mirror of
https://github.com/flokoe/bash-hackers-wiki.git
synced 2024-11-01 23:03:05 +01:00
92 lines
3.3 KiB
Markdown
92 lines
3.3 KiB
Markdown
|
# Expansions and substitutions
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
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), 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) (it has its own [overview
|
||
|
|
section](/syntax/pe#overview))
|
||
|
|
- `$WORD`
|
||
|
|
- `${STUFF...}`
|
||
|
|
- [Pathname expansion](/syntax/expansion/globs)
|
||
|
|
- `*.txt`
|
||
|
|
- `page_1?.html`
|
||
|
|
- [Arithmetic expansion](/syntax/expansion/arith)
|
||
|
|
- `$(( EXPRESSION ))`
|
||
|
|
- `$[ EXPRESSION ]`
|
||
|
|
- [Command substitution](/syntax/expansion/cmdsubst)
|
||
|
|
- `$( COMMAND )`
|
||
|
|
- `` ` COMMAND ` ``
|
||
|
|
- [Tilde expansion](/syntax/expansion/tilde)
|
||
|
|
- `~`
|
||
|
|
- `~+`
|
||
|
|
- `~-`
|
||
|
|
- [Brace expansion](/syntax/expansion/brace)
|
||
|
|
- `{X,Y,Z}`
|
||
|
|
- `{X..Y}`
|
||
|
|
- `{X..Y..Z}`
|
||
|
|
- [Process substitution](/syntax/expansion/proc_subst)
|
||
|
|
- `<( 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)
|
||
|
|
- [Tilde expansion](/syntax/expansion/tilde)
|
||
|
|
- The following expansions happen at the same time, in a left-to-right
|
||
|
|
fashion on the commandline (see below)
|
||
|
|
- [Parameter expansion](/syntax/pe)
|
||
|
|
- [Arithmetic expansion](/syntax/expansion/arith)
|
||
|
|
- [Command substitution](/syntax/expansion/cmdsubst)
|
||
|
|
- [Word splitting](/syntax/expansion/wordsplit)
|
||
|
|
- [Pathname expansion](/syntax/expansion/globs)
|
||
|
|
|
||
|
|
[Process substitution](/syntax/expansion/proc_subst) is performed
|
||
|
|
**simultaneously** with [parameter expansion](/syntax/pe), [command
|
||
|
|
substitution](/syntax/expansion/cmdsubst) and [arithmetic
|
||
|
|
expansion](/syntax/expansion/arith). It is only performed when the
|
||
|
|
underlying operating system supports it.
|
||
|
|
|
||
|
|
The 3 steps [parameter expansion](/syntax/pe), [arithmetic
|
||
|
|
expansion](/syntax/expansion/arith) and [command
|
||
|
|
substitution](/syntax/expansion/cmdsubst) 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`.
|