man
Most commands have built-in help info, usually with the --help or -h option, e.g. $ ls --help will give you more info about the command ls.
Or open the manual by calling man, e.g. $ man ls will show the manual page of ls.
Notice that at the very beginning of the manual, the command often has a number associated with it, like LS(1), the number is the section of the manual:
- Section 1: user commands
- Section 2: system calls
- Section 3: library functions
- Section 4: special files
- Section 5: file formats
- Section 6: games
- Section 7: conventions and miscellany
- Section 8: administration and privileged commands
- Section L: math library functions
- Section N: tcl functions
This info can be found in the manual of manual: $ man man.
The same entry may appear in different sections, e.g. to check manual for mount in system calls vs admin command:
$ man 2 mount # the system call
$ man 8 mount # the admin command
Use man -k to search the man pages, e.g. $ man -k mount will not only return mount but also umount, cgroupfs-mount, etc. This is equivalent to apropos mount
Use man -f to list all sections that have the term.
$ man -f mount
mount (8) - mount a filesystem
mount (2) - mount filesystem