# Expansions and substitutions

![](keywords>bash shell scripting expansion substitution text variable filename macro wildcard)

Before executing your commands, Bash checks whether there are any syntax
elements in the command line that should be interpreted rather than
taken literally. After splitting the command line into tokens (words),
Bash scans for these special elements and interprets them, resulting in
a changed command line: the elements are said to be **expanded** to or
**substituted** to **new text and maybe new tokens** (words).

The most simple example of this behaviour is a referenced variable:

    mystring="Hello world"
    echo "$mystring"

The `echo` program definitely doesn\'t care about what a shell variable
is. It is Bash\'s job to deal with the variable. Bash **expands** the
string \"`$mystring`\" to \"`Hello world`\", so that `echo` will only
see `Hello world`, not the variable or anything else!

After all these expansions and substitutions are done, all quotes that
are not meant literally (i.e., [the quotes that marked contiguous
words](../../syntax/quoting.md), as part of the shell syntax) are removed from
the commandline text, so the called program won\'t see them. This step
is called **quote-removal**.

## Overview

Saw a possible expansion syntax but don\'t know what it is? Here\'s a
small list.

-   [Parameter expansion](../../syntax/pe.md) (it has its own [overview
    section](../../syntax/pe.md#overview))
    -   `$WORD`
    -   `${STUFF...}`
-   [Pathname expansion](../../syntax/expansion/globs.md)
    -   `*.txt`
    -   `page_1?.html`
-   [Arithmetic expansion](../../syntax/expansion/arith.md)
    -   `$(( EXPRESSION ))`
    -   `$[ EXPRESSION ]`
-   [Command substitution](../../syntax/expansion/cmdsubst.md)
    -   `$( COMMAND )`
    -   `` ` COMMAND ` ``
-   [Tilde expansion](../../syntax/expansion/tilde.md)
    -   `~`
    -   `~+`
    -   `~-`
-   [Brace expansion](../../syntax/expansion/brace.md)
    -   `{X,Y,Z}`
    -   `{X..Y}`
    -   `{X..Y..Z}`
-   [Process substitution](../../syntax/expansion/proc_subst.md)
    -   `<( COMMAND )`
    -   `>( COMMAND )`

## Order

Bash performs expansions and substitutions in a defined order. This
explains why globbing (pathname expansion), for example, is safe to use
on filenames with spaces (because it happens **after** the final word
splitting!).

The order is (from first to last):

-   [Brace expansion](../../syntax/expansion/brace.md)
-   [Tilde expansion](../../syntax/expansion/tilde.md)
-   The following expansions happen at the same time, in a left-to-right
    fashion on the commandline (see below)
    -   [Parameter expansion](../../syntax/pe.md)
    -   [Arithmetic expansion](../../syntax/expansion/arith.md)
    -   [Command substitution](../../syntax/expansion/cmdsubst.md)
-   [Word splitting](../../syntax/expansion/wordsplit.md)
-   [Pathname expansion](../../syntax/expansion/globs.md)

[Process substitution](../../syntax/expansion/proc_subst.md) is performed
**simultaneously** with [parameter expansion](../../syntax/pe.md), [command
substitution](../../syntax/expansion/cmdsubst.md) and [arithmetic
expansion](../../syntax/expansion/arith.md). It is only performed when the
underlying operating system supports it.

The 3 steps [parameter expansion](../../syntax/pe.md), [arithmetic
expansion](../../syntax/expansion/arith.md) and [command
substitution](../../syntax/expansion/cmdsubst.md) happen at the same time in a
left-to-right fashion on nthe commandline. This means

    i=1
    echo $i $((i++)) $i

will output `1 1 2` and not `1 1 1`.