What Is /bin/sh

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Last updated: April 11, 2026

Quick Answer: /bin/sh is the Bourne shell, a POSIX-compliant command interpreter created by Stephen Bourne in 1977 and traditionally the default system shell on Unix and Unix-like systems. On modern systems, /bin/sh is typically a symbolic link to another shell like bash or dash, but maintains compatibility with POSIX shell scripts and standards.

Key Facts

Stephen Bourne created the original Bourne shell in 1977 at AT&T Bell Labs
POSIX (Portable Operating System Interface) established /bin/sh as the standard shell specification
On Linux systems, /bin/sh is often symlinked to dash (Debian Almquist Shell) or bash
System scripts and cron jobs use /bin/sh as the default interpreter for portability
The Bourne shell introduced features like pipes, redirection, and shell scripting that remain standard today

Overview

/bin/sh is the system shell interpreter, a command-line program that reads and executes user commands and shell scripts on Unix and Unix-like systems. Created by Stephen Bourne in 1977 at AT&T Bell Labs, the Bourne shell became the standard interactive and scripting shell for decades and influenced most modern shell designs.

On contemporary Linux and Unix systems, /bin/sh typically exists as a symbolic link pointing to another shell such as bash (Bourne Again Shell), dash (Debian Almquist Shell), or other POSIX-compliant shells. The POSIX standard formally specified /bin/sh as the location where a standards-compliant shell must be available, making it essential for system portability and script compatibility.

How It Works

The /bin/sh shell operates through a straightforward process when you execute a command or script:

Command Reading: /bin/sh reads input from the terminal (interactive mode) or from a script file (non-interactive mode), processing each line sequentially.
Parsing and Tokenization: The shell parses the input into tokens, identifying commands, arguments, operators, and special characters like pipes (|) and redirections (>, <).
Variable Expansion: /bin/sh expands variables, wildcards, and special parameters before executing commands, allowing dynamic script behavior and file pattern matching.
Command Execution: The shell locates the executable command in the system PATH directories and executes it with the provided arguments, managing file descriptors for input/output redirection.
Exit Status Management: /bin/sh captures the exit status of executed commands, allowing scripts to use conditional logic based on success or failure of previous operations.

Key Comparisons

Feature	/bin/sh (POSIX)	bash (Bourne Again)	zsh
POSIX Compliant	Yes (by definition)	Yes, with extensions	No, custom syntax
Script Portability	Maximum portability across Unix systems	Portable with bashisms	Requires zsh installed
Performance	Lightweight and fast execution	Slightly heavier, more features	More resource intensive
Default System Usage	Standard for system scripts	Common interactive shell	Advanced user preference
Interactive Features	Minimal, POSIX-only	History, completion, advanced editing	Extended features and themes

Why It Matters

System Stability: Using /bin/sh for system scripts ensures reliability across different Unix variants and distributions, as all systems must provide a POSIX-compliant shell at this location.
Portability Guarantee: Scripts written for /bin/sh will execute identically on Linux, BSD, macOS, Solaris, and other Unix-like systems without modification.
Performance Critical: System administrators and developers choose /bin/sh for automated tasks like cron jobs, startup scripts, and deployment processes because of its minimal overhead and universal availability.
Security Consideration: The standardized nature of /bin/sh makes it a target for security audits and hardening, with vulnerabilities receiving immediate attention across the entire Unix ecosystem.

/bin/sh remains foundational to Unix philosophy and system administration. Whether you're writing shell scripts, configuring system automation, or troubleshooting Unix systems, understanding /bin/sh provides essential knowledge for effective system interaction and scripting. The shell's longevity since 1977 demonstrates the enduring importance of its core design principles.