Quoting

The shell provides a mechanism called quoting to selectively suppress unwanted expansions.

Double Quotes

作用

除了以下的特殊字符，其他特殊字符都会被当作一般字符处理：

1. $ dollar sign: parameter expansion, arithmetic expansion
2. \ backslash: escape
3. ``` backtick or backquote: command substitution

   Examples

• Word splitting for spaces is suppressed by double quotes. ```bash $ echo this is a test this is a test

$ echo “this is a test” this is a test

- Word splitting for newlines is suppressed by double quotes.
```bash
$ echo $(cal)
Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
$ echo "$(cal)"
February 2020
Su Mo Tu We Th Fr Sa
                                      1
 2  3  4  5  6  7  8
 9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29

Single Quotes

作用

Single quotes suppress all expansions, even escape character().

Examples

 echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER \n'
 text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER \n

Escaping Characters

作用

Escape character suppresses only a _single _character.

Examples

$ echo "The balance for user $USER is: \$5.00"
The balance for user ronnie is: $5.00

$ mv bad[\&\$\!]filename good_filename

$ echo \\ # escape a escape character.
\

Backslash Escape Sequences

$ sleep 10; echo -e "Time's up\a"  # -e enable interpretation of escape sequences.

$ sleep 10; echo "Time's up" $'\a'