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How to get octal file permissions on Linux/Unix command line

I am a new Linux command line user. How do I find file permissions in octal format such as 0644 from the bash command line running on a Linux or Unix operating systems? How can I get the octal permissions for a file? By default the ls command will not display the permissions on a file in octal form, which is useful for many commands such as chmod command and other sysadmin tasks.