Brace Expansion

Brace expansion takes a list of strings separated by commas and expands those strings into separate arguments for you. The list is enclosed by braces, the symbols { and }, and there should be no spaces around the commas. For example:

$ echo {one,two,red,blue}
one two red blue

Using brace expansion as illustrated in this simple example doesn't offer too much to the user. In fact, the above example requires typing two more characters than simply typing:

echo one two red blue

which produces the same result. However, brace expansion becomes quite useful when the brace-enclosed list occurs immediately before, after or inside another string:

$ echo {one,two,red,blue}fish
onefish twofish redfish bluefish

$ echo fish{one,two,red,blue}
fishone fishtwo fishred fishblue

$ echo fi{one,two,red,blue}sh
fionesh fitwosh firedsh fibluesh

Notice that there are no spaces inside the brackets or between the brackets and the adjoining strings. If you include spaces, it breaks things:

$ echo {one, two, red, blue }fish
{one, two, red, blue }fish

$ echo "{one,two,red,blue} fish"
{one,two,red,blue} fish

However, you can use spaces if they're enclosed in quotes outside the braces or within an item in the comma-separated list:

$ echo {"one ","two ","red ","blue "}fish
one fish two fish red fish blue fish

$ echo {one,two,red,blue}" fish"
one fish two fish red fish blue fish

You also can nest braces, but you must use some caution here too:

$ echo {{1,2,3},1,2,3}
1 2 3 1 2 3

$ echo {{1,2,3}1,2,3}
11 21 31 2 3

Now, after all these examples, you might be thinking to yourself, “Gee, those are great parlor tricks, but why should I care about brace expansion?”

Brace expansion becomes useful when you need to make a backup of a file. This is why it's my favorite shell trick. I use it almost every day when I need to make a backup of a config file before changing it. For example, if I'm making a change to my Apache configuration, I can do the following and save some typing:

$ cp /etc/httpd/conf/httpd.conf{,.bak}

Notice that there is no character between the opening brace and the first comma. It's perfectly acceptable to do this and is useful when adding characters to an existing filename or when one argument is a substring of the other. Then, if I need to see what changes I made later in the day, I use the diff command and reverse the order of the strings inside the braces:

$ diff /etc/httpd/conf/httpd.conf{.bak,}
test
> # I added this comment earlier