CODEWITHSUNDEEP
bash
Jeff
Jeff

Reputation: 926

Why does Bash treat undefined variables as true in an 'if' statement?

In the below simple example, if CONDITION is not set in the script, running the script prints "True". However, one would expect it to print "False". What is the reason for this behavior?

# Set CONDITION by using true or false:
# CONDITION=true or false

if $CONDITION; then
    echo "True"
else
    echo "False"
fi

Upvotes: 62

Views: 5233

Answers (1)

Barmar
Barmar

Reputation: 780724

The argument to if is a statement to execute, and its exit status is tested. If the exit status is 0 the condition is true and the statements in then are executed.

When $CONDITION isn't set, the statement is an empty statement, and empty statements always have a zero exit status, meaning success. So the if condition succeeds.

This behavior is explained in the POSIX shell specification:

If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. Otherwise, the command shall complete with a zero exit status.

Upvotes: 81

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