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bash-reference

chapter_title stringclasses 9 values	section_title stringlengths 6 77	num_words int64 7 5.33k	num_chars int64 47 32.6k	text stringlengths 47 32.6k
Introduction	What is Bash?	157	954	Bash is the shell, or command language interpreter, for the gnu operating system. The name is an acronym for the ‘Bourne-Again SHell’, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition Bell Labs Research version of Unix. Bash is largely compat...
Introduction	What is a shell?	350	2,378	At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface t...
Definitions	2 Definitions	427	2,569	These definitions are used throughout the remainder of this manual. POSIX A family of open system standards based on Unix. Bash is primarily concerned with the Shell and Utilities portion of the posix 1003.1 standard. blank A space or tab character. whitespace A character belonging to the space character class in t...
Basic Shell Features	3 Basic Shell Features	87	569	Bash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, and the rules for evaluation and quoting are taken from the posix specification for the ‘standard’ Unix shell. This chapter br...
Basic Shell Features	Shell Syntax	113	720	When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the rest of that line. Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting...
Basic Shell Features	Shell Operation	138	870	The following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following: 1. Reads its input from a file, from a string supplied as an argument to the -c invocation option, or from the user’s terminal. 2. Breaks the input into words and operators, obeyi...
Basic Shell Features	Quoting	103	675	Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. Each of the shell metacharacters has special meaning to the she...
Basic Shell Features	Escape Character	63	378	A non-quoted backslash ‘\’ is the Bash escape character. It preserves the literal value of the next character that follows, removing any special meaning it has, with the exception of newline. If a \newline pair appears, and the backslash itself is not quoted, the \newline is treated as a line continuation (that is, it ...
Basic Shell Features	Single Quotes	31	195	Enclosing characters in single quotes (‘’’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Basic Shell Features	Double Quotes	164	1,025	Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘‘’, ‘\’, and, when history expansion is enabled, ‘!’. When the shell is in posix mode, the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. The...
Basic Shell Features	ANSI-C Quoting	167	1,050	Character sequences of the form $’string’ are treated as a special kind of single quotes. The sequence expands to string, with backslash-escaped characters in string replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows: \a alert (bell) \b backspace \e \E An ...
Basic Shell Features	Locale-Specific Translation	780	4,915	Prefixing a double-quoted string with a dollar sign (‘$’), such as $"hello, world", causes the string to be translated according to the current locale. The gettext infrastructure performs the lookup and translation, using the LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN shell variables, as explained below. See the gettex...
Basic Shell Features	Comments	88	569	In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled, a word beginning with ‘#’ introduces a comment. A word begins at the beginning of a line, after unquoted whitespace, or after an operator. The comment causes that word and all remaining characte...
Basic Shell Features	Shell Commands	64	359	A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces. More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditiona...
Basic Shell Features	Reserved Words	101	522	Reserved words are words that have special meaning to the shell. They are used to begin and end the shell’s compound commands. The following words are recognized as reserved when unquoted and the first word of a command (see below for exceptions): if then elif else fi time for in until while do done case esac coproc se...
Basic Shell Features	Simple Commands	78	449	A simple command is the kind of command that’s executed most often. It’s just a sequence of words separated by blanks, terminated by one of the shell’s control operators. The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments. The return status of a simple...
Basic Shell Features	Pipelines	466	2,742	A pipeline is a sequence of one or more commands separated by one of the control operators ‘\|’ or ‘\|&’. The format for a pipeline is [time [-p]] [!] command1 [ \| or \|& command2 ]... The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previ...
Basic Shell Features	Lists of Commands	272	1,560	A list is a sequence of one or more pipelines separated by one of the operators ‘;’, ‘&’, ‘&&’, or ‘\|\|’, and optionally terminated by one of ‘;’, ‘&’, or a newline. Of these list operators, ‘&&’ and ‘\|\|’ have equal precedence, followed by ‘;’ and ‘&’, which have equal precedence. A sequence of one or more newlines may ...
Basic Shell Features	Compound Commands	101	639	Compound commands are the shell programming language constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections associated with a compound command apply to all commands within that compound command unless explicitly overrid...
Basic Shell Features	Looping Constructs	332	1,986	Bash supports the following looping constructs. Note that wherever a ‘;’ appears in the description of a command’s syntax, it may be replaced with one or more newlines. until The syntax of the until command is: until test-commands; do consequent-commands; done Execute consequent-commands as long as test-commands has a...
Basic Shell Features	Conditional Constructs	2,104	12,805	if The syntax of the if command is: if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fi The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status,...
Basic Shell Features	Grouping Commands	210	1,202	Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream. () ( list ) Placing a list of commands between parentheses forces ...
Basic Shell Features	Coprocesses	387	2,330	A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the ‘&’ control operator, with a two-way pipe established between the executing shell and the coprocess. The syntax for a coprocess is: coproc [NAME] comm...
Basic Shell Features	GNU Parallel	121	777	There are ways to run commands in parallel that are not built into Bash. GNU Parallel is a tool to do just that. GNU Parallel, as its name suggests, can be used to build and run commands in parallel. You may run the same command with different arguments, whether they are filenames, usernames, hostnames, or lines read f...
Basic Shell Features	Shell Functions	1,230	7,428	Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" simple command. When the name of a shell function is used as a simple command name, the shell executes the list of commands associated with that function name. Shell functions are e...
Basic Shell Features	Shell Parameters	677	4,121	A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see the description of the declare builtin i...
Basic Shell Features	Positional Parameters	135	807	A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell’s arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameter N may be referenced as ${N}, or as $N when N consists of a singl...
Basic Shell Features	Special Parameters	486	2,844	The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. Special parameters are denoted by one of the following characters. * ($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional ...
Basic Shell Features	Shell Expansions	178	1,187	Expansion is performed on the command line after it has been split into tokens. Bash performs these expansions: • brace expansion • tilde expansion • parameter and variable expansion • command substitution • arithmetic expansion • word splitting • filename expansion • quote removal The order of expansions is: brace ex...
Basic Shell Features	Brace Expansion	480	2,990	Brace expansion is a mechanism to generate arbitrary strings sharing a common prefix and suffix, either of which can be empty. This mechanism is similar to filename expansion , but the filenames generated need not exist. Patterns to be brace expanded are formed from an optional preamble, followed by either a series of ...
Basic Shell Features	Tilde Expansion	405	2,460	If a word begins with an unquoted tilde character (‘~’), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as ...
Basic Shell Features	Shell Parameter Expansion	2,859	17,201	The ‘$’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the n...
Basic Shell Features	Command Substitution	493	3,169	Command substitution allows the output of a command to replace the command itself. The standard form of command substitution occurs when a command is enclosed as follows: $(command) or (deprecated) ‘command‘. Bash performs command substitution by executing command in a subshell environment and replacing the command sub...
Basic Shell Features	Arithmetic Expansion	140	943	Arithmetic expansion evaluates an arithmetic expression and substitutes the result. The format for arithmetic expansion is: $(( expression )) The expression undergoes the same expansions as if it were within double quotes, but unescaped double quote characters in expression are not treated specially and are removed. Al...
Basic Shell Features	Process Substitution	143	873	Process substitution allows a process’s input or output to be referred to using a filename. It takes the form of <(list) or >(list) The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion. If the...
Basic Shell Features	Word Splitting	342	2,134	The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. Words that were not expanded are not split. The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into fields us...
Basic Shell Features	Filename Expansion	437	2,584	After word splitting, unless the -f option has been set, Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with a sorted list of filenames matching the pattern, subject to the value of the GLOBSORT shell...
Basic Shell Features	Pattern Matching	774	4,939	Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The nul character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to b...
Basic Shell Features	Quote Removal	26	158	After the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘’’, and ‘"’ that did not result from one of the above expansions are removed.
Basic Shell Features	Redirections	529	3,334	Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. When used with the exec bui...
Basic Shell Features	Redirecting Input	39	231	Redirecting input opens the file whose name results from the expansion of word for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified. The general format for redirecting input is: [n]<word
Basic Shell Features	Redirecting Output	131	743	Redirecting output opens the file whose name results from the expansion of word for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. The general format for redirecting output is: [n]>[...
Basic Shell Features	Appending Redirected Output	51	293	Redirecting output in this fashion opens the file whose name results from the expansion of word for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created. The general format for appending output is: [n]>>word
Basic Shell Features	Redirecting Standard Output and Standard Error	86	530	This construct redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word. There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms, the first is preferred. This is semantica...
Basic Shell Features	Appending Standard Output and Standard Error	51	327	This construct appends both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word. The format for appending standard output and standard error is: &>>word This is semantically equivalent to >>word 2>&1 (see Duplicating File Descriptors ...
Basic Shell Features	Here Documents	246	1,557	This type of redirection instructs the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). All of the lines read up to that point then become the standard input (or file descriptor n if n is specified) for a command. The format of here-documents is: [n]...
Basic Shell Features	Here Strings	61	392	A variant of here documents, the format is: [n]<<< word The word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Filename expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the...
Basic Shell Features	Duplicating File Descriptors	158	884	The redirection operator [n]<&word is used to duplicate input file descriptors. If word expands to one or more digits, file descriptor n is made to be a copy of that file descriptor. It is a redirection error if the digits in word do not specify a file descriptor open for input. If word evaluates to ‘-’, file descripto...
Basic Shell Features	Moving File Descriptors	57	364	The redirection operator [n]<&digitmoves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n. Similarly, the redirection operator [n]>&digitmoves the file descriptor digit to file descriptor n, or the standard outpu...
Basic Shell Features	Opening File Descriptors for Reading and Writing	42	226	The redirection operator [n]<>word opens the file whose name is the expansion of word for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.
Basic Shell Features	Simple Command Expansion	282	1,805	When the shell executes a simple command, it performs the following expansions, assignments, and redirections, from left to right, in the following order. 1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. 2. The words that...
Basic Shell Features	Command Search and Execution	348	1,982	After a command has been split into words, if it results in a simple command and an optional list of arguments, the shell performs the following actions. 1. If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked. 2. If the name d...
Basic Shell Features	Command Execution Environment	466	2,808	The shell has an execution environment, which consists of the following: • Open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin. • The current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation. • The file creation mode mask as se...
Basic Shell Features	Environment	256	1,587	When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form name=value. Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking i...
Basic Shell Features	Exit Status	311	1,808	The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under ce...
Basic Shell Features	Signals	602	3,514	When Bash is interactive, in the absence of any traps, it ignores SIGTERM (so that ‘kill 0’ does not kill an interactive shell), and catches and handles SIGINT (so that the wait builtin is interruptible). When Bash receives a SIGINT, it breaks out of any executing loops. In all cases, Bash ignores SIGQUIT. If job contr...
Basic Shell Features	Shell Scripts	730	4,357	A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the -c nor -s option is supplied, Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. If the filename does no...
Basic Shell Features	Bourne Shell Builtins	3,821	21,987	The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the posix standard. : (a colon) : [arguments] Do nothing beyond expanding arguments and performing redirections. The return status is zero. . (a period) . [-p path] filename [arguments] The. command ...
Basic Shell Features	Bash Builtin Commands	4,958	29,771	This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the posix standard. alias alias [-p] [name[=value]...] Without arguments or with the -p option, alias prints the list of aliases on the standard output in a form that allows them to be reus...
Basic Shell Features	The Set Builtin	1,408	8,401	This builtin is so complicated that it deserves its own section. set allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables. set set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [argument...] set [+abefhkmnptuvxBCEHPT] [+o option-nam...

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