Differences Between Bourne and C Shell Quoting

This article explains quoting in C-type shells by comparing them to Bourne-type shell quoting. If you haven't read Section 27.12 about Bourne shell quoting, please do so now.

As in the Bourne shell, the overall idea of C shell quoting is this: quoting turns off (disables) the special meaning of characters. There are three quoting characters: a single quote ('), a double quote ("), and a backslash (\).

Special Characters

The C shell has a few more special characters in addition to the original Bourne shell:

! { } ~

How Quoting Works

Table 27-2 summarizes the rules; you might want to look back at it while you read the examples.

Table 27-2. C shell quoting characters

Quoting character	Explanation
'`xxx`'	Disable all special characters in `xxx` except `!`.
"`xxx`"	Disable all special characters in `xxx` except `$`, ', and `!`.
\`x`	Disable special meaning of character `x`. At end of line, a `\` treats the newline character like a space (continues line).

The major differences between C and Bourne shell quoting are the following:

The exclamation point (!) character can be quoted only with a backslash. That's true inside and outside single or double quotes. So you can use history substitution (Section 30.8) inside quotes. For example:
```
% grep intelligent engineering file*.txt
grep: engineering: No such file or directory
% grep '!:1-2' !:3
grep 'intelligent engineering' file*.txt
    ...
```
In the Bourne shell, inside double quotes, a backslash (\) stops variable and command substitution (it turns off the special meaning of $ and ').
In the C shell, you can't disable the special meaning of $ or ' inside double quotes. You'll need a mixture of single and double quotes. For example, searching for the string use the `-c' switch takes some work:
```
% fgrep "use the \`-c' switch" *.txt
Unmatched \`.
% fgrep 'use the \`-c\' switch' *.txt
Unmatched '.
% fgrep "use the "'`-c'"' switch" *.txt
hints.txt:Be sure to use the `-c' switch.
```
Section 29.10 shows an amazing pair of aliases that automate complicated C shell quoting problems like this.
In the Bourne shell, single and double quotes include newline characters. Once you open a single or double quote, you can type multiple lines before the closing quote.
In the C shell, if the quotes on a command line don't match, the shell will print an error unless the line ends with a backslash. In other words, to quote more than one line, type a backslash at the end of each line before the last line. Inside single or double quotes, the backslash-newline becomes a newline. Unquoted, backslash-newline is an argument separator:
```
% echo "one\
? two" three\
? four
one
two three four
```