apropos and whatis commands

As you know, each Unix/Linux distro comes with a massive set of man pages – helpful manuals for using pretty much every standard command found in your OS.

Quite often you don’t know the command though, but know what it should do. In such a scenario, apropos and whatis commands may come in handy.

Man pages database

There’s a special database (index) of man pages on your system, which indexes man pages and keeps a list of short 1-line descriptions of each command documented in a man page. This is how such a database looks in text form on a Ubuntu 16.04 Linux system:

...
ldap.conf (5) - LDAP configuration file/environment variables
adduser.conf (5) - configuration file for adduser(8) and addgroup(8) .
mailcap.order (5) - the mailcap ordering specifications
interfaces (5) - network interface configuration for ifup and ifdown
Compose (5) - X client mappings for multi-key input sequences
PAM (7) - Pluggable Authentication Modules for Linux
[ (1) - check file types and compare values
access.conf (5) - the login access control table file
accessdb (8) - dumps the content of a man-db database in a human readable format
add-apt-repository (1) - Adds a repository into the /etc/apt/sources.list or /etc/apt/sources.lis...
add-shell (8) - add shells to the list of valid login shells
addgroup (8) - add a user or group to the system
addpart (8) - tell the kernel about the existence of a partition
adduser (8) - add a user or group to the system
agetty (8) - alternative Linux getty
apropos (1) - search the manual page names and descriptions
apt (8) - command-line interface
...

Left side of the output lists command names, right side of the outout shows a brief command description. apropos and whatis commands work with these fields and allow you to search them.

Using whatis command

This command is useful when you want to confirm what a particular Unix command does. It searches man pages, but focuses specifically on the command names, rather than their descriptions.

For instance, if I know ls command, I would use whatis like this to find out more:

root@vps1:~# whatis ls
ls (1) - list directory contents

if I want to find similar commands, I can use the wildcard (in this example: all commands starting with ls combination):

root@vps1:~# whatis -w 'ls*'
ls (1) - list directory contents
lsattr (1) - list file attributes on a Linux second extended file system
lsb_release (1) - print distribution-specific information
lsblk (8) - list block devices
lscpu (1) - display information about the CPU architecture
lsinitramfs (8) - list content of an initramfs image
lsipc (1) - show information on IPC facilities currently employed in the system
lslocks (8) - list local system locks
lslogins (1) - display information about known users in the system
lsmod (8) - Show the status of modules in the Linux Kernel
lsof (8) - list open files
lspgpot (1) - extracts the ownertrust values from PGP keyrings and list them in GnuPG ow..

Using apropos command

apropos is useful when you don’t remember the command but may have a few keywords describing its functionality.

Using ls command from the previous examples, we can find it using “directory” keyword. Of coruse, searching for “directory” will find all the commands which have anything to do with directories, as shown below:

root@vps1:~# apropos directory
basename (1) - strip directory and suffix from filenames
bindtextdomain (3) - set directory containing message catalogs
chroot (8) - run command or interactive shell with special root directory
dbus-cleanup-sockets (1) - clean up leftover sockets in a directory
depmod.d (5) - Configuration directory for depmod
dir (1) - list directory contents
find (1) - search for files in a directory hierarchy
grub-macbless (8) - bless a mac file/directory
grub-mknetdir (1) - prepare a GRUB netboot directory.
helpztags (1) - generate the help tags file for directory
ls (1) - list directory contents
mklost+found (8) - create a lost+found directory on a mounted Linux second extended file system
mktemp (1) - create a temporary file or directory
modprobe.d (5) - Configuration directory for modprobe
mountpoint (1) - see if a directory or file is a mountpoint
pam_mkhomedir (8) - PAM module to create users home directory
pwd (1) - print name of current/working directory
pwdx (1) - report current working directory of a process
run-parts (8) - run scripts or programs in a directory
vdir (1) - list directory contents