Difference between quoting variables in shell script "if" statements?

Question

What is the difference between these two Bash if-statements? e.g.

if [ "$FOO" = "true" ]; then

vs

if [ $FOO = "true" ]; then

What is the difference? It seems that both statements work the same.

Solution 1

If the value of $FOO is a single word that doesn't contain a wildcard character \[*?, then the two are identical.

If $FOO is unassigned, or empty, or more than one word (i.e., contains whitespace or $IFS), then the unquoted version is a syntax error. If it happens to be just the right sequence of words (such as 0 -eq 0 -o false), the result could be arbitrary. Therefore, it is good practice to always quote variables in shell scripts.

Incidentally, "true" does not need to be quoted.

Solution 2

To illustrate what problems it might cause, here a few examples.

Let's say we have two variables as follow:

FOO="some value"
BAR="some value"

Now we have two variables holding exactly the same string/value. If we did some if statements to test the result, in your case:

if [ $FOO = "$BAR" ]; then echo "match"; else echo "no match"; fi

At this point you will get bash: [: too many arguments. The unquoted $FOO holds now three values, namely '[ , some , value'. [ test keyword doesn't know what to execute because it is expecting the first or second argument to be an operator.

When we quote "$FOO" we explicitly tell if to look at the right values where no word splitting takes place.

Another example:

my_file="A random file.txt"

• doing rm $my_file means removing 'A' 'random' 'file.txt' which makes it three files.
• doing rm "$my_file" will remove "A random file.txt" which makes one file.

Hope I've not confused you with these examples.

Comments

[Admin]

In Bash, use double square brackets and you don't need the quotes and you get additional benefits.

[Admin]

@DennisWilliamson Not quite, you do need quotes on the right-hand side of the (in)equality and pattern matching operators.

[Admin]

See also $VAR vs ${VAR} and to quote or not to quote, When is double-quoting necessary?, Why do I need to quote variable for if, but not for echo?, and other quoting questions.

[Admin]

of being incorporated literally as here). For glob-style pattern matching, the glob characters must not be quoted, whether literal or in a variable: [[ $a == foo* ]]. Can you provide any additional examples of a requirement for quoting besides my literal string example?