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bash-hackers-wiki/docs/syntax/expansion/intro.md

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---
tags:
- bash
- shell
- scripting
- expansion
- substitution
- text
- variable
- filename
- macro
- wildcard
---
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# Expansions and substitutions
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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 "$mystring"
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The `echo` program definitely doesn't care about what a shell variable
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is. It is Bash's job to deal with the variable. Bash **expands** the
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string \"`$mystring`\" to \"`Hello world`\", so that `echo` will only
see `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
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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
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is called **quote-removal**.
## 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
section](../../syntax/pe.md#overview))
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- `$WORD`
- `${STUFF...}`
Fix hyperlinks of markdown pages at depth 3 find docs/ -depth 3 -name '*.md' | xargs grep '(.*/' -l | \ xargs -I{} \ sed -i '' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2.md#\3)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2/\3.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2/\3.md#\4)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\))%](../../\1.md)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\)#\([^:.>)#][^:.>)#]*\))%](../../\1.md#\2)%g' \ {} Related to https://github.com/flokoe/bash-hackers-wiki/issues/10
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- [Pathname expansion](../../syntax/expansion/globs.md)
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- `*.txt`
- `page_1?.html`
Fix hyperlinks of markdown pages at depth 3 find docs/ -depth 3 -name '*.md' | xargs grep '(.*/' -l | \ xargs -I{} \ sed -i '' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2.md#\3)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2/\3.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2/\3.md#\4)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\))%](../../\1.md)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\)#\([^:.>)#][^:.>)#]*\))%](../../\1.md#\2)%g' \ {} Related to https://github.com/flokoe/bash-hackers-wiki/issues/10
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- [Arithmetic expansion](../../syntax/expansion/arith.md)
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- `$(( EXPRESSION ))`
- `$[ EXPRESSION ]`
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- [Command substitution](../../syntax/expansion/cmdsubst.md)
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- `$( COMMAND )`
- `` ` COMMAND ` ``
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- [Tilde expansion](../../syntax/expansion/tilde.md)
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- `~`
- `~+`
- `~-`
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- [Brace expansion](../../syntax/expansion/brace.md)
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- `{X,Y,Z}`
- `{X..Y}`
- `{X..Y..Z}`
Fix hyperlinks of markdown pages at depth 3 find docs/ -depth 3 -name '*.md' | xargs grep '(.*/' -l | \ xargs -I{} \ sed -i '' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2.md#\3)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)\(.md\)\{0\})%(../../\1/\2/\3.md)%g' \ -e 's%(/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)/\([^/#).][^/#).]*\)#\([a-zA-Z_-][0-9a-zA-Z_-]*\))%(../../\1/\2/\3.md#\4)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\))%](../../\1.md)%g' \ -e 's%](\([^:.>)#][^:.>)#]*\)#\([^:.>)#][^:.>)#]*\))%](../../\1.md#\2)%g' \ {} Related to https://github.com/flokoe/bash-hackers-wiki/issues/10
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- [Process substitution](../../syntax/expansion/proc_subst.md)
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- `<( 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):
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- [Brace expansion](../../syntax/expansion/brace.md)
- [Tilde expansion](../../syntax/expansion/tilde.md)
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- The following expansions happen at the same time, in a left-to-right
fashion on the commandline (see below)
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- [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
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underlying operating system supports it.
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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
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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`.
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