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5.1. Quoting VariablesWhen 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 -- the variable name [1] -- 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. 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, St�phane Chazelas. Tip 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 #!/bin/bash
# weirdvars.sh: Echoing weird variables.
var="'(]\\{}\$\""
echo $var # '(]\{}$"
echo "$var" # '(]\{}$" Doesn't make a difference.
echo
IFS='\'
echo $var # '(] {}$" \ converted to space. Why?
echo "$var" # '(]\{}$"
# Examples above supplied by Stephane Chazelas.
exit 0Single 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"). Note 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. echo "Why can't I write 's between single quotes" echo # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' # |-------| |----------| |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between. # This example courtesy of St�phane Chazelas. | ||
It also has side-effects on the value of the variable (see below)
Encapsulating "!" within double quotes gives an error when used from the command line. This is interpreted as a history command. Within a script, though, this problem does not occur, since the Bash history mechanism is disabled then.
Of more concern is the inconsistent behavior of "\" within double quotes.
bash$ echo hello\! hello! bash$ echo "hello\!" hello\! bash$ echo -e x\ty xty bash$ echo -e "x\ty" x y
"Word splitting", in this context, means dividing a character string into a number of separate and discrete arguments.