1.4 KiB
1.4 KiB
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Expansions and substitutions
Bash is weird in that parentheses, braces and brackets are used not just as markers for different code blocks but as the designators of commands in their own right. The type of bracket you use effects how your input is interpreted.
Below are the main forms of expansion and substitution:
| Representation | Name |
|---|---|
~ . |
Tilde expansion |
{...} |
Brace expansion |
${...} |
Parameter expansion |
$(...) |
Command substitution |
$((...)) |
Arithmetic expansion |
Brace expansion
Brace expansion is for changing a smaller part of a greater whole programmatically. This is best understood by looking at examples:
echo c{a,o,u}t
cat cot cut
echo /tmp/{1..3}/file.txt
/tmp/1/file.txt /tmp/2/file.txt /tmp/3/file.txt
echo {1..5}
1 2 3 4 5
echo {5..1}
5 4 3 2 1
echo {a...c}
a b c
We can also set sequences. If we wanted to count to twenty in intervals of two:
echo {1..20..2}
1 3 5 7 9 11 13 15 17 19
Note that we type two dots not an elipsis
Example use case
We might use brace expansion to generate sequential file names, eg.
touch file_{01..12}{a..d}
ls
file_01a
file_01b
file_01c
file_01d
file_02a
file_02b
...
file_12d