Getting Help: man, --help and whatis

You don't need to memorise everything

Linux has hundreds of commands, and each has many options. Experienced users don't remember them all; they know how to look things up. Linux has the documentation built in.

Quick help with --help

Most commands print a short summary of their options when you add --help:

mkdir --help

In the terminal

student@vulntech:~$ mkdir --help
Usage: mkdir [OPTION]... DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

Mandatory arguments to long options are mandatory for short options too.
  -m, --mode=MODE   set file mode (as in chmod), not a=rwx - umask
  -p, --parents     no error if existing, make parent directories as needed,
                    with their file modes unaffected by any -m option.
  -v, --verbose     print a message for each created directory
  -Z                   set SELinux security context of each created directory
                         to the default type
      --context[=CTX]  like -Z, or if CTX is specified then set the SELinux
                         or SMACK security context to CTX
      --help        display this help and exit
      --version     output version information and exit

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Report any translation bugs to <https://translationproject.org/team/>
Full documentation <https://www.gnu.org/software/coreutils/mkdir>
or available locally via: info '(coreutils) mkdir invocation'

whatis: one line about a command

whatis tells you in one line what a command does:

whatis ls
whatis date

In the terminal

student@vulntech:~$ whatis ls
ls(1) - list directory contents
student@vulntech:~$ whatis date
date(1) - print or set the system date and time

man: the full manual

man (short for manual) opens the complete manual page of a command:

man ls

The manual opens full-screen. A man page always has the same sections:

Moving around a man page

Tip: To find which option does something, open the man page and search. For example, in man ls type /size and press Enter to jump to options about file sizes.

Searching all manuals

Don't know the command's name? apropos (or man -k) searches the one-line descriptions of every manual page:

apropos directory

In the terminal

student@vulntech:~$ apropos directory
basename(1) - strip directory and suffix from filenames
chroot(1) - run command or interactive shell with special root directory
dir(1) - list directory contents
find(1) - search for files in a directory hierarchy
ls(1) - list directory contents
mktemp(1) - create a temporary file or directory
mountpoint(1) - see if a directory or file is a mountpoint
pwd(1) - print name of current/working directory
pwdx(1) - report current working directory of a process
vdir(1) - list directory contents
chdir, fchdir(2) - change working directory
chroot(2) - change root directory
execveat(2) - execute program relative to a directory file descriptor
futimesat(2) - change timestamps of a file relative to a directory file descriptor
getcwd, get_current_dir_name, getwd(2, 3) - get current working directory
getdents, getdents64(2) - get directory entries
lookup_dcookie(2) - return a directory entry's path
mkdir, mkdirat(2) - create a directory
readdir(2) - read directory entry
rmdir(2) - delete a directory
VFAT_IOCTL_READDIR_BOTH, VFAT_IOCTL_READDIR_SHORT(2const) - read filenames of a directory in a FAT filesystem
closedir(3) - close a directory
dirfd(3) - get directory stream file descriptor
getdirentries(3) - get directory entries in a filesystem-independent format
mkdtemp(3) - create a unique temporary directory
opendir, fdopendir(3) - open a directory
readdir(3, 3am) - directory input parser for gawk
readdir(3) - read a directory
readdir_r(3) - read a directory
remove(3) - remove a file or directory
rewinddir(3) - reset directory stream
scandir, alphasort, scandirat, versionsort(3) - scan a directory for matching entries
seekdir(3) - set the position of the next readdir() call in the directory stream.
telldir(3) - return current location in directory stream
proc_pid_cwd, /proc/pid/cwd(5) - symbolic link to current working directory
proc_pid_root, /proc/pid/root/(5) - symbolic link to root directory

Is it a real program?

type tells you what a command is, and which shows where its program file lives:

type ls
type cd
which date

In the terminal

student@vulntech:~$ type ls
ls is aliased to `ls --color=auto'
student@vulntech:~$ type cd
cd is a shell builtin
student@vulntech:~$ which date
/bin/date

cd is a shell builtin: it's built into bash itself, so it has no program file and no man page of its own. For builtins, use help, for example help cd.

Remember: --help for a quick reminder, man for everything, whatis and apropos to find the right command. Press q to leave a man page.