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dash

man page of dash

dash: command interpreter (shell)

NAME
dash - command interpreter (shell)
SYNOPSIS
dash [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name] [+o option_name] [command_file [argument ...]] dash -c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name] [+o option_name] command_string [command_name [argument ...]] dash -s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name] [+o option_name] [argument ...]

DESCRIPTION

dash is the standard command interpreter for the system. The current version of dash is in the process of being changed to conform with the POSIX 1003.2 and 1003.2a specifications for the shell. This version has many features which make it appear similar in some respects to the Korn shell, but it is not a Korn shell clone (see ksh(1)). Only features designated by POSIX, plus a few Berkeley extensions, are being incorporated into this shell. This man page is not intended to be a tutorial or a complete specification of the shell. Overview The shell is a command that reads lines from either a file or the terminal, interprets them, and generally executes other commands. It is the program that is running when a user logs into the system (although a user can select a different shell with the chsh(1) command). The shell implements a language that has flow control constructs, a macro facility that provides a variety of features in addition to data storage, along with built in history and line editing capabilities. It incorporates many features to aid interactive use and has the advantage that the interpretative language is common to both interactive and non-interactive use (shell scripts). That is, commands can be typed directly to the running shell or can be put into a file and the file can be executed directly by the shell. Invocation If no args are present and if the standard input of the shell is connected to a terminal (or if the -i flag is set), and the -c option is not present, the shell is considered an interactive shell. An interactive shell generally prompts before each command and handles programming and command errors differently (as described below). When first starting, the shell inspects argument 0, and if it begins with a dash '-', the shell is also considered a login shell. This is normally done automatically by the system when the user first logs in. A login shell first reads commands from the files /etc/profile and .profile if they exist. If the environment variable ENV is set on entry to an interactive shell, or is set in the .profile of a login shell, the shell next reads commands from the file named in ENV. Therefore, a user should place commands that are to be executed only at login time in the .profile file, and commands that are executed for every interactive shell inside the ENV file. To set the ENV variable to some file, place the following line in your .profile of your home directory ENV=$HOME/.shinit; export ENV substituting for ''.shinit'' any filename you wish. If command line arguments besides the options have been specified, then the shell treats the first argument as the name of a file from which to read commands (a shell script), and the remaining arguments are set as the positional parameters of the shell ($1, $2, etc). Otherwise, the shell reads commands from its standard input. Argument List Processing All of the single letter options that have a corresponding name can be used as an argument to the -o option. The set -o name is provided next to the single letter option in the description below. Specifying a dash ''-'' turns the option on, while using a plus ''+'' disables the option. The following options can be set from the command line or with the set builtin (described later). -a allexport Export all variables assigned to. -c Read commands from the command_string operand instead of from the standard input. Special parameter 0 will be set from the command_name operand and the positional parameters ($1, $2, etc.) set from the remaining argument operands. -C noclobber Don't overwrite existing files with ''>''. -e errexit If not interactive, exit immediately if any untested command fails. The exit status of a command is considered to be explicitly tested if the command is used to control an if, elif, while, or until; or if the command is the left hand operand of an ''&&'' or ''||'' operator. -f noglob Disable pathname expansion. -n noexec If not interactive, read commands but do not execute them. This is useful for checking the syntax of shell scripts. -u nounset Write a message to standard error when attempting to expand a variable that is not set, and if the shell is not interactive, exit immediately. -v verbose The shell writes its input to standard error as it is read. Useful for debugging. -x xtrace Write each command to standard error (preceded by a '+ ') before it is executed. Useful for debugging. -I ignoreeof Ignore EOF's from input when interactive. -i interactive Force the shell to behave interactively. -l Make dash act as if it had been invoked as a login shell. -m monitor Turn on job control (set automatically when interactive). -s stdin Read commands from standard input (set automatically if no file arguments are present). This option has no effect when set after the shell has already started running (i.e. with set). -V vi Enable the built-in vi(1) command line editor (disables -E if it has been set). -E emacs Enable the built-in emacs(1) command line editor (disables -V if it has been set). -b notify Enable asynchronous notification of background job completion. (UNIMPLEMENTED for 4.4alpha) Lexical Structure The shell reads input in terms of lines from a file and breaks it up into words at whitespace (blanks and tabs), and at certain sequences of characters that are special to the shell called ''operators''. There are two types of operators: control operators and redirection operators (their meaning is discussed later). Following is a list of operators: Control operators: & && ( ) ; ;; | || <newline> Redirection operators: < > >| << >> <& >& <<- <> Quoting Quoting is used to remove the special meaning of certain characters or words to the shell, such as operators, whitespace, or keywords. There are three types of quoting: matched single quotes, matched double quotes, and backslash. Backslash A backslash preserves the literal meaning of the following character, with the exception of <newline>. A backslash preceding a <newline> is treated as a line continuation. Single Quotes Enclosing characters in single quotes preserves the literal meaning of all the characters (except single quotes, making it impossible to put single-quotes in a single-quoted string). Double Quotes Enclosing characters within double quotes preserves the literal meaning of all characters except dollarsign ($), backquote ('), and backslash (\). The backslash inside double quotes is historically weird, and serves to quote only the following characters: $ ' " \ <newline>. Otherwise it remains literal. Reserved Words Reserved words are words that have special meaning to the shell and are recognized at the beginning of a line and after a control operator. The following are reserved words: ! elif fi while case else for then { } do done until if esac Their meaning is discussed later. Aliases An alias is a name and corresponding value set using the alias(1) builtin command. Whenever a reserved word may occur (see above), and after checking for reserved words, the shell checks the word to see if it matches an alias. If it does, it replaces it in the input stream with its value. For example, if there is an alias called ''lf'' with the value ''ls -F'', then the input: lf foobar <return> would become ls -F foobar <return> Aliases provide a convenient way for naive users to create shorthands for commands without having to learn how to create functions with arguments. They can also be used to create lexically obscure code. This use is discouraged. Commands The shell interprets the words it reads according to a language, the specification of which is outside the scope of this man page (refer to the BNF in the POSIX 1003.2 document). Essentially though, a line is read and if the first word of the line (or after a control operator) is not a reserved word, then the shell has recognized a simple command. Otherwise, a complex command or some other special construct may have been recognized. Simple Commands If a simple command has been recognized, the shell performs the following actions: 1. Leading words of the form ''name=value'' are stripped off and assigned to the environment of the simple command. Redirection operators and their arguments (as described below) are stripped off and saved for processing. 2. The remaining words are expanded as described in the section called ''Expansions'', and the first remaining word is considered the command name and the command is located. The remaining words are considered the arguments of the command. If no command name resulted, then the ''name=value'' variable assignments recognized in item 1 affect the current shell. 3. Redirections are performed as described in the next section. Redirections Redirections are used to change where a command reads its input or sends its output. In general, redirections open, close, or duplicate an existing reference to a file. The overall format used for redirection is: [n] redir-op file where redir-op is one of the redirection operators mentioned previously. Following is a list of the possible redirections. The [n] is an optional number, as in '3' (not '[3]'), that refers to a file descriptor. [n]> file Redirect standard output (or n) to file. [n]>| file Same, but override the -C option. [n]>> file Append standard output (or n) to file. [n]< file Redirect standard input (or n) from file. [n1]<&n2 Duplicate standard input (or n1) from file descriptor n2. [n]<&- Close standard input (or n). [n1]>&n2 Duplicate standard output (or n1) to n2. [n]>&- Close standard output (or n). [n]<> file Open file for reading and writing on standard input (or n). The following redirection is often called a ''here-document''. [n]<< delimiter here-doc-text ... delimiter All the text on successive lines up to the delimiter is saved away and made available to the command on standard input, or file descriptor n if it is specified. If the delimiter as specified on the initial line is quoted, then the here-doc-text is treated literally, otherwise the text is subjected to parameter expansion, command substitution, and arithmetic expansion (as described in the section on ''Expansions''). If the operator is ''<<-'' instead of ''<<'', then leading tabs in the here-doc- text are stripped. Search and Execution There are three types of commands: shell functions, builtin commands, and normal programs -- and the command is searched for (by name) in that order. They each are executed in a different way. When a shell function is executed, all of the shell positional parameters (except $0, which remains unchanged) are set to the arguments of the shell function. The variables which are explicitly placed in the environment of the command (by placing assignments to them before the function name) are made local to the function and are set to the values given. Then the command given in the function definition is executed. The positional parameters are restored to their original values when the command completes. This all occurs within the current shell. Shell builtins are executed internally to the shell, without spawning a new process. Otherwise, if the command name doesn't match a function or builtin, the command is searched for as a normal program in the file system (as described in the next section). When a normal program is executed, the shell runs the program, passing the arguments and the environment to the program. If the program is not a normal executable file (i.e., if it does not begin with the "magic number" whose ASCII representation is "#!", so execve(2) returns ENOEXEC then) the shell will interpret the program in a subshell. The child shell will reinitialize itself in this case, so that the effect will be as if a new shell had been invoked to handle the ad-hoc shell script, except that the location of hashed commands located in the parent shell will be remembered by the child. Note that previous versions of this document and the source code itself misleadingly and sporadically refer to a shell script without a magic number as a "shell procedure". Path Search When locating a command, the shell first looks to see if it has a shell function by that name. Then it looks for a builtin command by that name. If a builtin command is not found, one of two things happen: 1. Command names containing a slash are simply executed without performing any searches. 2. The shell searches each entry in PATH in turn for the command. The value of the PATH variable should be a series of entries separated by colons. Each entry consists of a directory name. The current directory may be indicated implicitly by an empty directory name, or explicitly by a single period. Command Exit Status Each command has an exit status that can influence the behaviour of other shell commands. The paradigm is that a command exits with zero for normal or success, and non-zero for failure, error, or a false indication. The man page for each command should indicate the various exit codes and what they mean. Additionally, the builtin commands return exit codes, as does an executed shell function. If a command consists entirely of variable assignments then the exit status of the command is that of the last command substitution if any, otherwise 0. Complex Commands Complex commands are combinations of simple commands with control operators or reserved words, together creating a larger complex command. More generally, a command is one of the following: +

EXIT STATUS

Errors that are detected by the shell, such as a syntax error, will cause the shell to exit with a non-zero exit status. If the shell is not an interactive shell, the execution of the shell file will be aborted. Otherwise the shell will return the exit status of the last command executed, or if the exit builtin is used with a numeric argument, it will return the argument.

ENVIRONMENT

HOME Set automatically by login(1) from the user's login directory in the password file (passwd(4)). This environment variable also functions as the default argument for the cd builtin. PATH The default search path for executables. See the above section Path Search. CDPATH The search path used with the cd builtin. MAIL The name of a mail file, that will be checked for the arrival of new mail. Overridden by MAILPATH. MAILCHECK The frequency in seconds that the shell checks for the arrival of mail in the files specified by the MAILPATH or the MAIL file. If set to 0, the check will occur at each prompt. MAILPATH A colon '':'' separated list of file names, for the shell to check for incoming mail. This environment setting overrides the MAIL setting. There is a maximum of 10 mailboxes that can be monitored at once. PS1 The primary prompt string, which defaults to ''$ '', unless you are the superuser, in which case it defaults to ''# ''. PS2 The secondary prompt string, which defaults to ''> ''. PS4 Output before each line when execution trace (set -x) is enabled, defaults to ''+ ''. IFS Input Field Separators. This is normally set to <space>, <tab>, and <newline>. See the White Space Splitting section for more details. TERM The default terminal setting for the shell. This is inherited by children of the shell, and is used in the history editing modes. HISTSIZE The number of lines in the history buffer for the shell. PWD The logical value of the current working directory. This is set by the cd command. OLDPWD The previous logical value of the current working directory. This is set by the cd command. PPID The process ID of the parent process of the shell.

FILES

$HOME/.profile /etc/profile
SEE ALSO
csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1), getopt(3), passwd(5), environ(7), sysctl(8)
HISTORY
dash is a POSIX-compliant implementation of /bin/sh that aims to be as small as possible. dash is a direct descendant of the NetBSD version of ash (the Almquist SHell), ported to Linux in early 1997. It was renamed to dash in 2002.

BUGS

Setuid shell scripts should be avoided at all costs, as they are a significant security risk. PS1, PS2, and PS4 should be subject to parameter expansion before being displayed. DASH(1)
 
 
 

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