$ Linux Command

Shell

sh

The POSIX shell command interpreter.

shellcommandinterpreterposixscripting

Additional Notes

sh is the standard command language interpreter for POSIX-compliant Unix systems. On most Linux distributions, /bin/sh is a symbolic link to either bash (Bourne Again SHell) running in POSIX mode, dash (Debian Almquist Shell), or ash. The POSIX shell standard (IEEE 1003.1) defines the minimum functionality that all Unix shells must provide.

The shell is both an interactive command interpreter and a scripting language. It supports variables, control structures (if/else, for, while, case), functions, input/output redirection, pipelines, command substitution, and job control. As a scripting language, it is used for system initialization scripts, package management scripts, and automation tasks throughout the operating system.

Syntax

sh [options] [script_file] [arguments...]
sh -c 'command_string' [arguments...]

Parameters

  • script_file: A file containing shell commands to execute.
  • command_string: A string containing shell commands to execute (with -c).
  • arguments: Arguments passed to the script or command string ($0, $1, $2, ...).

Common Options

  • -c string: Read commands from the specified string.
  • -s: Read commands from standard input.
  • -i: Interactive mode (with job control and prompt).
  • -l, --login: Login shell (sources login profile files).
  • -x: Print commands and their arguments as they are executed (debugging).
  • -e: Exit immediately if a command exits with a non-zero status.
  • -n: Read and parse commands but do not execute them (syntax check).
  • -u: Treat unset variables as an error when substituting.
  • -f: Disable pathname expansion (globbing).
  • -v: Print shell input lines as they are read.
  • -a: Mark variables that are modified or created for export.
  • --restricted: Run in restricted mode (rbash, rsh).

Examples

sh

Start an interactive POSIX shell session.

sh script.sh

Execute script.sh in a new shell.

sh -c 'echo "Hello, $USER"'

Execute a command string.

sh -n script.sh

Check script.sh for syntax errors without executing it.

sh -x script.sh

Run script.sh with debug tracing (shows each command before execution).

#!/bin/sh
echo "Hello, World!"

Standard POSIX script shebang line.

Shell Scripting Basics

Variables:

name="World"
echo "Hello, $name"

Conditionals:

if [ -f "$file" ]; then
    echo "File exists"
fi

Loops:

for f in *.txt; do
    echo "$f"
done

Functions:

hello() {
    echo "Hello, $1"
}
hello "World"

Practical Notes

  • Scripts using #!/bin/sh should avoid bash-specific syntax like [[ ]], arrays, and (( )).
  • The POSIX test command [ requires spaces around operators and variables to be quoted.
  • The $() command substitution syntax is POSIX-compliant (prefer it over backticks).
  • Always quote variables ("$var") to avoid word splitting and glob expansion.
  • Use set -e for stricter error handling in scripts.
  • The dash shell (Debian/Ubuntu default /bin/sh) is faster but has fewer features than bash.