How can I declare and use Boolean variables in a shell script?

I tried to declare a Boolean variable in a shell script using the following syntax:

variable=$false

variable=$true

Is this correct? Also, if I wanted to update that variable would I use the same syntax? Finally, is the following syntax for using Boolean variables as expressions correct?

if [ $variable ]

if [ !$variable ]

Solution 1:

Revised Answer (Feb 12, 2014)

the_world_is_flat=true
# ...do something interesting...
if [ "$the_world_is_flat" = true ] ; then
    echo 'Be careful not to fall off!'
fi

Original Answer

Caveats: https://stackoverflow.com/a/21210966/89391

the_world_is_flat=true
# ...do something interesting...
if $the_world_is_flat ; then
    echo 'Be careful not to fall off!'
fi

From: Using boolean variables in Bash

The reason the original answer is included here is because the comments before the revision on Feb 12, 2014 pertain only to the original answer, and many of the comments are wrong when associated with the revised answer. For example, Dennis Williamson's comment about bash builtin true on Jun 2, 2010 only applies to the original answer, not the revised.

Solution 2:

TL;DR

my_bool=true

if [ "$my_bool" = true ]

Issues with Miku's (original) answer

I do not recommend the accepted answer¹. Its syntax is pretty, but it has some flaws.

Say we have the following condition.

if $var; then
  echo 'Muahahaha!'
fi

In the following cases², this condition will evaluate to true and execute the nested command.

# Variable var not defined beforehand. Case 1
var=''  # Equivalent to var="".      # Case 2
var=                                 # Case 3
unset var                            # Case 4
var='<some valid command>'           # Case 5

Typically you only want your condition to evaluate to true when your "Boolean" variable, var in this example, is explicitly set to true. All the other cases are dangerously misleading!

The last case (#5) is especially naughty because it will execute the command contained in the variable (which is why the condition evaluates to true for valid commands^{3, 4}).

Here is a harmless example:

var='echo this text will be displayed when the condition is evaluated'
if $var; then
  echo 'Muahahaha!'
fi

# Outputs:
# this text will be displayed when the condition is evaluated
# Muahahaha!

Quoting your variables is safer, e.g. if "$var"; then. In the above cases, you should get a warning that the command is not found. But we can still do better (see my recommendations at the bottom).

^{Also see Mike Holt's explanation of Miku's original answer.}

Issues with Hbar's answer

This approach also has unexpected behavior.

var=false
if [ $var ]; then
  echo "This won't print, var is false!"
fi

# Outputs:
# This won't print, var is false!

You would expect the above condition to evaluate to false, thus never executing the nested statement. Surprise!

Quoting the value ("false"), quoting the variable ("$var"), or using test or [[ instead of [, do not make a difference.

What I do recommend:

Here are ways I recommend you check your "Booleans". They work as expected.

my_bool=true

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

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

if test "$my_bool" = true; then
if test "$my_bool" = "true"; then

They're all pretty much equivalent. You'll have to type a few more keystrokes than the approaches in the other answers⁵, but your code will be more defensive.

Footnotes

Miku's answer has since been edited and no longer contains (known) flaws.
Not an exhaustive list.
A valid command in this context means a command that exists. It doesn't matter if the command is used correctly or incorrectly. E.g. man woman would still be considered a valid command, even if no such man page exists.
For invalid (non-existent) commands, Bash will simply complain that the command wasn't found.
If you care about length, the first recommendation is the shortest.

Solution 3:

There seems to be some misunderstanding here about the Bash builtin true, and more specifically, about how Bash expands and interprets expressions inside brackets.

The code in miku's answer has absolutely nothing to do with the Bash builtin true, nor /bin/true, nor any other flavor of the true command. In this case, true is nothing more than a simple character string, and no call to the true command/builtin is ever made, neither by the variable assignment, nor by the evaluation of the conditional expression.

The following code is functionally identical to the code in the miku's answer:

the_world_is_flat=yeah
if [ "$the_world_is_flat" = yeah ]; then
    echo 'Be careful not to fall off!'
fi

The only difference here is that the four characters being compared are 'y', 'e', 'a', and 'h' instead of 't', 'r', 'u', and 'e'. That's it. There's no attempt made to call a command or builtin named yeah, nor is there (in miku's example) any sort of special handling going on when Bash parses the token true. It's just a string, and a completely arbitrary one at that.

Update (2014-02-19): After following the link in miku's answer, now I see where some of the confusion is coming from. Miku's answer uses single brackets, but the code snippet he links to does not use brackets. It's just:

the_world_is_flat=true
if $the_world_is_flat; then
  echo 'Be careful not to fall off!'
fi

Both code snippets will behave the same way, but the brackets completely change what's going on under the hood.

Here's what Bash is doing in each case:

No brackets:

Expand the variable $the_world_is_flat to the string "true".
Attempt to parse the string "true" as a command.
Find and run the true command (either a builtin or /bin/true, depending on the Bash version).
Compare the exit code of the true command (which is always 0) with 0. Recall that in most shells, an exit code of 0 indicates success and anything else indicates failure.
Since the exit code was 0 (success), execute the if statement's then clause

Brackets:

Expand the variable $the_world_is_flat to the string "true".
Parse the now-fully-expanded conditional expression, which is of the form string1 = string2. The = operator is bash's string comparison operator. So...
Do a string comparison on "true" and "true".
Yep, the two strings were the same, so the value of the conditional is true.
Execute the if statement's then clause.

The no-brackets code works, because the true command returns an exit code of 0, which indicates success. The bracketed code works, because the value of $the_world_is_flat is identical to the string literal true on the right side of the =.

Just to drive the point home, consider the following two snippets of code:

This code (if run with root privileges) will reboot your computer:

var=reboot
if $var; then
  echo 'Muahahaha! You are going down!'
fi

This code just prints "Nice try." The reboot command is not called.

var=reboot
if [ $var ]; then
  echo 'Nice try.'
fi

Update (2014-04-14) To answer the question in the comments regarding the difference between = and ==: AFAIK, there is no difference. The == operator is a Bash-specific synonym for =, and as far as I've seen, they work exactly the same in all contexts.

Note, however, that I'm specifically talking about the = and == string comparison operators used in either [ ] or [[ ]] tests. I'm not suggesting that = and == are interchangeable everywhere in bash.

For example, you obviously can't do variable assignment with ==, such as var=="foo" (well technically you can do this, but the value of var will be "=foo", because Bash isn't seeing an == operator here, it's seeing an = (assignment) operator, followed by the literal value ="foo", which just becomes "=foo").

Also, although = and == are interchangeable, you should keep in mind that how those tests work does depend on whether you're using it inside [ ] or [[ ]], and also on whether or not the operands are quoted. You can read more about that in Advanced Bash Scripting Guide: 7.3 Other Comparison Operators (scroll down to the discussion of = and ==).

Solution 4:

Use arithmetic expressions.

#!/bin/bash

false=0
true=1

((false)) && echo false
((true)) && echo true
((!false)) && echo not false
((!true)) && echo not true

Output:

true
not false