110 lines
2.4 KiB
Markdown
110 lines
2.4 KiB
Markdown
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---
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tags:
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- shell
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---
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# Expansions and substitutions
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Bash is weird in that parentheses, braces and brackets are used not just as
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markers for different code blocks but as the designators of commands in their
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own right. The type of bracket you use effects how your input is interpreted.
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Below are the main forms of expansion and substitution:
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| Representation | Name |
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| -------------- | -------------------- |
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| `~` . | Tilde expansion |
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| `{..}` | Brace expansion |
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| `${...}` | Parameter expansion |
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| `$(...)` | Command substitution |
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| `$((...))` | Arithmetic expansion |
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## Brace expansion: `{..}`
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Brace expansion is for changing a smaller part of a greater whole
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programmatically. This is best understood by looking at examples:
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```
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echo c{a,o,u}t
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cat cot cut
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```
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```
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echo /tmp/{1..3}/file.txt
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/tmp/1/file.txt /tmp/2/file.txt /tmp/3/file.txt
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```
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```bash
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echo {1..5}
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1 2 3 4 5
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echo {5..1}
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5 4 3 2 1
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echo {a...c}
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a b c
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```
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We can also set sequences. If we wanted to count to twenty in intervals of two
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```bash
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echo {1..20..2}
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1 3 5 7 9 11 13 15 17 19
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```
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> Note that we type _two_ dots **not** an elipsis
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### Example use case
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We might use brace expansion to generate sequential file names using a
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pre-defined naming scheme, eg.
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```
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touch file_{01..12}{a..d}
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ls
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file_01a
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file_01b
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file_01c
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file_01d
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file_02a
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file_02b
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...
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file_12d
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```
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The syntax here basically means: for each of the elements in the first list, run
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the second list against them.
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## Parameter expansion: `${...}`
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We use most frequently for returning the value of stored
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[variables](Variables_and_data_types_in_Bash.md).
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Techically we do not have to use the braces, we can retrieve with just `$var`
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however it's better to use them to minimise interpretation fuck-ups which happen
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a lot.
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When the braces are used, this allows us to transform the values before they are
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returned such as only returning from the 6th character: `${var:6}`.
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## Command substition: `$(...)`
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Command substitution (circle-brackets) allows us to put the output of one
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command inside another. Bash runs the bracketed command in a
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[sub-shell](Shell_sessions.md) and then returns it
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to the main user shell.
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For example:
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```bash
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echo "The current directory is $(pwd)."
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```
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## Arithemtic expansion: `$((...))`
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We use arithmetic expansion when we want to calculate numerical values
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See
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[Working with numbers in Bash](Working_with_numbers_in_Bash.md)
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for more.
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