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5.2 Writing full configuration files directly

grub.cfg is written in GRUB’s built-in scripting language, which has a syntax quite similar to that of GNU Bash and other Bourne shell derivatives.

Words

A word is a sequence of characters considered as a single unit by GRUB. Words are separated by metacharacters, which are the following plus space, tab, and newline:

{ } | & $ ; < >

Quoting may be used to include metacharacters in words; see below.

Reserved words

Reserved words have a special meaning to GRUB. The following words are recognised as reserved when unquoted and either the first word of a simple command or the third word of a for command:

! [[ ]] { }
case do done elif else esac fi for function
if in menuentry select then time until while

Not all of these reserved words have a useful purpose yet; some are reserved for future expansion.

Quoting

Quoting is used to remove the special meaning of certain characters or words. It can be used to treat metacharacters as part of a word, to prevent reserved words from being recognised as such, and to prevent variable expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of newline.

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.

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of ‘$’ and ‘\’. The ‘$’ character retains its special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: ‘$’, ‘"’, ‘\’, or newline. A backslash-newline pair is treated as a line continuation (that is, it is removed from the input stream and effectively ignored6). A double quote may be quoted within double quotes by preceding it with a backslash.

Variable expansion

The ‘$’ character introduces variable expansion. The variable name 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.

Normal variable names begin with an alphabetic character, followed by zero or more alphanumeric characters. These names refer to entries in the GRUB environment (see Environment).

Positional variable names consist of one or more digits. They represent parameters passed to function calls, with ‘$1’ representing the first parameter, and so on.

The special variable name ‘?’ expands to the exit status of the most recently executed command. When positional variable names are active, other special variable names ‘@’, ‘*’ and ‘#’ are defined and they expand to all positional parameters with necessary quoting, positional parameters without any quoting, and positional parameter count respectively.

Comments

A word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored.

Simple commands

A simple command is a sequence of words separated by spaces or tabs and terminated by a semicolon or a newline. The first word specifies the command to be executed. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status. If the reserved word ! precedes the command, then the return value is instead the logical negation of the command’s exit status.

Compound commands

A compound command is one of the following:

for name in word …; do list; done

The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. The return value is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.

if list; then list; [elif list; then list;] … [else list;] fi

The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.

while cond; do list; done
until cond; do list; done

The while command continuously executes the do list as long as the last command in cond returns an exit status of zero. The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in cond returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.

function name { command; … }

This defines a function named name. The body of the function is the list of commands within braces, each of which must be terminated with a semicolon or a newline. This list of commands will be executed whenever name is specified as the name of a simple command. Function definitions do not affect the exit status in $?. When executed, the exit status of a function is the exit status of the last command executed in the body.

menuentry title [--class=class …] [--users=users] [--unrestricted] [--hotkey=key] { command; … }

See menuentry.

Built-in Commands

Some built-in commands are also provided by GRUB script to help script writers perform actions that are otherwise not possible. For example, these include commands to jump out of a loop without fully completing it, etc.

break [n]

Exit from within a for, while, or until loop. If n is specified, break n levels. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless n is not greater than or equal to 1.

continue [n]

Resume the next iteration of the enclosing for, while or until loop. If n is specified, resume at the nth enclosing loop. n must be greater than or equal to 1. If n is greater than the number of enclosing loops, the last enclosing loop (the top-level loop) is resumed. The return value is 0 unless n is not greater than or equal to 1.

return [n]

Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed in the function body. If used outside a function the return status is false.

shift [n]

The positional parameters from n+1 … are renamed to $1…. Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0.


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