= Shell Expansion =

<<TableOfContents>>

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== Tilde Expansion ==

The tilde (`~`) expands to a directory. Which directory depends on what, if any, characters follow it.

||'''Tilde Prefix'''||'''Expansion'''                                                        ||
||`~`               ||`$HOME`                                                                ||
||`~/foo`           ||`$HOME/foo`                                                            ||
||`~me/foo`         ||subdirectory `foo` within the home directory of user `me`              ||
||`~+/foo`          ||`$PWD/foo`                                                             ||
||`~-/foo`          ||if `$OLDPWD` is set, `$OLDPWD/foo`                                     ||
||`~N`              ||for an integer `N`, the Nth element in the directory stack (`dirs +N`) ||
||`~+N`             ||for an integer `N`, the Nth element in the directory stack (`dirs +N`) ||
||`~-N`             ||for an integer `N`, the -Nth element in the directory stack (`dirs -N`)||

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== Parameter Expansion ==

The simplest form of parameter expansion is `${parameter}`.



=== Conditional Parameter Expansion ===

Conditional parameter expansions provide default values if the parameter is unset or empty.

Suppose that:

{{{
export a=a
export b=
unset c
}}}

||'''Operator'''||'''Parameter Expansion'''||'''Value'''||'''Side Effects'''||
||`:-`          ||`${a:-d}`                ||`a`        ||                  ||
||`:-`          ||`${b:-d}`                ||`d`        ||                  ||
||`:-`          ||`${c:-d}`                ||`d`        ||                  ||
||`-`           ||`${a-d}`                 ||`a`        ||                  ||
||`-`           ||`${b-d}`                 ||``         ||                  ||
||`-`           ||`${c-d}`                 ||`d`        ||                  ||
||`:=`          ||`${a:=d}`                ||`a`        ||                  ||
||`:=`          ||`${b:=d}`                ||`d`        ||`b` is set to `d` ||
||`:=`          ||`${c:=d}`                ||`d`        ||`c` is set to `d` ||
||`=`           ||`${a=d}`                 ||`a`        ||                  ||
||`=`           ||`${b=d}`                 ||``         ||                  ||
||`=`           ||`${c=d}`                 ||`d`        ||`c` is set to `d` ||
||`:?`          ||`${a:?d}`                ||`a`        ||                  ||
||`:?`          ||`${b:?d}`                ||`d`        ||shell exits       ||
||`:?`          ||`${c:?d}`                ||`d`        ||shell exits       ||
||`?`           ||`${a?d}`                 ||`a`        ||                  ||
||`?`           ||`${b?d}`                 ||``         ||                  ||
||`?`           ||`${c?d}`                 ||`d`        ||shell exits       ||
||`:+`          ||`${a:+d}`                ||`d`        ||                  ||
||`:+`          ||`${b:+d}`                ||``         ||                  ||
||`:+`          ||`${c:+d}`                ||``         ||                  ||
||`+`           ||`${a+d}`                 ||`d`        ||                  ||
||`+`           ||`${b+d}`                 ||`d`        ||                  ||
||`+`           ||`${c+d}`                 ||``         ||                  ||

The general rules are that:

 * including a colon (`:`) checks for unset and empty values; excluding it checks for only unset values
 * the subtraction sign (`-`) uses the left value value if possible, otherwise uses the right hand value
 * the equals sign (`=`) uses the left value value if possible, otherwise uses the right hand value ''and'' set the left hand value to be equal to the right hand value
 * the question mark (`?`) uses the left value value if possible, otherwise throws an error and exits the shell
 * the plus sign (`+`) uses the right value value if the left hand value is available, otherwise is empty

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== Command Expansion ==

`$(date)` expands to the output of `date`.

The commands are executed in a subshell, so all side-effects are discarded.

Command expansion can be nested.

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== Arithmetic Expansion ==

`$(( 1 + 1 ))` expands to 2. For a description of all arithmetic operators, see [[Shell/Arithmetic|here]].

The tokens within an arithmetic expansion undergo all of the above and below expansions individually. Arithmetic expansion can be nested.

Variables are expanded, though an empty or unset variable expands to 0. If `$x` expands to a valid number, then `$(($x+1))` and `$((x+1))` are equivalent.

A string literal will not expand, and in fact will raise an error.

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== Word Splitting ==

A command is split into tokens using `$IFS`. (The default is spaces, tabs, and newlines.) Leading and trailing instances of `$IFS` are ignored. Repeated instances of any member of `$IFS` are treated as a singular delimiter for splitting.

Literal null tokens (`""` or `''`) are kept and passed to commands. On the other hand, unquoted null tokens (as resulting from expansions) are removed. The exception is if such an expansion happens ''within'' double quotes (such as `"$var"`), in which case a null token is kept. Note that when such an expansion is part of a larger, non-null token (such as `"x$var"`), the null expansion part is removed.

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== Filename Expansion ==

The unquoted characters `*`, `?`, and `[` trigger filename expansion.

`*` matches any 0 or more characters.

`?` matches any 1 character.

`[...]` matches any character contained within the brackets, except if a forward slash (`/`) is included.



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