Word expansions

In tilde expansion, parts of words that start with a tilde (~) are substituted with particular pathnames. The part of each word that gets substituted is from the beginning of the word, which is a tilde, up to (but not including) the first slash (/) in the word. If the word does not contain a slash, the whole word is substituted. If any character in the substituted part is quoted, tilde expansion is not performed on the word.

The results of expansion are determined by the format of the substituted part:

When tilde expansion is performed on the value of a variable assignment that occurs during execution of a simple command, the value is considered as a colon-separated list of words and those words are each subject to tilde expansion. For example, the variable assignment

is equivalent to

if the value of HOME variable is /home/foo.

The POSIX standard does not prescribe how the shell should behave when it encounters an error during tilde expansion (e.g., when the HOME variable is not defined). Yash silently ignores any errors during tilde expansion; the part of the word that would be substituted is left intact.

In the POSIXly-correct mode, tilde expansion supports the formats of ~ and ~username only.

Parameter expansion expands to the value of a parameter.

The syntax of typical, simple parameter expansion is ${parameter}, which expands to the value of the parameter whose name is parameter. You can omit the braces (e.g., $parameter) if

If parameter is none of a special parameter, positional parameter, and variable, it is a syntax error. (Some shells other than yash may treat such a case as an expansion error.)

If the unset option is disabled and the parameter is an undefined variable, it is an expansion error. If the unset option is enabled, an undefined variable expands to the empty string.

More complex syntax of parameter expansion allows modifying the value of a parameter.

The spaces in the syntax definition above are for readability only and must be omitted. You can omit prefix, index, and/or modifier.

Command substitution expands to output of commands specified.

When command substitution is evaluated, commands are executed by a subshell with output pipelined to the shell. When the commands finished, command substitution is substituted with the output of the commands. Any trailing newline characters in the output are ignored.

When command substitution of the form $(commands) is parsed, the commands are parsed carefully so that complex commands such as nested command substitution are parsed correctly. If commands start with (, you should put a space before commands so that the whole command substitution is not confused with arithmetic expansion. If the shell is in the POSIXly-correctly mode, the commands are parsed each time the command substitution is expanded; otherwise, commands are parsed only when the command substitution is parsed.

If command substitution is of the form `commands`, the commands are not parsed when the command substitution is parsed; the commands are parsed each time the command substitution is expanded. The end of commands is detected by the first backquote character (`) after the beginning of commands that is not quoted by a backslash. Backquotes that are part of commands (typically used for nested command substitution) must be quoted by backslashes. In commands, backslashes are treated as quotes only when preceding a dollar ($), backquote, newline, or another backslash. Additionally, if the command substitution occurs inside double quotes, double quotes in commands must be quoted with a backslash. Those backslashes are removed before commands are parsed.

Arithmetic expansion evaluates an arithmetic expression and expands to the value of the expression.

When arithmetic expansion is expanded, the expression is subject to parameter expansion, command substitution, and (nested) arithmetic expansion. The expression is parsed in (almost) same manner as an expression of the C programming language.

Yash allows an expression to be either an integer (of the long type in C) or a floating-point number (of the double type in C). An operation on integers yields an integer and an operation involving a floating-point number yields a floating-point number. In the POSIXly-correct mode, you can use integers only.

The following operators are available (in the order of precedence):

The ++ and -- operators cannot be used in the POSIXly-correct mode.

An atomic expression can be one of an integer literal, a floating-point number literal, and a variable. Literals are parsed in the same manner as in C. An octal integer literal starts with 0, and hexadecimal with 0x. A floating-point number literal may have an exponent (i.e. 1.23e+6). A variable with a non-numeric value will result in an error when parsed as a number. An unset variable is treated as a value of zero if the unset option is enabled.

In the POSIXly-correct mode, variables are always parsed as numbers. Otherwise, variables are parsed only when they are used as numbers in computation. Unparsed variables are left intact.

It is an expansion error if the result of an expression is not defined in C.

Brace expansion expands to several split words with preceding and succeeding portions duplicated to each split words. Brace expansion is expanded only when the brace-expand option is enabled.

Comma-separated brace expansion is expanded to each comma-separated word. For example, a{1,2,3}b is expanded to the three words a1b, a2b, and a3b.

Range brace expansion is expanded to integers in the range defined by start and end. The difference between each integer can be defined by delta. If start is larger than end, the results will be in descending order. When ..delta is omitted, it defaults to 1 or -1. For example, a{1..3}b is expanded to the three words a1b, a2b, and a3b; and a{1..7..2}b to the four words a1b, a3b, a5b, and a7b.

Multiple brace expansions can be used in one word. Brace expansions can also be nested. You can quote braces and/or commas to prevent them from being treated as brace expansion.

Any errors in brace expansion are silently ignored.

In field splitting, words are split at predefined separators.

Field splitting can occur only within parts of words that resulted from parameter expansion, command substitution, and arithmetic expansion that are not between double-quotation marks. Expansion results of the @ special parameter are exceptionally split even between double-quotation marks.

Separators used in field splitting are defined by the value of the IFS variable. If the variable does not exist, the value is assumed to be the three characters of space, tab, and newline.

Characters included in the value of the IFS variable are called IFS characters. IFS characters that are any of space, tab, and newline are called IFS whitespace and other IFS characters are called IFS non-whitespace.

Field splitting is performed as follows:

Finally, the last word is removed from the results if:

Pathname expansion performs pattern matching and expands to pathnames matched by the pattern.

A word subjected to pathname expansion is treated as a pattern. If one or more pathnames are found that are matched by the pattern, the pathnames become the results of the pathname expansion.

Pathname expansion is not performed when the glob option is disabled.

The shell searches readable directories for matching pathnames. Unreadable directories are silently ignored.

The following options affect the behavior of pathname expansion:

Any errors in pathname expansion are silently ignored. If the word is an invalid pattern, it just becomes the result. The results depend on the null-glob option when no matching pathnames are found.

Pattern matching is done for each filename (or pathname component) of pathnames. The shell skips matching for literal patterns that contain no wildcards or bracket expressions. As a result, the patterns /*/foo and /*/fo[o] may yield different expansion results when the case-glob option is disabled; for example, the pattern /*/fo[o] matches the pathname /bar/FOO but the pattern /*/foo does not because matching is skipped for foo.