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Quoting a single character with the backslash

Quoting a single character with the backslash

You can prevent the shell from interpreting a character by placing a backslash ("\") in front of it. Here is a shell script that can delete any files that contain an asterisk:

echo This script removes all files that 
echo contain an asterisk in the name.
echo
echo Are you sure you want to remove these files\?
rm -i *\**

The backslash was also necessary before the question mark, which is also a shell meta-character. Without it, the shell would look for all files that match the pattern "files?." If you had the files "files1" and "files2" the script would print out

Are you sure you want to remove these files1 files2

which is not what you want.

The backslash is the "strongest" method of quotation. It works when every other method fails. If you want to place text on two or more lines for readability, but the program expects one line, you need a line continuation character. Just use the backslash as the last character on the line:

% echo This could be \
a very \
long line\!
This could be a very long line!
%

This escapes or quotes the end of line character, so it no longer has a special meaning. In the above example, I also put a backslash before the exclamation point. This is necessary if you are using the C shell, which treats the "!" as a special character. If you are using some other shell, it might not be necessary.

Strong Quoting with the Single Quotes

When you need to quote several character at once, you could use several backslashes:

% echo a\ \ \ \ \ \ \ b

(There are 7 spaces between 'a' and 'b'.) This is ugly but works. It is easier to use pairs of quotation marks to indicate the start and end of the characters to be quoted:

% echo 'a       b'

(The HTML ruins the formatting. Imagine that there are 7 spaces between the a and b. -Bruce) Inside the single quotes, you can include almost all meta-characters:

% echo 'What the *heck* is a $ doing here???'
What the *heck* is a $ doing here???

The above example uses asterisks, dollar signs, and question marks meta-characters. The single quotes should be used when you want the text left alone. If you are using the C shell, the "!" character may need a backslash before it. It depends on the characters next to it. If it is surrounded by spaces, you don't need to use a backslash.

Weak Quotes with the Double Quotes

Sometimes you want a weaker type of quoting: one that doesn't expand meta-characters like "*" or "?," but does expand variables and does command substitution. This can be done with the double quote characters:

% echo "Is your home directory $HOME?"
Is your home directory /home/kreskin/u0/barnett?
% echo "Your current directory is `pwd`"
Your current directory is /home/kreskin/u0/barnett

Once you learn the difference between single quotes and double quotes, you will have mastered a very useful skill. It's not hard. The single quotes are stronger than the double quotes. Got it? Okay. And the backslash is the strongest of all.

Using quotes to include spaces and characters in filenames

If you want to work with files with spaces or special characters in the filename, you may have to use quotes. For instance, if you wanted to create a file with a space in the name, you could use the following:

% cp /dev/null 'a file with spaces in the name'

Normally, the shell uses spaces to determine the end of each argument. Quoting changes that, and the above example only has two arguments. You can also use a backslash before the character. The example below will rename a file with a space in the name, changing the space to an underscore:

% mv a\ file a_file

Using the same techniques, you can deal with any character in a filename:

% mv a 'a?'

At worst, a space in a file makes it difficult to use as an argument. Other characters are very dangerous to use in a filename. In particular, using "?" and "*" in a filename is playing with fire. If you want to delete the file "a?" you may end up deleting more than the single file.

Quotes within Quotes

While having two types of quotes (three if you count the backslash) might seem confusing, in reality it provides you with several ways to solve the same problems. You can put either quotes inside the other. If you want to quote single quotes, use double quotes around it. To quote double quotes, use single quotes. Heck, it's easier to show you:

% echo "Don't do that"
Don't do that
% echo 'The quote of the day is: "TGIF"'
The quote of the day is: "TGIF"
%

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