Chapter 6. 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, such as the wild card character, *.)
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 |
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Certain programs and utilities can still reinterpret or expand special characters in a quoted string. This is an important use of quoting, protecting a command-line parameter from the shell, but still letting the calling program expand it.
Of course, grep [Ff]irst *.txt would not work. |
When referencing a variable, it is generally advisable in enclose it in double quotes (" "). This preserves all special characters within the variable name, except $, ` (backquote), and \ (escape). Keeping $ as a special character permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 5-1, above).
Use double quotes to prevent word splitting. [1] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.
variable1="a variable containing five words" COMMAND This is $variable1 # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words" COMMAND "This is $variable1" # Executes COMMAND with 1 argument: # "This is a variable containing five words" variable2="" # Empty. COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments. COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments. COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces). # Thanks, S.C. |
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Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting is an issue. |
Example 6-1. Echoing Weird Variables
#!/bin/bash
# weirdvars.sh: Echoing weird variables.
var="'(]\\{}\$\""
echo $var # '(]\{}$"
echo "$var" # '(]\{}$" Doesn't make a difference.
IFS='\'
echo $var # '(]\{}$" \ converted to space.
echo "$var" # '(] {}$"
# Examples above supplied by S.C.
exit 0
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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").
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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.
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Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.
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With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character. |
Special meanings of certain escaped characters
- used with echo and sed
- \n
-
means newline
- \r
-
means return
- \t
-
means tab
- \v
-
means vertical tab
- \b
-
means backspace
- \a
-
means "alert" (beep or flash)
- \0xx
-
translates to the octal ASCII equivalent of 0xx
Example 6-2. Escaped Characters
#!/bin/bash # escaped.sh: escaped characters echo; echo echo "\v\v\v\v" # Prints \v\v\v\v # Must use the -e option with 'echo' to print escaped characters. echo -e "\v\v\v\v" # Prints 4 vertical tabs. echo -e "\042" # Prints " (quote, octal ASCII character 42). # Bash, version 2 and later, permits using the $'\xxx' construct. echo $'\n' echo $'\a' echo $'\t \042 \t' # Quote (") framed by tabs. # Assigning ASCII characters to a variable. # ---------------------------------------- quote=$'\042' # " assigned to a variable. echo "$quote This is a quoted string, $quote and this lies outside the quotes." echo # Concatenating ASCII chars in a variable. triple_underline=$'\137\137\137' # 137 is octal ASCII code for "_". echo "$triple_underline UNDERLINE $triple_underline" ABC=$'\101\102\103\010' # 101, 102, 103 are octal A, B, C. echo $ABC echo; echo escape=$'\033' # 033 is octal for escape. echo "\"escape\" echoes as $escape" echo; echo exit 0See Example 35-1 for another example of the $' ' string expansion construct.
- \"
-
gives the quote its literal meaning
echo "Hello" # Hello echo "\"Hello\", he said." # "Hello", he said.
- \$
-
gives the dollar sign its literal meaning (variable name following \$ will not be referenced)
echo "\$variable01" # results in $variable01
- \\
-
gives the backslash its literal meaning
echo "\\" # results in \
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The behavior of \ depends on whether it is itself escaped, quoted, or appearing within a here document.
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Escaping a space can prevent word splitting in a command's argument list.
file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7" # List of files as argument(s) to a command. # Add two files to the list, and list all. ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list echo "-------------------------------------------------------------------------" # What happens if we escape a couple of spaces? ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list # Error: the first three files concatenated into a single argument to 'ls -l' # because the two escaped spaces prevent argument (word) splitting. |
The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line.
(cd /source/directory && tar cf - . ) | \ (cd /dest/directory && tar xpvf -) # Repeating Alan Cox's directory tree copy command, # but split into two lines for increased legibility. # As an alternative: tar cf - -C /source/directory | tar xpvf - -C /dest/directory # See note below. # (Thanks, Stephane Chazelas.) |
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If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line. |
echo "foo bar" #foo #bar echo echo 'foo bar' # No difference yet. #foo #bar echo echo foo\ bar # Newline escaped. #foobar echo echo "foo\ bar" # Same here, as \ still interpreted as escape within weak quotes. #foobar echo echo 'foo\ bar' # Escape character \ taken literally because of strong quoting. #foor\ #bar # Examples suggested by Stephane Chazelas. |