ladybird/Base/usr/share/man/man5/Shell.md

Name

The Shell Command Language

Introduction

The shell operates according to the following general steps:

• Some string is read from a source, be it a file, the standard input, or a command string (see Shell(1))
• The shell parses the input to an abstract syntax tree
• The shell performs various expansions and/or resolutions on the nodes
• The shell performs various type checks and syntactic checks
• The shell interprets the AST, evaluating commands as needed
• For each given command, the shell flattens all the string/list arguments
• For each given command, the shell records the applicable redirections
• Should a command be executed, the shell applies the redirections, and executes the command with the flattened argument list
• Should a command need waiting, the shell shall wait for the command to finish, and continue execution

Any text below is superceded by the formal grammar defined in the formal grammar section.

General Token Recognition

This section describes the general tokens the language accepts, it should be noted that due to nature of the language, some tokens are valid only in a specific context.

Bareword

String of characters that are not Special or Syntactic Elements

Glob

String of characters containing at least one of *? in bareword position

Single Quoted String

Any sequence of characters between two single quotes (')

Double Quoted String

Any sequence of Double Quoted String Part tokens:

• Barewords
• Single Quotes
• Variable References
• Evaluate expressions
• Escaped sequences

Variable Reference

Any sequence of Identifier characters, or a Special Variable follwing a $

Evaluate expression

Any expression following a $ that is not a variable reference:

• Inline execution: A syntactic list following a $:
• Dynamic evaluation: Any other expression following a $

Lists

Any two expressions joined by the Join operator ( [whitespace]), or a variable reference referring to a list value

• Syntactic Lists: Any list enclosed in parentheses (( and ))

Comments

Any text following a # in bareword position, up to but not including a newline

Keywords

The following tokens:

• for in command name position
• in as a syntactic element of a for expression
• if in command name position, or after the else keyword
• else after a partial if expression

Special characters

Any of the following:

• ; in bareword position
• \\n (a newline) in bareword position
• Any of (){}
• Any of *? not in glob position

Tilde

Any initial path segment starting with the character ~ in bareword position, Optionally followed by a bareword for the username

Redirections

The shell can create various redirections to file descriptors of a command before executing it, the general syntax for redirections is an optional file descriptor, followed by a redirection operator, followed by a destination.

There are four redirection operators corresponding to various file descriptor open modes: Read, Write, WriteAppend and ReadWrite, respectively <, >, >> and <>.

A special syntactic element &fd can reference a file descriptor as a destination.

Redirections take two main forms, Read/Write redirections, and fd closure redirections.

Read/Write

• Allowed operators: all
• Allowed destinations: file paths (any shell expression) and file descriptor references

Close

• Allowed operators: Write (>)
• Allowed destinations: the special "close" reference &-

Examples

# Redirect the standard error to a file, and close the standard input
$ 2> foo 1>&-

# Redirect a file as read-write into the standard input
$ 1<>foo

# Redirect the standard output to /dev/null
$ >/dev/null

Expansions

The shell performs various expansions, in different stages.

• Glob Expansion: Globs shall be expanded to a list.

• Variable Expansion: Variables shall be expanded preserving their types.

• Juxtaposition Expansion: Juxtapositions shall be expanded as list products.

• Other expansions: Tildes, Evaluate expressions, etc. shall be expanded as needed.

Juxtapositions

Any two expressions joined without any operator are considered to be in a Juxtaposition, with the resulting value being the list product of two expressions. For instance, (1 2)(3 4) shall be evaluated to (13 14 23 24) by calculating the list product of the two expressions (1 2) and (3 4).

Tildes

Any bareword starting with a tilde (~) and spanning up to the first path separator (/) - or EOL - is considered to be a tilde expansion with the text between the tilde and the separator being the username, which shall be expanded to a single string containing the home directory of the given username (or the current user if no username is provided).

Evaluate

Evaluate expressions take the general form of a dollar sign ($) followed by some expression, which is evaluated by the rules below.

• Should the expression be a string, it shall be evaluated as a dynamic variable lookup by first evaluating the string, and then looking up the given variable.
• Should the expression be a list or a command, it shall be converted to a command, whose output (from the standard output) shall be captured, and split to a list with the shell local variable IFS (or the default splitter \n (newline, 0x0a)). It should be noted that the shell option inline_exec_keep_empty_segments will determine whether empty segments in the split list shall be preserved when this expression is evaluated, this behaviour is disabled by default.

Commands

A Command is a single simple command, containing arguments and redirections for a single program, or a compound command containing a shell control structure. The shell can evaluate a sequence of commands, a conditional relation between commands, or various semantic elements composed of commands and intrinsics.

Commands can be either calls to Shell builtins, or external programs.

Shell Semantic Elements

The commands can be composed into semantic elements, producing composite commands:

Sequences

A sequence of commands, executed serially independent of each other: Commanad ; Command ; Command ...

It should be noted that a newline (\\n) can be substituted for the semicolon (;).

Example

# Do one thing, then do another
echo foo; echo bar

Logical Relations

A sequence of commands whose execution depends somehow on the result of another

Command && Command && Command ... (AND)

Short-circuiting command evaluations, will cancel the entire chain should any command fails (have a non-zero exit code)

Command || Command || Command ... (OR)

Short-circuiting command evaluation, will continue down the chain if any command fails.

It should be noted that And chains bind more tightly than Or chains, so an expression of the form C1 && C2 || C3 is understood as "evaluate C1, if successful, evaluate C2, if not successful, evaluate C3".

Examples

# Create file if not found
test -f foo.txt || touch foo.txt

# Announce execution status of a command
rm test && echo "deleted!" || echo "failed with $?"

Control Structures

Conditionals

Conditionals can either be expressed with the Logical Relations, or via explicit if expressions. An if expression contains at least a condition and a then clause, and optionally the else keyword followed by an else clause. An else clause may contain another if expression instead of a normal block.

The then clause must be surrounded by braces, but the else clause may also be another if expression.

An if expression evaluates either the then clause or (if available) the else clause, based on the exit code of the condition; should the exit code be zero, the then clause will be executed, and if not, the else clause will.

Examples

# Remove a file if it exists, create it otherwise
if test -e the_file {
    rm the_file
} else {
    touch the_file
}

# Cond chain (if-elseif-else)
if A {
    echo A
} else if B {
    echo B
} else {
    echo C
}

For Loops

For Loops evaluate a sequence of commands once per element in a given list. The shell has two forms of for loops, one with an explicitly named iteration variable, and one with an implicitly named one. The general syntax follows the form for name in expr { sequence }, and allows omitting the name in part to implicitly name the variable it.

A for-loop evaluates the sequence once per every element in the expr, seetting the local variable name to the element being processed.

The Shell shall cancel the for loop if two consecutive commands are interrupted via SIGINT (^C), and any other terminating signal aborts the loop entirely.

Examples

# Iterate over every non-hidden file in the current directory, and prepend '1-' to its name.
$ for * { mv $it 1-$it }

# Iterate over a sequence and write each element to a file
$ for i in $(seq 1 100) { echo $i >> foo }

Subshells

Subshells evaluate a given block in a new instance (fork) of the current shell process. to create a subshell, any valid shell code can be enclosed in braces.

Examples

# Run a block of code in the background, in a subshell, then detach it from the current shell
$ { for * { te $it } }&
$ disown

Functions

A function is a user-defined entity that can be used as a simple command to execute a compound command, optionally with some parameters. Such a function is defined via the syntax below:

function_name(explicitly_named_arguments...) { compound_command }

The function is named function_name, and has some explicitly named arguments explicitly_named_arguments..., which must be supplied by the caller, failure to do so will cause the command to exit with status 1.

The compound command shall be executed whenever the simple command function_name is executed. This execution shall be performed in a new local frame.

Additionally, should the simple command containing the function name be in a pipeline, or requested to be run in the background, this execution shall be moved to a subshell; naturally, in such a case any changes to the shell state (such as variables, aliases, etc) shall not be leaked to the parent shell process.

The passed arguments shall be stored in the special variables * and ARGV, and the explicitly named arguments shall be set, in order, from the first passed argument onwards.

The exit status of a function simple command shall be the exit status of the last command executed within the command, or 0 if the function has no commands. The declaration is not a command, and will not alter the exit status.

Examples

fn(a b c) {
    echo $a $b $c \( $* \)
}

$ fn 1 2 3 4
# 1 2 3 ( 1 2 3 4 )

Formal Grammar

Shell Grammar

toplevel :: sequence?

sequence :: variable_decls? or_logical_sequence terminator sequence
          | variable_decls? or_logical_sequence '&' sequence
          | variable_decls? or_logical_sequence
          | variable_decls? function_decl (terminator sequence)?
          | variable_decls? terminator sequence

function_decl :: identifier '(' (ws* identifier)* ')' ws* '{' toplevel '}'

or_logical_sequence :: and_logical_sequence '|' '|' and_logical_sequence
                     | and_logical_sequence

and_logical_sequence :: pipe_sequence '&' '&' and_logical_sequence
                      | pipe_sequence

terminator :: ';'
            | '\n'

variable_decls :: identifier '=' expression (' '+ variable_decls)? ' '*
                | identifier '=' '(' pipe_sequence ')' (' '+ variable_decls)? ' '*

pipe_sequence :: command '|' pipe_sequence
               | command
               | control_structure '|' pipe_sequence
               | control_structure

control_structure :: for_expr
                   | if_expr
                   | subshell

for_expr :: 'for' ws+ (identifier ' '+ 'in' ws*)? expression ws+ '{' toplevel '}'

if_expr :: 'if' ws+ or_logical_sequence ws+ '{' toplevel '}' else_clause?

else_clause :: else '{' toplevel '}'
             | else if_expr

subshell :: '{' toplevel '}'

command :: redirection command
         | list_expression command?

redirection :: number? '>'{1,2} ' '* string_composite
             | number? '<' ' '* string_composite
             | number? '>' '&' number
             | number? '>' '&' '-'

list_expression :: ' '* expression (' '+ list_expression)?

expression :: evaluate expression?
            | string_composite expression?
            | comment expession?
            | '(' list_expression ')' expression?

evaluate :: '$' '(' pipe_sequence ')'
          | '$' expression          {eval / dynamic resolve}

string_composite :: string string_composite?
                  | variable string_composite?
                  | bareword string_composite?
                  | glob string_composite?

string :: '"' dquoted_string_inner '"'
        | "'" [^']* "'"

dquoted_string_inner :: '\' . dquoted_string_inner?       {concat}
                      | variable dquoted_string_inner?    {compose}
                      | . dquoted_string_inner?
                      | '\' 'x' digit digit dquoted_string_inner?
                      | '\' [abefrn] dquoted_string_inner?

variable :: '$' identifier
          | '$' '$'
          | '$' '?'
          | '$' '*'
          | '$' '#'
          | ...

comment :: '#' [^\n]*

bareword :: [^"'*$&#|()[\]{} ?;<>] bareword?
          | '\' [^"'*$&#|()[\]{} ?;<>] bareword?

bareword_with_tilde_expansion :: '~' bareword?

glob :: [*?] bareword?
      | bareword [*?]

digit :: <native hex digit>
number :: <number in base 10>
identifier :: <string of word characters>