

Commands are executed sequentially, one per line.

Each line typically contains a single command or a control structure.

Semicolons (;) can separate multiple commands on a single line.

Example:
command1; command2

Use exit n to exit the script with status n.

A status of 0 usually indicates success, while a non-zero status indicates failure.

Variables

Variable Assignment

variable=value (No spaces around =).

Example:
name="John Doe"

Variable Access

$variable or ${variable} (safer).

Example:
echo "Hello, $name!"

Read-only Variables

readonly variable

Example:
readonly name

Unsetting Variables

unset variable

Example:
unset name

Special Variables

$0: Script name
$1, $2, …: Arguments
$#: Number of arguments
$?: Exit status of last command
$$: Process ID
$!: PID of last background command

Input and Output

echo message - Prints a message to standard output.

Example:
echo "Hello, world!"

read variable - Reads input from standard input and assigns it to a variable.

Example:
read name

cat filename - Displays the content of a file.

Example:
cat myfile.txt

printf format arguments - Formatted output (like C’s printf).

Example:
printf "Name: %s, Age: %d\n" "John" 30

Control Structures

Conditional Statements (if/then/else/fi)

if condition; then commands [elif condition; then commands] [else commands] fi

Example:

if [ "$name" = "John" ]; then
  echo "Hello, John!"
else
  echo "Hello, stranger!"
fi

Conditions are often enclosed in square brackets [ ]. Note the spaces around the brackets and the condition.

Example:
[ -f "myfile.txt" ] (checks if the file exists)

String comparison: = (equal), != (not equal)
Integer comparison: -eq (equal), -ne (not equal), -lt (less than), -le (less than or equal), -gt (greater than), -ge (greater than or equal)
File tests: -f (file exists), -d (directory exists), -r (readable), -w (writable), -x (executable)

Looping (for/while/until)

for variable in word1 word2 ...; do commands done

Example:

for i in 1 2 3; do
  echo "Number: $i"
done

while condition; do commands done

Example:

i=1
while [ $i -le 3 ]; do
  echo "Number: $i"
  i=$((i + 1))
done

until condition; do commands done

Example:

i=1
until [ $i -gt 3 ]; do
  echo "Number: $i"
  i=$((i + 1))
done

break - Exits the loop.
continue - Skips the current iteration.

Case Statements

case variable in pattern1) commands ;; pattern2) commands ;; *) commands ;; esac

Example:

case "$1" in
  start) echo "Starting service" ;;
  stop) echo "Stopping service" ;;
  *) echo "Usage: $0 {start|stop}" ;;
esac

The *) pattern is the default case, similar to default in other languages.

Commands and Utilities

File Manipulation

ls

List directory contents

mkdir directory

Create a directory

rm file

Remove a file

rmdir directory

Remove an empty directory

cp source destination

Copy a file

mv source destination

Move or rename a file

touch file

Create an empty file or update its timestamp

Text Processing

grep pattern file

Search for a pattern in a file

sed 's/old/new/g' file

Replace text in a file

awk '{print $1}' file

Print the first field of each line in a file

sort file

Sort the lines in a file

uniq file

Remove duplicate lines from a file

cut -d',' -f1 file

Cut out sections of each line of a file

Process Control

ps

List running processes

kill pid

Terminate a process

sleep seconds

Pause execution for a specified number of seconds

command &

Run a command in the background

wait

Wait for all background processes to complete

Functions and Advanced Features

Functions

function_name() { commands } or function function_name { commands }

Example:

my_function() {
  echo "Hello from my_function!"
}

my_function # Call the function

Functions can accept arguments: $1, $2, etc.

Example:

greet() {
  echo "Hello, $1!"
}

greet "John"

return value - Returns a value from the function. The value should be between 0 and 255.

Local variables can be declared using local.

Example:

my_function() {
  local my_var="local value"
  echo $my_var
}

Command Substitution

$(command) or command (deprecated) - Executes a command and substitutes its output.

Example:

date_str=$(date +%Y-%m-%d)
echo "Today is $date_str"

Here Documents

<<DELIMITER text DELIMITER - Redirects multiple lines of input to a command.

Example:

cat <<EOF
Hello, this is a multi-line string.
EOF

Signal Handling

trap 'command' SIGNAL - Executes a command when a signal is received.

Example:

trap 'echo "Exiting..." ; exit 1' SIGINT

Common signals: SIGINT (Ctrl+C), SIGTERM (termination signal), SIGKILL (kill signal, cannot be trapped).