[#]: author: (Paul Brown https://www.linux.com/users/bro66)
Using Square Brackets in Bash: Part 1
======
![square brackets][1]
This tutorial tackle square brackets and how they are used in different contexts at the command line.
[Creative Commons Zero][2]
After taking a look at [how curly braces (`{}`) work on the command line][3], now it’s time to tackle brackets (`[]`) and see how they are used in different contexts.
### Globbing
The first and easiest use of square brackets is in _globbing_. You have probably used globbing before without knowing it. Think of all the times you have listed files of a certain type, say, you wanted to list JPEGs, but not PNGs:
```
ls *.jpg
```
Using wildcards to get all the results that fit a certain pattern is precisely what we call globbing.
In the example above, the asterisk means " _zero or more characters_ ". There is another globbing wildcard, `?`, which means " _exactly one character_ ", so, while
```
ls d*k*
```
will list files called _darkly_ and _ducky_ (and _dark_ and _duck_ \-- remember `*` can also be zero characters),
```
ls d*k?
```
will not list _darkly_ (or _dark_ or _duck_ ), but it will list _ducky_.
Square brackets are used in globbing for sets of characters. To see what this means, make directory in which to carry out tests, `cd` into it and create a bunch of files like this:
```
touch file0{0..9}{0..9}
```
(If you don't know why that works, [take a look at the last installment that explains curly braces `{}`][3]).
This will create files _file000_ , _file001_ , _file002_ , etc., through _file097_ , _file098_ and _file099_.
Then, to list the files in the 70s and 80s, you can do this:
```
ls file0[78]?
```
To list _file022_ , _file027_ , _file028_ , _file052_ , _file057_ , _file058_ , _file092_ , _file097_ , and _file98_ you can do this:
```
ls file0[259][278]
```
Of course, you can use globbing (and square brackets for sets) for more than just `ls`. You can use globbing with any other tool for listing, removing, moving, or copying files, although the last two may require a bit of lateral thinking.
Let's say you want to create duplicates of files _file010_ through _file029_ and call the copies _archive010_ , _archive011_ , _archive012_ , etc..
You can't do:
```
cp file0[12]? archive0[12]?
```
Because globbing is for matching against existing files and directories and the _archive..._ files don't exist yet.
Doing this:
```
cp file0[12]? archive0[1..2][0..9]
```
won't work either, because `cp` doesn't let you copy many files to other many new files. Copying many files only works if you are copying them to a directory, so this:
```
mkdir archive
cp file0[12]? archive
```
would work, but it would copy the files, using their same names, into a directory called _archive/_. This is not what you set out to do.
However, if you look back at [the article on curly braces (`{}`)][3], you will remember how you can use `%` to lop off the end of a string contained in a variable.
Of course, there is a way you can also lop of the beginning of string contained in a variable. Instead of `%`, you use `#`.
For practice, you can try this:
```
myvar="Hello World"
echo Goodbye Cruel ${myvar#Hello}
```
It prints " _Goodbye Cruel World_ " because `#Hello` gets rid of the _Hello_ part at the beginning of the string stored in `myvar`.
You can use this feature alongside your globbing tools to make your _archive_ duplicates:
```
for i in file0[12]?;\
do\
cp $i archive${i#file};\
done
```
The first line tells the Bash interpreter that you want to loop through all the files that contain the string _file0_ followed by the digits _1_ or _2_ , and then one other character, which can be anything. The second line `do` indicates that what follows is the instruction or list of instructions you want the interpreter to loop through.
Line 3 is where the actually copying happens, and you use the contents of the loop variable _`i`_ **twice: First, straight out, as the first parameter of the `cp` command, and then you add _archive_ to its contents, while at the same time cutting of _file_. So, if _`i`_ contains, say, _file019_...
```
"archive" + "file019" - "file" = "archive019"
```
the `cp` line is expanded to this:
```
cp file019 archive019
```
Finally, notice how you can use the backslash `\` to split a chain of commands over several lines for clarity.
In part two, we’ll look at more ways to use square brackets. Stay tuned.
