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92 lines
3.3 KiB
Markdown
92 lines
3.3 KiB
Markdown
# Expansions and substitutions
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Before executing your commands, Bash checks whether there are any syntax
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elements in the command line that should be interpreted rather than
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taken literally. After splitting the command line into tokens (words),
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Bash scans for these special elements and interprets them, resulting in
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a changed command line: the elements are said to be **expanded** to or
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**substituted** to **new text and maybe new tokens** (words).
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The most simple example of this behaviour is a referenced variable:
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mystring="Hello world"
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echo "$mystring"
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The `echo` program definitely doesn't care about what a shell variable
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is. It is Bash's job to deal with the variable. Bash **expands** the
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string "`$mystring`" to "`Hello world`", so that `echo` will only see
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`Hello world`, not the variable or anything else!
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After all these expansions and substitutions are done, all quotes that
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are not meant literally (i.e., [the quotes that marked contiguous
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words](/syntax/quoting.md), as part of the shell syntax) are removed from
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the commandline text, so the called program won't see them. This step is
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called **quote-removal**.
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## Overview
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Saw a possible expansion syntax but don't know what it is? Here's a
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small list.
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- [Parameter expansion](/syntax/pe.md) (it has its own [overview
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section](/syntax/pe.md#overview))
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- `$WORD`
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- `${STUFF...}`
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- [Pathname expansion](/syntax/expansion/globs.md)
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- `*.txt`
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- `page_1?.html`
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- [Arithmetic expansion](/syntax/expansion/arith.md)
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- `$(( EXPRESSION ))`
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- `$[ EXPRESSION ]`
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- [Command substitution](/syntax/expansion/cmdsubst.md)
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- `$( COMMAND )`
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- `` ` COMMAND ` ``
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- [Tilde expansion](/syntax/expansion/tilde.md)
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- `~`
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- `~+`
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- `~-`
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- [Brace expansion](/syntax/expansion/brace.md)
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- `{X,Y,Z}`
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- `{X..Y}`
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- `{X..Y..Z}`
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- [Process substitution](/syntax/expansion/proc_subst.md)
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- `<( COMMAND )`
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- `>( COMMAND )`
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## Order
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Bash performs expansions and substitutions in a defined order. This
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explains why globbing (pathname expansion), for example, is safe to use
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on filenames with spaces (because it happens **after** the final word
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splitting!).
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The order is (from first to last):
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- [Brace expansion](/syntax/expansion/brace.md)
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- [Tilde expansion](/syntax/expansion/tilde.md)
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- The following expansions happen at the same time, in a left-to-right
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fashion on the commandline (see below)
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- [Parameter expansion](/syntax/pe.md)
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- [Arithmetic expansion](/syntax/expansion/arith.md)
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- [Command substitution](/syntax/expansion/cmdsubst.md)
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- [Word splitting](/syntax/expansion/wordsplit.md)
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- [Pathname expansion](/syntax/expansion/globs.md)
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[Process substitution](/syntax/expansion/proc_subst.md) is performed
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**simultaneously** with [parameter expansion](/syntax/pe.md), [command
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substitution](/syntax/expansion/cmdsubst.md) and [arithmetic
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expansion](/syntax/expansion/arith.md). It is only performed when the
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underlying operating system supports it.
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The 3 steps [parameter expansion](/syntax/pe.md), [arithmetic
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expansion](/syntax/expansion/arith.md) and [command
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substitution](/syntax/expansion/cmdsubst.md) happen at the same time in a
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left-to-right fashion on nthe commandline. This means
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i=1
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echo $i $((i++)) $i
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will output `1 1 2` and not `1 1 1`.
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