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Saija Grzegorek
Bash contains features that appear in other popular shells, and somefeatures that only appear in Bash. Some of the shells that Bash hasborrowed concepts from are the Bourne Shell (sh), the Korn Shell(ksh), and the C-shell (csh and its successor,tcsh). The following menu breaks the features up intocategories, noting which features were inspired by other shells andwhich are specific to Bash.
Bash is largely compatible with sh and incorporates usefulfeatures from the Korn shell ksh and the C shell csh.It is intended to be a conformant implementation of the IEEEPOSIX Shell and Tools portion of the IEEE POSIXspecification (IEEE Standard 1003.1).It offers functional improvements over sh for both interactive andprogramming use.
While the GNU operating system provides other shells, includinga version of csh, Bash is the default shell.Like other GNU software, Bash is quite portable. It currently runson nearly every version of Unix and a few other operating systems -independently-supported ports exist for MS-DOS, OS/2,and Windows platforms.
A Unix shell is both a command interpreter and a programminglanguage. As a command interpreter, the shell provides the userinterface to the rich set of GNU utilities. The programminglanguage features allow these utilities to be combined.Files containing commands can be created, and becomecommands themselves. These new commands have the same status assystem commands in directories such as /bin, allowing usersor groups to establish custom environments to automate their commontasks.
Shells also provide a small set of built-incommands (builtins) implementing functionality impossibleor inconvenient to obtain via separate utilities.For example, cd, break, continue, andexec cannot be implemented outside of the shell becausethey directly manipulate the shell itself.The history, getopts, kill, or pwdbuiltins, among others, could be implemented in separate utilities,but they are more convenient to use as builtin commands.All of the shell builtins are described insubsequent sections.
While executing commands is essential, most of the power (andcomplexity) of shells is due to their embedded programminglanguages. Like any high-level language, the shell providesvariables, flow control constructs, quoting, and functions.
Shells offer features geared specifically forinteractive use rather than to augment the programming language. These interactive features include job control, command lineediting, command history and aliases. Each of these features isdescribed in this manual.
Quoting is used to remove the special meaning of certaincharacters or words to the shell. Quoting can be used todisable special treatment for special characters, to preventreserved words from being recognized as such, and to preventparameter expansion.
Once you have the MO files, you decide where to install them and use theTEXTDOMAINDIR shell variable to tell the gettext tools where they are.Make sure to use the same message domain to name the MO files as you did for the PO files when you install them.
If you used that last convention, and you wanted to store the messagecatalog files with Spanish (es) and Esperanto (eo) translations into alocal directory you use for custom translation files, you could run
When all of this is done, and the message catalog files containing thecompiled translations are installed in the correct location,your users will be able to see translated stringsin any of the supported languages by setting the LANG orLC_MESSAGES environment variables before running your script.
More complex shell commands are composed of simple commands arranged togetherin a variety of ways: in a pipeline in which the output of one commandbecomes the input of a second, in a loop or conditional construct, or insome other grouping.
When the shell is in POSIX mode (see Bash POSIX Mode), timemay be followed by a newline. In this case, the shell displays thetotal user and system time consumed by the shell and its children.The TIMEFORMAT variable may be used to specify the format ofthe time information.
Each command in a multi-command pipeline,where pipes are created,is executed in its own subshell, which is aseparate process (see Command Execution Environment).If the lastpipe option is enabled using the shopt builtin(see The Shopt Builtin),the last element of a pipeline may be run by the shell processwhen job control is not active.
Compound commands are the shell programming language constructs.Each construct begins with a reserved word or control operator and isterminated by a corresponding reserved word or operator.Any redirections (see Redirections) associated with a compound commandapply to all commands within that compound command unless explicitly overridden.
Execute consequent-commands as long astest-commands has an exit status which is not zero.The return status is the exit status of the last command executedin consequent-commands, or zero if none was executed.
First, the arithmetic expression expr1 is evaluated accordingto the rules described below (see Shell Arithmetic).The arithmetic expression expr2 is then evaluated repeatedlyuntil it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands areexecuted and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1.The return value is the exit status of the last command in commandsthat is executed, or false if any of the expressions is invalid.
The arithmetic expression is evaluated according to the rulesdescribed below (see Shell Arithmetic).The expression undergoes the same expansionsas if it were within double quotes,but double quote characters in expression are not treated speciallyare removed.If the value of the expression is non-zero, the return status is 0;otherwise the return status is 1.
You can quote any part of the patternto force the quoted portion to be matched literallyinstead of as a regular expression (see above).If the pattern is stored in a shell variable, quoting the variableexpansion forces the entire pattern to be matched literally.
Bracket expressions in regular expressions can be sources of errors as well,since characters that are normally special in regular expressionslose their special meanings between brackets.However, you can use bracket expressions to match special pattern characterswithout quoting them, so they are sometimes useful for this purpose.
The array variable BASH_REMATCH records which parts of the stringmatched the pattern.The element of BASH_REMATCH with index 0 contains the portion ofthe string matching the entire regular expression.Substrings matched by parenthesized subexpressions within the regularexpression are saved in the remaining BASH_REMATCH indices.The element of BASH_REMATCH with index n is the portion of thestring matching the nth parenthesized subexpression.
Bash provides two ways to group a list of commands to be executedas a unit. When commands are grouped, redirections may be appliedto the entire command list. For example, the output of all thecommands in the list may be redirected to a single stream.
Placing a list of commands between parentheses forces the shell to createa subshell (see Command Execution Environment), and eachof the commands in list is executed in that subshell environment.Since the list is executed in a subshell, variable assignments do notremain in effect after the subshell completes.
In addition to the creation of a subshell, there is a subtle differencebetween these two constructs due to historical reasons. The bracesare reserved words, so they must be separated from the listby blanks or other shell metacharacters.The parentheses are operators, and arerecognized as separate tokens by the shell even if they are not separatedfrom the list by whitespace.
This creates a coprocess named NAME.command may be either a simple command (see Simple Commands)or a compound command (see Compound Commands).NAME is a shell variable name.If NAME is not supplied, the default name is COPROC.
If command is a compound command, NAME is optional. Theword following coproc determines whether that word is interpretedas a variable name: it is interpreted as NAME if it is not areserved word that introduces a compound command.If command is a simple command, NAME is not allowed; thisis to avoid confusion between NAME and the first word of the simplecommand.
When the coprocess is executed, the shell creates an array variable(see Arrays)named NAME in the context of the executing shell.The standard output of commandis connected via a pipe to a file descriptor in the executing shell,and that file descriptor is assigned to NAME[0].The standard input of commandis connected via a pipe to a file descriptor in the executing shell,and that file descriptor is assigned to NAME[1].This pipe is established before any redirections specified by thecommand (see Redirections).The file descriptors can be utilized as arguments to shell commandsand redirections using standard word expansions.Other than those created to execute command and process substitutions,the file descriptors are not available in subshells.
GNU Parallel, as its name suggests, can be used to build and run commandsin parallel. You may run the same command with different arguments, whetherthey are filenames, usernames, hostnames, or lines read from files. GNUParallel provides shorthand references to many of the most common operations(input lines, various portions of the input line, different ways to specifythe input source, and so on). Parallel can replace xargs or feedcommands from its input sources to several different instances of Bash.
Shell functions are a way to group commands for later executionusing a single name for the group. They are executed just likea "regular" command.When the name of a shell function is used as a simple command name,the list of commands associated with that function name is executed.Shell functions are executed in the currentshell context; no new process is created to interpret them.
The exit status of a function definition is zero unless a syntax erroroccurs or a readonly function with the same name already exists.When executed, the exit status of a function is the exit status of thelast command executed in the body.
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