This chapter briefly summarizes the shell’s ‘building blocks’: commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.

A shell allows execution of gnu commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asyn- chronous commands continue to execute in parallel with the shell while it reads and executes

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 (see Invoking Bash ), Bash reads and executes commands from the file, then exits.

Conditional expressions are used by the [[compound command (see Conditional Constructs) and the test and [builtin commands (see Bourne Shell Builtins). The test and [ commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.

Sep 19, 2022 · Some of the shells that Bash has borrowed 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 into categories, noting which features were inspired by other shells and which are specific to Bash.

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 name.

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 …

The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words (the expansions that would occur if the words were enclosed in double quotes).

This command allows one to pipe input from a shell command into pattern space. Without parameters, the e command executes the command that is found in pattern space and replaces the pattern space with the output; a trailing newline is suppressed.

At shell startup, set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous simple command executed in the foreground, after expansion.