true

What does true do?

The true command simply exits with a return code of 0, signifying success. This might seem trivial, but its significance lies in how it’s used in conditional statements and shell scripting. In essence, it acts as a placeholder for a command that’s always meant to succeed.

true in Conditional Statements

Conditional statements like if depend on the exit status of a command. A return code of 0 indicates success, while a non-zero code indicates failure. Because true always returns 0, it’s perfect for situations where you want a block of code to always execute.

Example 1: Always executing a command

if true; then
  echo "This will always be printed."
fi

This script will always print “This will always be printed,” regardless of any external factors.

Example 2: Creating an infinite loop (use cautiously!)

While not recommended for production code without a clear exit condition, true can be used to create an infinite loop. This is useful for testing or for processes that need to run indefinitely, checking for certain conditions.

while true; do
  echo "This loop runs forever."
  sleep 1  # Pauses for 1 second
done

To stop this loop, you’ll need to manually interrupt it using Ctrl+C.

true in Cron Jobs and Automation

In automated tasks, true is often used as a placeholder when a command isn’t needed but a successful return code is required. Imagine a cron job designed to run a backup script. If the backup completes successfully, you might want the cron job to log the success. If the backup fails, you want a different action (perhaps sending an email alert). Using true in the success case ensures a 0 return code to avoid triggering the failure handling.

Example 3: Cron job with success logging:

Let’s say your backup script is backup_script.sh. A cron job might look like this:

0 0 * * *  if ./backup_script.sh; then echo "Backup successful" >> /var/log/backup.log; else echo "Backup failed!" >> /var/log/backup.log; fi

If you always want to log the success of the cron job itself, even if backup_script.sh fails:

0 0 * * * ./backup_script.sh && echo "Cron job ran" >> /var/log/cron.log || (echo "Backup failed!"; echo "Cron job ran with error" >> /var/log/cron.log)

true as a No-op Command

true can be used as a no-operation (noop) command, essentially doing nothing. This can be a placeholder in scripts or when you need a command that doesn’t produce any output or side effects.

Example 4: A simple noop:

true

This command executes without any visible effect.

Using false for Contrasting Behavior

The opposite of true is the false command, which always returns a non-zero exit code (typically 1), indicating failure. This is useful for testing error handling in scripts or intentionally causing a conditional statement to fail. This command behaves much the same as true, except the opposite result.

Example 5: Always failing condition:

if false; then
  echo "This will never be printed."
fi

This will never print the message because false always returns a failure code.