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Unix shell
Unix shell
Unix shell
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tcsh and sh shell windows on a Mac OS X Leopard[1] desktop

A Unix shell is a shell that provides a command-line user interface for a Unix-like operating system. A Unix shell provides a command language that can be used either interactively or for writing a shell script.[2] A user typically interacts with a Unix shell via a terminal emulator; however, direct access via serial hardware connections or Secure Shell are common for server systems. Although use of a Unix shell is popular with some users, others prefer to use a graphical shell in a windowing system, such as those provided in desktop Linux distributions or macOS, instead of a command-line interface.

A user may have access to multiple Unix shells with one configured to run by default when the user logs in interactively. The default selection is typically stored in a user's profile; for example, in the local passwd file or in a distributed configuration system such as NIS or LDAP. A user may use other shells nested inside the default shell.

A Unix shell may provide many features including: variable definition and substitution, command substitution, filename wildcarding, stream piping, control flow structures (condition-testing and iteration), working directory context, and here document.

Early shells

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The first Unix shell was the Thompson shell, sh, written by Ken Thompson at Bell Labs and distributed with Versions 1 through 6 of Unix, from 1971 to 1975.[3] Though rudimentary by modern standards, it introduced many of the basic features common to all later Unix shells, including piping, simple control structures using if and goto, and filename wildcarding. Though not in current use, it is still available as part of some Ancient UNIX systems.

It was modeled after the Multics shell, developed in 1965 by American software engineer Glenda Schroeder. Schroeder's Multics shell was itself modeled after the RUNCOM program Louis Pouzin showed to the Multics Team. The "rc" suffix on some Unix configuration files (e.g. ".bashrc" or ".vimrc"), is a remnant of the RUNCOM ancestry of Unix shells.[1][4]

The PWB shell or Mashey shell, sh, was an upward-compatible version of the Thompson shell, augmented by John Mashey and others and distributed with the Programmer's Workbench UNIX, circa 1975–1977. It focused on making shell programming practical, especially in large shared computing centers. It added shell variables (precursors of environment variables, including the search path mechanism that evolved into $PATH), user-executable shell scripts, and interrupt-handling. Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, and while/end/break/continue. As shell programming became widespread, these external commands were incorporated into the shell itself for performance.

But the most widely distributed and influential of the early Unix shells were the Bourne shell and the C shell. Both shells have been used as the coding base and model for many derivative and work-alike shells with extended feature sets.[5]

Bourne shell

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The Bourne shell, sh, was a new Unix shell by Stephen Bourne at Bell Labs.[6] Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the later Unix shells, including here documents, command substitution, more generic variables and more extensive builtin control structures. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68.[7] Traditionally, the Bourne shell program name is sh and its path in the Unix file system hierarchy is /bin/sh. But a number of compatible work-alikes are also available with various improvements and additional features. On many systems, sh may be a symbolic link or hard link to one of these alternatives:

The POSIX standard specifies its standard shell as a strict subset of the Korn shell, an enhanced version of the Bourne shell. From a user's perspective the Bourne shell was immediately recognized when active by its characteristic default command line prompt character, the dollar sign ($).

C shell

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The C shell, csh, was modeled on the C programming language, including the control structures and the expression grammar. It was written by Bill Joy as a graduate student at University of California, Berkeley, and was widely distributed with BSD Unix.[9][better source needed]

The C shell also introduced many features for interactive work, including the history and editing mechanisms, aliases, directory stacks, tilde notation, cdpath, job control and path hashing. On many systems, csh may be a symbolic link or hard link to TENEX C shell (tcsh), an improved version of Joy's original version. Although the interactive features of csh have been copied to most other shells, the language structure has not been widely copied. The only work-alike is Hamilton C shell, written by Nicole Hamilton, first distributed on OS/2 in 1988 and on Windows since 1992.[10]

Configuration files

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Shells read configuration files in various circumstances. These files usually contain commands for the shell and are executed when loaded; they are usually used to set important variables used to find executables, like $PATH, and others that control the behavior and appearance of the shell. The table in this section shows the configuration files for popular shells.[11]

Configuration file sh ksh csh tcsh bash zsh
/etc/.login login login
/etc/csh.cshrc yes yes
/etc/csh.login login login
~/.tcshrc yes
~/.cshrc yes yes[a]
/etc/ksh.kshrc int.
/etc/sh.shrc int.[b]
$ENV (typically ~/.kshrc)[12] int.[c][d] int. int.[e]
~/.login login login
~/.logout login login
/etc/profile login login login login[f]
~/.profile login login login[g] login[f]
~/.bash_profile login[g]
~/.bash_login login[g]
~/.bash_logout login
~/.bashrc int.+n/login
/etc/zshenv yes
/etc/zprofile login
/etc/zshrc int.
/etc/zlogin login
/etc/zlogout login
~/.zshenv yes
~/.zprofile login
~/.zshrc int.
~/.zlogin login
~/.zlogout login

Explanation:

  • blank means a file is not read by a shell at all.
  • "yes" means a file is always read by a shell upon startup.
  • "login" means a file is read if the shell is a login shell.
  • "n/login" means a file is read if the shell is not a login shell.
  • "int." means a file is read if the shell is interactive.
  1. ^ only if ~/.tcshrc not found
  2. ^ Newer versions of the Bourne Shell only
  3. ^ Available on systems that support the "User Portability Utilities option"; value of the variable must be an absolute path, and it is ignored "if the user's real and effective user IDs or real and effective group IDs are different."[13]
  4. ^ $ENV is $HOME/.shrc in newer versions of the Bourne Shell
  5. ^ Same behavior as sh, but only if invoked as sh (bash 2+) or, since bash 4.2, also if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix).[14]
  6. ^ a b Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)
  7. ^ a b c The first readable file in order of ~/.bash_profile, ~/.bash_login and ~/.profile; and only ~/.profile if invoked as sh or, as of at least Bash 4.2, if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix)

Other shells

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Variations on the Unix shell concept that don't derive from Bourne shell or C shell include the following:[15]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Unix shell

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from Grokipedia
The Unix shell is a command-line interpreter that serves as the primary interface between users and operating systems, enabling the execution of commands, management of processes, and automation through scripting. It functions both interactively, where users enter commands at a prompt, and non-interactively, by processing scripts containing sequences of commands, variables, control structures, and operations. As a core component of Unix systems, the shell interprets user input, expands variables and wildcards, handles redirections and pipelines, and invokes other programs, thereby providing a flexible environment for system administration and application development. The history of the Unix shell traces back to the early 1970s, with developing the initial shell in 1971 for early versions of Unix, which supported basic command execution, redirection, and piping but lacked advanced scripting capabilities. Stephen Bourne developed the (sh), which was released with in 1979, adding essential scripting features such as loops, conditionals, variables, and command substitution, making it the foundational standard for Unix shells. Subsequent developments included Bill Joy's (csh) in 1978 for BSD Unix, which introduced C-like syntax, command history, and aliases to enhance interactivity. The Korn shell (ksh), developed by David Korn in the 1980s, extended the with advanced features like functions, associative arrays, and job control, while maintaining . Key features of Unix shells, as standardized by , include tokenization of input into commands, parameter and command substitution expansions, field splitting for argument handling, pathname expansion for file matching, and support for compound commands like pipelines, loops, and conditionals. Redirection operators allow control over input and output streams, such as < for standard input and > for standard output, while quoting mechanisms preserve literal text or enable expansions. Major implementations continue to evolve; the Bourne-Again Shell (Bash), released in 1989 as part of the GNU Project, combines elements of sh and ksh with additions like command-line editing and support, becoming the default shell on most distributions. These shells adhere to the POSIX Shell Command Language standard, ensuring portability across compliant systems.

Overview

Definition and purpose

The Unix shell is a command-line interpreter that provides a text-based interface to the operating system kernel in systems. It functions by reading lines of input from the user, parsing them into commands and arguments, and executing the corresponding programs or system calls. The primary purposes of the Unix shell are to execute user commands, manage processes by spawning new ones for each invoked program, and enable scripting to automate repetitive tasks through sequences of commands stored in files. As an intermediary layer, the shell translates high-level user instructions into kernel interactions, distinguishing it from the kernel itself—which handles core system resources and —and from graphical user interfaces (GUIs), which use visual metaphors like windows and icons for interaction rather than text-based prompts. The shell's design draws inspiration from the command language of the operating system, including its general functions and nomenclature, with the first implementation created by in 1971 as part of the inaugural version of Unix at .

Usage modes

Unix shells operate in two primary modes: interactive and non-interactive, each tailored to different user needs and system operations. In interactive mode, the shell reads commands directly from the user's input, typically via a terminal, and provides immediate feedback through prompts such as PS1 (defaulting to "$ ") for primary input and PS2 (defaulting to "> ") for continuations. This mode supports real-time interaction, where users enter commands one at a time and receive instant execution results, error messages, or output. Common features include command history for recalling previous inputs and programmable completion for suggesting arguments or filenames, enhancing efficiency during sessions. Unlike non-interactive execution, interactive shells do not automatically exit on certain errors, such as syntax issues in special built-ins, allowing users to correct mistakes on the fly. Non-interactive mode, in contrast, involves the shell reading and executing a sequence of commands from a file or string without user input, ideal for and . Here, the shell processes scripts sequentially, setting up an execution environment that supports portability across systems by adhering to standards like . Error handling is stricter; for instance, non-interactive shells exit with a non-zero status upon encountering expansion errors, redirection failures, or errors from special built-ins like cd or exit. This mode enables reliable , as scripts can run unattended, such as in jobs, without requiring terminal access. Mode switching occurs through specific invocation methods. For example, the -c option forces non-interactive execution of a single command string, as in sh -c 'echo Hello', which reads the command from the argument rather than standard input. Scripts typically use a shebang line, such as #!/bin/[sh](/page/.sh), at the file's beginning to specify the shell interpreter, allowing the system to execute the file directly in non-interactive mode while ensuring portability (though results for shebang are implementation-defined). Interactive mode is the default when the shell starts with standard input connected to a terminal without non-option arguments. The benefits of interactive mode lie in its support for exploratory work and real-time corrections, with features like history reducing repetition and completion minimizing typing errors. Non-interactive mode excels in and consistency, where predefined error handling prevents partial failures in scripted tasks, promoting reliability in system administration and . Together, these modes enable the shell to serve both ad-hoc user interactions and structured, repeatable processes.

History

Origins in early Unix

The Unix shell originated with the development of the Unix operating system at Bell Labs in the late 1960s. Ken Thompson implemented the initial shell, known as the Thompson shell, in 1971 as part of the first edition of Unix, running on a PDP-11/20 computer. This early shell functioned primarily as a basic command interpreter, reading user input lines, parsing them into commands and arguments, locating executable files typically in the /bin directory, and executing them in a forked process before returning control to the user with a prompt. It supported rudimentary wildcard expansion for filename patterns, such as using * to match multiple files, I/O redirection with < for input, > for output, and >> for append, as well as piping with | to chain commands, but lacked support for variables, control structures like loops or conditionals, and advanced scripting capabilities. The 's design drew significant influence from the operating system, on which Thompson and had previously worked. Multics introduced concepts like hierarchical file systems, which Unix adopted to organize directories and files in a tree structure accessible via pathnames, and interactive command-line interfaces that emphasized user-friendly syntax for file manipulation. Additionally, Multics' dynamic I/O stream redirection inspired early Unix features, which the Thompson shell implemented in a basic form. As Unix evolved in the 1970s, particularly with the Programmer's Workbench (PWB) version around 1975–1977, the shell saw incremental enhancements that introduced simple scripting capabilities. The PWB shell, also known as the Mashey shell and based on Thompson's design, added support for single-letter variables, control structures such as and while loops, and switch statements, enabling basic command files for sequential execution with parameter substitution. These developments built on the core interpreter, including its existing redirection and , to facilitate automation of routine tasks, such as , while maintaining compatibility with earlier versions. Early Unix shells, including the Thompson implementation, had key limitations that constrained their utility for complex interactions. Notably, they lacked job control features, such as the ability to run processes in the background (&), suspend them (Ctrl-Z), or manage multiple jobs from a single terminal session, requiring users to wait for each command to complete or use separate logins for parallelism. These shortcomings were later addressed in subsequent shells, such as the , which expanded on redirection and introduced variables and .

Bourne shell era

The Bourne shell was developed by Stephen Bourne at Bell Laboratories in 1977 and released as part of the Seventh Edition of Unix in 1979, replacing the earlier as the default command interpreter. This shell introduced significant advancements in scripting capabilities, establishing it as the first Unix shell to support variables, control structures such as statements and for loops, and user-defined functions, which allowed for more sophisticated command sequencing and automation. These features transformed the shell from a simple command executor into a programmable environment, enabling the creation of reusable scripts for system administration and user tasks. Key innovations in the Bourne shell included support for positional parameters, accessible via $1 for the first argument, ∗forallargumentsasasinglestring,and* for all arguments as a single string, and @ for all arguments as separate quoted strings, facilitating argument passing in scripts. Arithmetic operations were handled through the external expr command, while signal handling was provided by the trap builtin, allowing scripts to respond to interrupts or other signals, such as cleaning up temporary files upon termination. These elements laid the groundwork for modern shell programming, emphasizing portability and integration with Unix commands. The Bourne shell quickly became the standard /bin/sh on Unix systems, serving as the foundation for AT&T's System V Unix and influencing subsequent commercial distributions. Its syntax, inspired by , prioritized scripting efficiency but drew criticism for being less intuitive for interactive use, lacking features like command history and job control that would enhance at the terminal. This limitation prompted the development of alternative shells better suited to everyday interaction, though the Bourne shell's design endured as the basis for standards.

C shell and diversification

The (csh) was developed by William Joy at the , between 1978 and 1979 as part of the Berkeley Software Distribution (BSD) of Unix. It introduced a syntax inspired by for control structures such as conditionals and loops, making it more familiar to C programmers compared to the Bourne shell's scripting-oriented design. Key interactive enhancements included a history mechanism allowing command reuse via substitutions like ! (e.g., !! to repeat the last command), and job control features such as suspending processes with Ctrl-Z and managing background jobs with %job notation, which were contributed by Jim Kulp. In the 1980s, the Tenex C shell (tcsh) emerged as a popular extension of csh, originally developed by Ken Greer at in the late 1970s and further refined into the 1980s. maintained backward compatibility with csh while adding advanced interactive capabilities, including programmable filename completion (e.g., Tab to expand partial paths), command-line editing with or Vi key bindings, and enhanced alias support for abbreviating complex commands. These features addressed user demands for more ergonomic daily interaction, contrasting with the Bourne shell's emphasis on portable scripting. This period marked a diversification in Unix shell designs, driven by tensions between scripting portability—favored in System V Unix for its commercial focus—and interactive usability, which appealed to academic and research environments. The became the default login shell in BSD systems starting with 4BSD releases, reinforcing the divide between BSD's innovative, user-centric extensions and System V's adherence to the original for consistency across vendors. This split highlighted evolving user preferences, with csh and its derivatives prioritizing features like history and job control to streamline terminal sessions in multi-user settings.

Standardization and evolution

The standardization of the Unix shell emerged in the late 1980s as part of broader efforts to promote portability across Unix variants, culminating in the POSIX.2 standard developed by IEEE Working Group 1003.2. This standard, formally known as IEEE Std 1003.2-1992, was ratified in September 1992 after approximately six years of development and defined a minimal, compliant command for the (sh), ensuring consistent behavior for core scripting elements like variable expansion, quoting, and pipelines. Key additions included enhancements to the test command for more robust conditional evaluations, such as support for string comparisons and file tests, and the getopts utility for standardized parsing of command-line options in scripts. It also incorporated job control mechanisms, such as the bg, fg, and jobs utilities, derived from earlier innovations in BSD and Korn shells, enabling users to manage multiple processes asynchronously from interactive sessions. Following its initial publication, the POSIX shell specification evolved through ongoing revisions managed jointly by IEEE and The Open Group, which assumed maintenance responsibilities and integrated it into the . Subsequent updates, including POSIX.1-2001 and POSIX.1-2008, refined the core language while emphasizing portability without mandating advanced constructs like named arrays, prioritizing a lean baseline over expansive features to avoid fragmentation. In the post-1990s era, the standard profoundly influenced open-source ecosystems, particularly through the Project's Bash shell, which implemented full compliance in its POSIX mode and became the default in most distributions, facilitating widespread adoption of standardized scripting practices. This integration supported the proliferation of systems while allowing extensions for contemporary needs, such as improved and utility alignments. A persistent challenge in this evolution has been balancing backward compatibility with legacy scripts against the introduction of new capabilities, such as coprocesses for bidirectional , which remain non-standard extensions in specific implementations rather than core features. Standardization bodies have navigated this by focusing on minimal viable enhancements, ensuring scripts remain portable across compliant environments without requiring proprietary additions.

Core features

Command execution

The Unix shell processes user input by first reading a line from standard input or a script file and it into , which are sequences of characters treated as words or operators. This tokenization follows rules that distinguish between quoted strings, variables, and literal text, ensuring that spaces and other delimiters separate arguments correctly. For instance, the input ls -l file.txt is tokenized into the command "ls", the option "-l", and the argument "file.txt". After tokenization and initial expansions like variable substitution, the shell performs field splitting on the results using the IFS (Internal Field Separator) variable, which defaults to space, tab, and newline, to further divide words into fields. Following parsing, the shell applies filename expansion, also known as globbing, to match patterns against existing files and directories in the current . Patterns such as * (matching any string), ? (matching a single character), and [abc] (matching one of a set) are expanded into lists of matching pathnames; if no matches exist, the original pattern is retained unless the nullglob option is set in some implementations. For example, entering echo *.txt expands to the names of all files ending in ".txt" if any exist, otherwise it echoes the literal "*.txt". This expansion occurs after field splitting but before command execution, and it requires appropriate read and search permissions on directories. To execute a command, the shell first checks if it is a built-in utility or requires searching external executables via the PATH environment variable, a colon-separated list of directories defining the search order. If the command name contains no slashes, the shell searches each directory in PATH for an file matching the name, starting from the leftmost directory; the first match is used. Some shells, such as Bash, maintain a to cache locations of recently executed commands, speeding up subsequent lookups by avoiding repeated PATH scans. If the command is not found, the shell reports an error like "command not found" and sets the to 127. Built-in commands are implemented directly within the shell and execute in the current environment without spawning a new , allowing efficient operations that affect the shell state, such as cd which changes the current working directory. In contrast, external commands are separate executable programs found via PATH, and the shell executes them by forking a and using the exec family of system calls (e.g., execl) to replace the image with the target program. For example, ls is typically an external command, requiring this fork-exec mechanism, while echo may be built-in in some shells for performance. distinguishes special built-ins (e.g., export, cd) that have side effects on the shell environment and may cause the shell to exit on errors, from regular built-ins which do not. The shell handles errors during execution by capturing the of the most recent command, accessible via the special parameter $?, which holds an integer value from 0 to 255 where 0 indicates success. Non-zero values signal failure; for instance, 126 means the command was found but not executable, and 127 indicates not found. Commands print diagnostic messages to for issues like permission denied, and the overall script or interactive session can use $? to implement conditional logic based on these statuses. In interactive mode, the shell typically continues after errors, but in scripts, unhandled non-zero exits may propagate to terminate the script.

Input/output redirection

Input/output redirection in the Unix shell provides mechanisms to alter the default sources and destinations of a command's input and output streams, enabling flexible data handling without modifying the commands themselves. This feature, a core aspect of shell functionality, supports directing standard input (stdin) from files or inline content, routing standard output (stdout) to files or other commands, and managing (stderr) separately or merged. These operators are applied to simple commands or pipelines, enhancing modularity in command-line operations. The basic redirection operators include < for stdin from a file, > for stdout to a file (overwriting if it exists), and >> for appending stdout to a file. For example, command < inputfile reads stdin from inputfile, while command > outputfile writes stdout to outputfile, truncating it if present; command >> outputfile appends instead. These operators default to file descriptor 0 for input and 1 for output but can be prefixed with a descriptor number, such as 2> errorfile to redirect stderr to errorfile. The >| variant forces overwriting even if the noclobber option is set, preventing accidental data loss. Unix shells use file descriptors to manage multiple streams, with 0 representing stdin, 1 for stdout, and 2 for stderr by convention. Redirections can duplicate or merge these, such as 2>&1 to send stderr to the same destination as stdout, thereby combining error messages with standard output, which is useful for logging all command output into a single file. For example, ls > dirlist 2>&1 directs both stdout and stderr to the file dirlist. or >&2 to redirect stdout to stderr. Closing a descriptor uses >&- or <&-, and opening a file for both reading and writing employs <> , as in exec 3<> sharedfile to assign descriptor 3. These capabilities allow precise control over error handling and logging in command sequences. Here documents provide a way to supply multi-line input directly in the command line using <<[delimiter](/page/Delimiter), where the shell reads subsequent lines until encountering the alone on a line, treating them as stdin for the command. For instance:

[cat](/page/Cat) <<EOF This is multi-line input for the command. EOF

[cat](/page/Cat) <<EOF This is multi-line input for the command. EOF

This outputs the enclosed text. The <<- variant strips leading tabs from the input lines and for indentation in scripts. If the is quoted, variable expansions and command substitutions are suppressed, ensuring literal input. Pipes, denoted by |, connect the stdout of one command to the stdin of the next, facilitating unidirectional data streaming and command composition. An example is ls | grep pattern, where ls output filters through grep for lines containing "pattern". Multiple pipes form chains, like ps aux | grep process | awk '{print &#36;1}', processing data sequentially across commands. This mechanism underpins the Unix philosophy of small, composable tools.

Scripting elements

Shell scripting in Unix shells incorporates fundamental programming constructs that allow for conditional execution, , and modular code organization, enabling the automation of complex tasks through scripts saved in files and executed via the shell interpreter. These elements are standardized in the shell command language, providing a portable foundation across compliant implementations. Control structures facilitate decision-making and repetition in scripts. The if statement evaluates a compound list and executes a corresponding then clause if its exit status is zero; multiple elif clauses allow for chained conditions, with an optional else for the default case, all terminated by fi. For multi-way branching, the case statement matches a word against patterns, executing the compound list associated with the first matching pattern and ending with esac. Iteration is supported by loops: a for loop assigns successive words from a list (or positional parameters if omitted) to a variable and executes a compound list for each, bounded by do and done; a loop repeats its body as long as the condition compound list succeeds (exit status zero); conversely, an until loop continues until the condition succeeds. Functions promote by defining named blocks of commands. They are declared using the syntax name() compound-list, where the compound list forms the body; upon invocation, the function executes in the current environment with access to positional parameters shifted accordingly. Within functions, the local special built-in utility declares variables local to the function's scope, preventing interference with the global . The of a function matches that of its last command or zero if none is executed. Arithmetic operations in shell scripts are limited to computations, with no native support for . The arithmetic expansion $((expression)) evaluates an expression—supporting operators like , , , division, and modulus—and substitutes the result, after performing parameter and command substitutions within the expression. Complementarily, the let special built-in evaluates one or more arithmetic expressions as arguments, setting variables to their results if applicable. For example, to compute the sum of two variables a and b into c, one might use let "c = a + b" or c=$((a + b)). These mechanisms handle signed integers within the shell's implementation-defined range. Quoting mechanisms control word expansion and substitution to preserve literal text or selectively enable features like variable . Single quotes ('text') treat all enclosed characters literally, suppressing expansions except for the inability to nest single quotes. Double quotes ("text") preserve literals but allow dollar-sign parameter expansion, backquote command substitution, and escapes for specific characters. A (\) immediately before a non-special character preserves its literal value, while before special characters like $ or `, it escapes their meaning; a trailing continues a line without executing it. These rules ensure precise control over script behavior, such as preventing globbing or word splitting in arguments.

Configuration

Startup files

When a Unix shell starts, it executes a of configuration files known as startup files to initialize the environment, set variables, and define user preferences. These files differ based on whether the shell is invoked as a login shell (typically at terminal login or with the --login option), an interactive non-login shell (such as opening a new terminal window), or a non-interactive shell (running scripts). The behavior varies across shell implementations, but common patterns exist for POSIX-compliant shells like and enhanced variants like Bash and . Global startup files, such as /etc/profile, provide system-wide configurations and are usually read first by login shells across many Unix implementations. This file sets default environment variables and paths applicable to all users. User-specific files, located in the (prefixed by ~), allow customization; for example, ~/.profile serves as the primary user file in -style shells. In practice, login shells execute /etc/profile followed by ~/.profile, though does not strictly mandate this sequence—it is a widely adopted convention in Unix systems. Shell-specific files address non-login interactive sessions or additional behaviors. For Bash, an interactive non-login shell sources ~/.bashrc after checking for its existence, while login shells prioritize ~/.bash_profile (or fallback to ~/.bash_login or ~/.profile if absent). The execution order for Bash login shells is: /etc/profile, then the first readable among ~/.bash_profile, ~/.bash_login, or ~/.profile. For the (csh or ), every shell invocation reads ~/.cshrc (after global /etc/csh.cshrc), but login shells additionally execute ~/.login (after global /etc/csh.login). Non-login s skip the .login files. Common customizations in these files include modifying the PATH environment variable to include user directories or binaries, and setting up aliases for frequently used commands, such as aliasing 'll' to 'ls -l' for convenience. These adjustments ensure a tailored interactive environment without altering system defaults. Environment variables initialized here, like PATH, influence subsequent shell behavior.

Environment setup

In Unix shells, the runtime environment is managed through environment variables, which are name-value pairs passed to processes and their children. These variables can be set using the assignment syntax VAR=value within the shell, but to make them available to child processes, the export special built-in command is used, such as export VAR=value or simply export VAR after assignment. Exported variables are inherited by subprocesses invoked by the shell, forming the environment that influences command execution and application behavior across the process hierarchy. Several common environment variables are widely used to customize the shell's interaction with the user and system. For instance, HOME specifies the path to the user's , USER identifies the current username (though not strictly mandated by POSIX, it is a conventional variable in many implementations), PS1 defines the format of the primary command prompt (e.g., expanded to include the and current directory), and MAIL points to the path of the user's mailbox file for mail notification checks. These variables are typically initialized during shell startup from profile files and can be exported to ensure persistence in child processes. Shell options provide fine-grained control over execution behavior and resource constraints, adjustable via the set special built-in command. The -x option enables by printing each command and its arguments as it is executed (before substitution), aiding in tracing script flow. Similarly, the -e option causes the shell to exit immediately if any simple command terminates with a non-zero status, promoting robust error handling in scripts. Resource limits, such as the maximum writable by the shell and its children, are managed with the ulimit (also accessible as a shell built-in), for example ulimit -f 1024 to restrict files to 1024 blocks. For temporary modifications without affecting the parent shell, the env utility allows overriding or unsetting variables in a subprocess environment. Invoked as env VAR=value command, it executes the specified command with the altered environment, isolating changes to that invocation and its descendants while preserving the original shell state. This approach is particularly useful for running applications with custom configurations, such as env PATH=/opt/bin:$PATH myapp, ensuring the modification does not propagate upward.

Shell variants

POSIX shells

POSIX shells refer to Unix command-line interpreters that implement the Shell and Utilities specification, as defined in IEEE Std 1003.1 and maintained by The Open Group, ensuring a standardized set of behaviors for command execution, scripting, and utility interfaces across compliant systems. This standard mandates a core command language for the sh utility, emphasizing portability without proprietary extensions. A prominent example is the Debian Almquist Shell (), a lightweight derivative of the original (), designed specifically to serve as the POSIX-compliant /bin/sh on systems like and . Other POSIX-compliant implementations include ash, commonly used in embedded systems for its minimal footprint. Key features of POSIX shells include a required set of built-in commands, such as read for input handling and test (or its synonym [) for conditional expressions, which must be available without relying on external binaries to support essential scripting constructs. These shells adhere strictly to the specification's syntax for pipelines, redirections, and variable expansions, but deliberately omit advanced features like associative arrays or non-POSIX globbing patterns to maintain compatibility. Special built-ins, including export, set, and trap, are also mandated to handle environment and signal management in a predictable manner. In practice, POSIX shells are the default choice for system scripts and portable code, invoked via the #!/bin/[sh](/page/.sh) shebang, which guarantees execution on any POSIX-conforming system regardless of the underlying shell implementation. Distributions like prioritize for /bin/sh due to its efficiency in startup time and resource usage compared to feature-rich alternatives, making it ideal for boot scripts and utilities. This focus on minimalism ensures broad interoperability, as scripts written for POSIX sh run unchanged on diverse environments, from embedded systems to servers. However, POSIX shells have limitations in interactive use, lacking enhancements such as command-line editing, history navigation, or tab completion, which are absent from the standard to preserve simplicity and avoid vendor-specific behaviors. As a result, while they excel in scripted, automated tasks requiring reliability across platforms, users often turn to extended shells like Bash or Zsh for daily interactive sessions where features are prioritized over strict adherence.

Enhanced and modern shells

The Korn shell (ksh), developed by David Korn at Bell Laboratories and first announced in 1983, introduced significant enhancements over earlier shells, including support for arrays, built-in , and advanced capabilities such as extended globbing with constructs like *(pattern), ?(pattern), and !(pattern) for more flexible file selection and string manipulation. The ksh93 version, released in 1993, further expanded these with associative arrays via the typeset -A command and improved floating-point precision using double or arithmetic where supported. The GNU Bourne-Again SHell (Bash), created in 1989 by Brian Fox as part of the GNU Project, became the de facto standard shell for distributions due to its POSIX compatibility combined with user-friendly extensions like brace expansion (e.g., {a..z} to generate sequences) and (e.g., <(command) to treat command output as a file). These features enable concise scripting for tasks such as generating file lists or integrating command outputs seamlessly, making Bash the default login shell in major distributions like , , and as of 2025. Z shell (Zsh), first released in 1990 by Paul Falstad, emphasizes extensibility through loadable modules for adding functionality like advanced editing modes and is renowned for its sophisticated system, which predicts arguments based on context, history, and man pages. It also supports theme customization for prompts and shared history across sessions, enhancing interactive use; the open-source Oh My Zsh framework, launched in 2009, has popularized these traits by providing thousands of plugins and themes, fostering widespread adoption among developers for its balance of power and ease. Among modern shells, (Friendly Interactive SHell), initially released in 2005 by Axel Liljencrantz, prioritizes user-friendliness with out-of-the-box , autosuggestions from history, and tab completions that require no initial configuration, diverging from traditional shells by embedding these as core behaviors rather than optional setups. Similarly, Nushell, which reached its first public release in 2019 and has evolved rapidly through the 2020s, revolutionizes data handling by treating pipeline outputs as structured records, lists, or tables instead of plain text, enabling queries like ls | get name to extract fields directly without parsing tools. is used in some Docker containers for its simplicity in workflows.

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