Linux true Command

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What is the true command

trueIt is one of the simplest commands in Linux/Unix systems, and it has only one function:Always return success (exit status code 0)。

This command seems simple, but it has important uses in shell script programming. It is equivalent to a "do nothing but always succeed" operation.

Basic Syntax

Examples

true [--help] [--version]

Options

Option Description
--help Display help information and exit
--version Display version information and exit

How It Works

trueThe execution process of the command is very simple:

  1. When you entertrueand press Enter
  2. The system executes this command
  3. The command completes immediately without doing anything
  4. Returns exit status code 0 (indicating success)

Practical Use Cases

1. As a Condition for an Infinite Loop

Examples

while true; do
    echo "This is an infinite loop"
    sleep 1
done

In this example,while truecreates an infinite loop becausetruealways returns success (0), so the loop condition is always true.

2. As a Default Successful Operation

Examples

command || true

This syntax means: ifcommandfails to execute, then executetrue(always succeeds), this ensures that the entire expression always returns a successful status.

3. As a Placeholder in Scripts

Examples

if [ condition ]; then
    :  # Do nothing
else
    true  # This can also be written to do nothing
fi

Heretrueserves as a substitute for a no-op, making the code structure clearer.

Comparison with the false Command

Linux also has a correspondingfalsecommand, which always returns failure (exit status code 1). The table below shows the differences between them:

Command Function Exit status code Typical use
true Does nothing, returns success 0 Infinite loops, ignoring errors
false Does nothing, returns failure 1 Testing error handling, conditional checks

Implementation Details

In most Linux systems,trueis a built-in command (shell builtin), but there is usually also a standalone binary version:

Examples

$ type -a true
true is a shell builtin
true is /usr/bin/true
true is /bin/true

Its implementation is extremely simple and can be represented in C as:

Examples

int main() {
    return 0;
}

Practical Exercises

Try running it directly in the terminaltrue, then check the exit status code:

Examples

true
echo $?  # This will output 0

Create an infinite loop script that outputs the current time every 5 seconds:

Examples

#!/bin/bash
while true; do
    date
    sleep 5
done

Testtruethe behavior in conditional statements:

Examples

if true; then
    echo "This will always execute"
fi

FAQ

Q: Does the true command really do nothing?A: Yes, it does not perform any actual operation, it simply returns a successful exit status.

Q: Why do we need such a seemingly useless command?A: In script programming, sometimes we need a placeholder or condition that always succeeds,trueand this command exactly meets that need.

Q: What is the difference between true and colon (:)?A: In most shells,:is also a command that does nothing, and its behavior is identicaltrueto true. However,:is a pure shell builtin, whiletruemay have a standalone binary version.

Summary

trueAlthough the command is simple, it is a practical tool in shell script programming. Its main features include:

  • Always returns success (exit status code 0)
  • Does not perform any actual operation
  • Often used for infinite loops, error handling, and code placeholders

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