Quoting special characters

Certain characters may have special meaning to the shell, depending on their context. If you want a command line to include any of the special characters that the shell processes, then you may have to quote these characters to force the shell to treat them as simple characters.

You must quote the following characters to avoid their special interpretation:

| $ ( " ) & ` ; \ ' Tab Newline Space

You might need to quote the following characters, depending on their context within a shell command:

* ? [ # ~ = %

In order to quote:	You can:
A single character	Precede the character with a single backslash (`\`) character
All special characters within a string of characters	Enclose the whole string in single quotes
All special characters within a string, except for `$`, `, and `\`	Enclose the whole string in double quotes

For example, these commands search for all occurrences of the string “realtime OS” in the chapter1.html file:

grep realtime\ OS chapter1.html
grep 'realtime OS' chapter1.html
grep "realtime OS" chapter1.html

However, note that:

grep realtime OS chapter1.html

doesn't do what you might expect, as it attempts to find the string “realtime” in the files named OS and chapter1.html.

Depending on the complexity of a command, you might have to nest the quoting. For example:

find -name "*.html" | xargs grep -l '"realtime.*OS"' | less

This command lists all the HTML files that contain a string consisting of realtime, followed by any characters, followed by OS. The command line uses find to locate all of the files with an extension of html and passes the list of files to the xargs command, which executes the given grep command on each file in turn. All of the output from xargs is then passed to less, which displays the output, one screenful at a time.

This command uses quoting in various ways to control when the special characters are processed, and by which process:

If you don't put quotes around the *.html, the shell interprets the *, and passes to find the list of files in the current directory with an extension of html. If you quote the *.html, the shell passes the string as-is to find, which then uses it to match all of the files in this directory and below in the filesystem hierarchy with that extension.
In a similar way, if you don't quote the realtime.*OS string at all, the shell generates a list of files that match the pattern. Quoting it once ("realtime.*OS") works for a single invocation of grep, but this example has the added complexity of the xargs command.
The xargs command takes a command line as its argument, and the shell interprets this command line for each item that's passed to xargs. If you don't want the realtime.*OS string to be interpreted by the shell at all, you need to put nested quotes around the pattern that you want to pass to grep:
```
xargs grep -l '"realtime.*OS"'
  
```
The quoting also indicates when you want to execute the less command. As given, the shell passes the output from all of the invocations of xargs to less. In contrast, this command:
```
find -name "*.html" | xargs 'grep -l "realtime.*OS" | less'
  
```
passes the command:
```
grep -l "realtime.*OS" | less
  
```
to xargs, which will have quite different results—if it works at all.

For more information, see “Quoting” in the entry for ksh in the Utilities Reference.