5.1. Variable Substitution
The name of a variable is a placeholder for its value, the data it holds. Referencing its value is called variable substitution.
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Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. The only time a variable appears "naked", without the $ prefix, is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see Example 30-4). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3).
Enclosing a referenced value in double quotes (" ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as "weak quoting". Using single quotes (' ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as "strong quoting". See Chapter 6 for a detailed discussion.
Note that $variable is actually a simplified alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see Section 9.3, below).
Example 5-1. Variable assignment and substitution
#!/bin/bash # Variables: assignment and substitution a=375 hello=$a #------------------------------------------------------------------------- # No space permitted on either side of = sign when initializing variables. # If "VARIABLE =value", #+ script tries to run "VARIABLE" command with one argument, "=value". # If "VARIABLE= value", #+ script tries to run "value" command with #+ the environmental variable "VARIABLE" set to "". #------------------------------------------------------------------------- echo hello # Not a variable reference, just the string "hello". echo $hello echo ${hello} #Identical to above. echo "$hello" echo "${hello}" echo hello="A B C D" echo $hello echo "$hello" # Now, echo $hello and echo "$hello" give different results. # Quoting a variable preserves whitespace. echo echo '$hello' # Variable referencing disabled by single quotes, #+ which causes the "$" to be interpreted literally. # Notice the effect of different types of quoting. hello= # Setting it to a null value. echo "\$hello (null value) = $hello" # Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below). # -------------------------------------------------------------- # It is permissible to set multiple variables on the same line, #+ if separated by white space. # Caution, this may reduce legibility, and may not be portable. var1=variable1 var2=variable2 var3=variable3 echo echo "var1=$var1 var2=$var2 var3=$var3" # May cause problems with older versions of "sh". # -------------------------------------------------------------- echo; echo numbers="one two three" other_numbers="1 2 3" # If whitespace within a variable, then quotes necessary. echo "numbers = $numbers" echo "other_numbers = $other_numbers" echo echo "uninitialized_variable = $uninitialized_variable" # Uninitialized variable has null value (no value at all). uninitialized_variable= # Declaring, but not initializing it #+ (same as setting it to a null value, as above). echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. uninitialized_variable=23 # Set it. unset uninitialized_variable # Unset it. echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. echo exit 0
An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will inevitably cause problems.