第38页 | Learning the Bash Shell | 阅读 ‧ 电子书库

Quoting

Sometimes you will want to use special characters literally, i.e., without their special meanings. This is called quoting. If you surround a string of characters with single quotation marks (or quotes), you strip all characters within the quotes of any special meaning they might have.

The most obvious situation where you might need to quote a string is with the echo command, which just takes its arguments and prints them to the standard output. What is the point of this? As you will see in later chapters, the shell does quite a bit of processing on command lines—most of which involves some of the special characters listed in Table 1-6. echo is a way of making the result of that processing available on the standard output.

What if we want to print the string 2 * 3 > 5 is a valid inequality? Suppose you type this:

$ echo 2 * 3 > 5 is a valid inequality.

You would get your shell prompt back, as if nothing happened! But then there would be a new file, with the name 5, containing "2", the names of all files in your current directory, and then the string 3 is a valid inequality. Make sure you understand why.[18]

However, if you type:

$ echo '2 * 3 > 5 is a valid inequality.'

the result is the string, taken literally. You needn't quote the entire line, just the portion containing special characters (or characters you think might be special, if you just want to be sure):

$ echo '2 * 3 > 5' is a valid inequality.

This has exactly the same result.

Notice that Table 1-6 lists double quotes (") as weak quotes. A string in double quotes is subjected to some of the steps the shell takes to process command lines, but not all. (In other words, it treats only some special characters as special.) You'll see in later chapters why double quotes are sometimes preferable; Chapter 7 contains the most comprehensive explanation of the shell's rules for quoting and other aspects of command-line processing. For now, though, you should stick to single quotes.