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). [1] 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. [2] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.
List="one two three" for a in $List # Splits the variable in parts at whitespace. do echo "$a" done # one # two # three echo "---" for a in "$List" # Preserves whitespace in a single variable. do # ^ ^ echo "$a" done # one two three |
A more elaborate example:
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, 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
#!/bin/bash # weirdvars.sh: Echoing weird variables. echo 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. echo var2="\\\\\"" echo $var2 # " echo "$var2" # \\" echo # But ... var2="\\\\"" is illegal. Why? var3='\\\\' echo "$var3" # \\\\ # Strong quoting works, though. # ************************************************************ # # As the first example above shows, nesting quotes is permitted. echo "$(echo '"')" # " # ^ ^ # At times this comes in useful. var1="Two bits" echo "\$var1 = "$var1"" # $var1 = Two bits # ^ ^ # Or, as Chris Hiestand points out ... if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]] # ^ ^ ^ ^ ^ ^ ^ ^ then ... fi # ************************************************************ # |
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.
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[1] | 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 apparently inconsistent behavior of \ within double quotes, and especially following an echo -e command.
Double quotes following an echo sometimes escape \. Moreover, the -e option to echo causes the "\t" to be interpreted as a tab. (Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and Daniel Barclay for explaining it.) | |
[2] | "Word splitting," in this context, means dividing a character string into separate and discrete arguments. |