Chapter 5. Quoting
Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an interpretation other than its literal meaning. For example, the asterisk * represents a wild card character in globbing and Regular Expressions).
bash$ ls -l [Vv]* -rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT -rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh -rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh bash$ ls -l '[Vv]*' ls: [Vv]*: No such file or directory |
Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it.
bash$ grep '[Ff]irst' *.txt file1.txt:This is the first line of file1.txt. file2.txt:This is the First line of file2.txt. |
Note that the unquoted grep [Ff]irst *.txt works under the Bash shell. [1]
Quoting can also suppress echo's "appetite" for newlines.
bash$ echo $(ls -l) total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh bash$ echo "$(ls -l)" total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh |
5.1. Quoting Variables
When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). [2] Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 4-1, above).
Use double quotes to prevent word splitting. [3] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.
1 List="one two three" 2 3 for a in $List # Splits the variable in parts at whitespace. 4 do 5 echo "$a" 6 done 7 # one 8 # two 9 # three 10 11 echo "---" 12 13 for a in "$List" # Preserves whitespace in a single variable. 14 do # ^ ^ 15 echo "$a" 16 done 17 # one two three |
A more elaborate example:
1 variable1="a variable containing five words" 2 COMMAND This is $variable1 # Executes COMMAND with 7 arguments: 3 # "This" "is" "a" "variable" "containing" "five" "words" 4 5 COMMAND "This is $variable1" # Executes COMMAND with 1 argument: 6 # "This is a variable containing five words" 7 8 9 variable2="" # Empty. 10 11 COMMAND $variable2 $variable2 $variable2 12 # Executes COMMAND with no arguments. 13 COMMAND "$variable2" "$variable2" "$variable2" 14 # Executes COMMAND with 3 empty arguments. 15 COMMAND "$variable2 $variable2 $variable2" 16 # Executes COMMAND with 1 argument (2 spaces). 17 18 # Thanks, Stéphane Chazelas. |
 | Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue. |
Example 5-1. Echoing Weird Variables
1 #!/bin/bash
2 # weirdvars.sh: Echoing weird variables.
3
4 echo
5
6 var="'(]\\{}\$\""
7 echo $var # '(]\{}$"
8 echo "$var" # '(]\{}$" Doesn't make a difference.
9
10 echo
11
12 IFS='\'
13 echo $var # '(] {}$" \ converted to space. Why?
14 echo "$var" # '(]\{}$"
15
16 # Examples above supplied by Stephane Chazelas.
17
18 echo
19
20 var2="\\\\\""
21 echo $var2 # "
22 echo "$var2" # \\"
23 echo
24 # But ... var2="\\\\"" is illegal. Why?
25 var3='\\\\'
26 echo "$var3" # \\\\
27 # Strong quoting works, though.
28
29
30 # ************************************************************ #
31 # As the first example above shows, nesting quotes is permitted.
32
33 echo "$(echo '"')" # "
34 # ^ ^
35
36
37 # At times this comes in useful.
38
39 var1="Two bits"
40 echo "\$var1 = "$var1"" # $var1 = Two bits
41 # ^ ^
42
43 # Or, as Chris Hiestand points out ...
44
45 if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]
46 # ^ ^ ^ ^ ^ ^ ^ ^
47 then
48 ...
49 fi
50 # ************************************************************ # |
Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting").
 | Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.
|