This Linux tutorial will explain the three “W” commands. The three “W”s are whatis, whereis and which commands. You already know how to use find
command to efficiently fo find a file.

Now, these three W commands will help you to locate more stuff from Linux command line.

I. Linux whatis Command

Whatis command is helpful to get brief information about Linux commands or functions. Whatis command displays man page single line description for command that matches string passed as a command line argument to whatis command. Whatis command searches for string
in its index databases which is maintained by mandb program. Whatis command picks short description of NAME section of man page of command that matches to input given to the whatis command.

Whatis provides several command line options to help user in getting brief information of specific Linux commands as per their need or interest.

Syntax:

$ whatis [-options]

For example, here is the output of whatis command, when it is run without any option.

$ whatis write

write (1)            - send a message to another user

write (2)            - write to a file descriptor

It displays brief information about “write” from man pages.

1. Get information from specific sections of man pages using -s option

If we want to get Linux command information from specific section of man pages, then we can provide sections list using “-s or —sections or –section” option. It will restrict whatis command to display brief information from specified man page section only.

$ whatis -s "1","2" open

open (1)             - start a program on a new virtual terminal (VT).

open (2)             - open and possibly create a file or device

It displays open command and function brief information from man page sections 1 and 2.

$ whatis -s "2" open

open (2)             - open and possibly create a file or device

It displays open function brief information from man page section 2.

2. Search information through wild-cards using -w option

If we want to search Linux commands or functions information using wild card, then whatis command gives “-w or –wildcard” option. It will make your search specific as per user’s need.

$ whatis -w 'ab*'

abort (3)            - cause abnormal process termination

abs (3)              - compute the absolute value of an integer

It displays brief information of Linux commands or functions which start from “ab”.

$ whatis -w 'ab?'

abs (3)              - compute the absolute value of an integer

It displays brief information of Linux commands or functions which start from “ab” and followed by any single character.

3. Search information through regular expressions using -r option

If we want to search Linux commands or functions information using regular expressions, then whatis command gives “-r or –regex” option. It will give flexibility to customize your search for Linux commands or functions throughout the Linux system.

$ whatis -r '^ab'

abort (3)            - cause abnormal process termination

abs (3)              - compute the absolute value of an integer

It displays brief information of Linux commands or functions which start from “ab”.

$ whatis -r 'ab$'

anacrontab (5)       - configuration file for anacron

baobab (1)           - A graphical tool to analyse disk usage

crontab (1)          - maintain crontab files for individual users (Vixie Cron)

crontab (5)          - tables for driving cron

fstab (5)            - static information about the filesystems

inittab (5)          - init daemon configuration

swab (3)             - swap adjacent bytes

tc-stab (8)          - Generic size table manipulations

It displays brief information of Linux commands or functions which ends with “ab”.

4. Disable trimmed output using -l option

Generally whatis command trims long output of Linux commands or functions information to avoid “Not good” output display on terminal that is going beyond screen. To allow whatis command to show complete output on screen, “-l or –long” option can be used.

$ whatis ssh-import-id

ssh-import-id (1)    - retrieve one or more public keys from a public keyserver (Launchpad.net by default) and append them to the current user's authorized_keys file (or some other specifie...

It displays trimmed output of brief information of Linux command.

$ whatis -l ssh-import-id

ssh-import-id (1)    - retrieve one or more public keys from a public keyserver (Launchpad.net by default) and append them to the current user's authorized_keys file (or some other specified file)

It displays complete output of brief information of Linux command.

5. Restrict search up to specified path using -M option

By default, whatis command uses $MANPATH environment variable. But whatis provides “-M or –manpath” option to restrict search up to specified path of man pages.

$ whatis -M /usr/share/man hexdump

hexdump (1)          - ASCII, decimal, hexadecimal, octal dump

It displays brief information of Linux hexdump command from man pages available at path /usr/share/man.

$ whatis -M /usr/man hexdump

hexdump: nothing appropriate.

It could not find brief information of Linux hexdump command from specified path /usr/man.

II. Linux whereis Command

Whereis command is helpful to locate binary, source and manual pages of commands in the Linux system. It is very simple utility and provides several options
which are given below with examples.

Syntax:

$ whereis [-options]

For example, whereis command is run without any option.

$ whereis open

open: /bin/open /usr/share/man/man1/open.1.gz /usr/share/man/man2/open.2.gz

It locates binary, source and man pages of “open” command and here it displayed paths where binary, man pages of open command is available in the system.

6. Locate binaries using -b option

If we want to locate binary of Linux command, use “-b” option.

$ whereis -b whereis

whereis: /usr/bin/whereis /usr/bin/X11/whereis

It locates binary of “whereis” command and displays paths where binary of command is available in the system.

7. Locate man pages for a command using -m option

If we want to locate man page of Linux command, use “-m” option.

$ whereis -m whereis

whereis: /usr/share/man/man1/whereis.1.gz

It locates man page of “whereis” command and displays path where man page of command is available in the system.

8. Locate source of a command using -s option

If we want to locate source of Linux command, use “-s” option.

$ whereis -s whereis

whereis:

It locates source of “whereis” command, but source of “whereis” command does not exist in the system, so it did not display path for source of command in the system.

9. Locate unusual entries using -u option

This option is something different that searches for unusual entries. These entries are those command whose source, binary or man page does not exist in the system as per options “[-bms]” specified along with “–u”.

$ whereis  -m  -u wcgrep

wcgrep:

It checks if specified command (i.e. wcgrep) man page does not exist in the system. Whereis command with options “-m and -u” locates for the commands in the system whose man page does not exist.

$ whereis  -m  -u grep

$

Here, whereis command with same options is applied on “grep” command whose man page exists in the system, so whereis returned nothing and exits normally.

10. Locate binaries in a specified path using -B option

If user wants to search for binary and wants to limit the scope of search for whereis command up to specified path, then use “-B” option.

$ whereis -B /bin -f for_loop

for_loop: /bin/for_loop

It locates binary of “for_loop” user program from path “/bin”.

$ whereis -B /usr -f open

open: /usr/share/man/man1/open.1.gz /usr/share/man/man2/open.2.gz

If open command’s binary is not found at specified path, then it is not shown but whereis command by default searches for other types (i.e. man page and source) of specified command (i.e. open) and displays them if found.

11. Locate man pages with limited scope using -M option

If user wants to search for man pages and wants to limit the scope of search for whereis command up to specified path, then use “-M” option.

$ whereis -M /usr/share/man/man1 -f open

open: /bin/open /usr/share/man/man1/open.1.gz

$ whereis -M /usr/share/man/man2 -f open

open: /bin/open /usr/share/man/man2/open.2.gz

$ whereis -M /usr/share/man/man3 -f open

open: /bin/open

Here, it is observed that whereis command is displaying man page of “open” command which is available in specified path only. But, whereis command by default searches for other types (i.e. binary and source) of specified command (i.e. open) and displays them
if found.

III. Linux which Command

Which command is very small and simple command to locate executables in the system. It allows user to pass several command names as arguments to get their paths in the system. “which” commands searches the path of executable in system paths set in $PATH environment
variable .

Syntax:

$ which [-option]

For example,

$ which ls gdb open grep

/bin/ls

/usr/bin/gdb

/bin/open

/bin/grep

It locates command names – “ls”, “gdb”, “open” and “grep” specified as arguments to “which” command and displays paths of each executable where it exists in the system.

12. Display all the paths using -a option

“which” command gives option “-a” that displays all paths of executable matching to argument.

$ which echo

/usr/sbin/echo

Above will search display the executable “echo” from all paths set in $PATH environment variable and displays the first path where echo executable is found. It may be case that executable is placed at other paths of $PATH environment variable as well. To get
all paths where executable is present in the system, “-a” option can be used.

$ which -a  echo

/usr/sbin/echo

/bin/echo