My Favorite bash Tips and Tricks

bash, or the Bourne again shell, is the default shell in most Linux distributions. The popularity of the bash shell amongst Linux and UNIX users is no accident. It has many features to enhance user-friendliness and productivity. Unfortunately, you can't take advantage of those features unless you know they exist.

When I first started using Linux, the only bash feature I took advantage of was going back through the command history using the up arrow. I soon learned additional features by watching others and asking questions. In this article, I'd like to share some bash tricks I've learned over the years.

This article isn't meant to cover all of the features of the bash shell; that would require a book, and plenty of books are available that cover this topic, including Learning the bash Shell from O'Reilly and Associates. Instead, this article is a summary of the bash tricks I use most often and would be lost without.

My favorite bash trick definitely is 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,}
1050a1051
> # I added this comment earlier

Another bash trick I like to use is command substitution. To use command substitution, enclose any command that generates output to standard output inside parentheses and precede the opening parenthesis with a dollar sign, $(command). Command substitution is useful when assigning a value to a variable. This is typical in shell scripts, where a common operation is to assign the date or time to a variable. It also is handy for using the output of one command as an argument to another command. If you want to assign the date to a variable, you can do this:

$ date +%d-%b-%Y
12-Mar-2004

$ today=$(date +%d-%b-%Y)

$ echo $today
12-Mar-2004

I often use command substitution to get information about several RPM packages at once. If I want a listing of all the files from all the RPM packages that have httpd in the name, I simply execute the following:

$ rpm -ql $(rpm -qa | grep httpd)