Conditionals in

About Conditionals

Truthiness in Bash

Bash does not have the concept of boolean values. There are strings and numbers, and arrays of strings or numbers. So how do conditional commands deal with true and false?

Exit Status

Commands produce an exit status when they end. An exit status is an integer between 0 and 255 inclusive. Bash considers a zero exit status to represent success. Any other exit status represents failure.

This applies to all commands, including bash builtin commands, keywords, and functions.

The "if" Command

The basic syntax of the if command is

if CONDITIONAL_COMMANDS; then TRUE_COMMANDS; else FALSE_COMMANDS; fi

It starts with if; the condition commands are separated from the "success" commands by then; and it is terminated with fi. The else clause is optional.

CONDITIONAL_COMMANDS can be a single command or it can be a list of commands.

The CONDITIONAL_COMMANDS are executed, and

• if the exit status is zero (success), then the TRUE_COMMANDS are executed.
• if the exit status is non-zero (failure), then the FALSE_COMMANDS are executed, if they are present.

Cascading branches can be given with elif

if CONDITIONAL_COMMANDS
then TRUE_COMMANDS
elif CONDITIONAL_COMMANDS_2
then TRUE_COMMANDS_2
# more elif branches ...
else FALSE_COMMANDS
fi

There must be a semicolon or a newline before the then, elif, else and fi words.

To emphasize: it is the exit status of the conditional commands that controls the flow. As an example, grep returns 0 if a match is found, and 1 if a match is not found. The -q option suppresses output, only producing the exit status.

if grep -q "my pattern" my_file; then echo "the pattern is found in the file"; fi

"[" and "test" Commands

There is no special syntax around the CONDITIONAL_COMMANDS. You may be used to seeing if statements that look like this:

if [ "$password" = "secure" ]; then
    echo "Welcome!"
fi

[ is not special syntax. It is a command that evaluates the conditional expression and exits with a success/failure status. Like all commands, whitespace is required between it and its arguments.

[ is actually a synonym for the test command. They are exactly the same, except that the last argument to [ must be ].

Conditional Expressions

Within [ and ], you write a conditional expression. Some typical conditional expressions include:

[ -f "$filename" ]             # file operations
[ "$string1" = "$string2" ]    # string comparisons
[ "$num1" -eq "$num2" ]        # arithmetic comparisons

There are many more operations available; they are listed in the Bash Conditional Expressions section of the manual.

str=""

# this prints "empty"
if [ -n "$str" ]; then echo "not empty"; else echo "empty"; fi

# leaving the variable unquoted results in incorrect "not empty" output
if [ -n $str ]; then echo "not empty"; else echo "empty"; fi

"[[" Keyword

The [[...]] conditional construct is not a command, it is a keyword. This means that, although it is handled like any other command, it has special parsing rules. What's special about [[ is that the variables expanded within it are not subject to word splitting or filename expansion. That means this command acts as you expect, even without quoting.

if [[ -n $str ]]; then echo "not empty"; else echo "empty"; fi

[[ supports all the conditional expressions that test and [ can handle. In addition, [[ provides

• the =~ regular-expression matching operator,
• == and != operate as a glob-pattern matching operator,
• && and || as logical operators (special parsing rule),
• < and > as "bare" string comparison operators (special parsing rule: because these are redirection symbols, in [ they must be escaped).

It is widely held that these special features offer so much benefit that [[ should be used exclusively. (For example, the Google Shell Style Guide.)

The "case" Command

case is another control flow command. It is like a "switch" statement in other languages.

case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac

The WORD is matched against each PATTERN. When one matches, the COMMANDS are executed.

read -p "Guess the secret word: " word
case "$word" in
    secret) echo "Yes, you guessed it!" ;;
    ??????) echo "That's the right number of letters." ;;
    s*)     echo "You guessed the first letter." ;;
    *)      echo "Not even close! Try again." ;;
esac

Each COMMANDS clause must end with two semicolons, ;;.