Where are all the terminal commands stored and how to view them?

Question

I forgot the terminal command for viewing the disk space. So I began with the resources for terminal commands like pdf and webpages.
Aren't those terminal commands stored somewhere in the file syster?
In that case where are they stored and how can I see the terminal commands?
PS: It will be like a single command which lists and displays all the rest of command that are stored in the system.

Answer 1

You could try using the apropos command to find a command knowing a basic idea of the command. For e.g, if you need to find a command for handling disk functions try: apropos disk. Advantage of using apropos is that it gives a short description of the command.

e.g:

$ apropos disk
arm_sync_file_range (2) - sync a file segment with disk
baobab (1)           - A graphical tool to analyze disk usage
cfdisk (8)           - display or manipulate disk partition table
cgdisk (8)           - Curses-based GUID partition table (GPT) manipulator
df (1)               - report file system disk space usage
dvd+rw-booktype (1)  - format DVD+-RW/-RAM disk with a logical format
dvd+rw-format (1)    - format DVD+-RW/-RAM disk
dvd+rw-mediainfo (1) - display information about dvd drive and disk
fdformat (8)         - low-level format a floppy disk
fdisk (8)            - manipulate disk partition table
gdisk (8)            - Interactive GUID partition table (GPT) manipulator
git-count-objects (1) - Count unpacked number of objects and their disk consumption
git-credential-store (1) - Helper to store credentials on disk
gnome-disk-image-mounter (1) - Attach and mount disk images
gnome-disks (1)      - the GNOME Disks application
grub-mkstandalone (1) - make a memdisk-based GRUB image
grub-render-label (1) - generate a .disk_label for Apple Macs.
hd (4)               - MFM/IDE hard disk devices
initrd (4)           - boot loader initialized RAM disk
mbadblocks (1)       - tests a floppy disk, and marks the bad blocks in the FAT
mcat (1)             - dump raw disk image
mcheck (1)           - verify all files on an MS-DOS formatted disk
memdiskfind (1)      - utility to search for a MEMDISK instance
mformat (1)          - add an MSDOS filesystem to a low-level formatted floppy disk
mkdiskimage (1)      - Create a blank MS-DOS formatted hard disk image
mmount (1)           - mount an MSDOS disk
mpartition (1)       - partition an MSDOS hard disk
mtools (1)           - utilities to access DOS disks in Unix.
mxtar (1)            - Wrapper for using GNU tar directly from a floppy disk
mzip (1)             - change protection mode and eject disk on Zip/Jaz drive
netscsid (1)         - write data to optical disk media
partx (8)            - tell the Linux kernel about the presence and numbering of on-disk partitions
quotactl (2)         - manipulate disk quotas
ram (4)              - ram disk device
sd (4)               - driver for SCSI disk drives
sfdisk (8)           - partition table manipulator for Linux
sgdisk (8)           - Command-line GUID partition table (GPT) manipulator for Linux and Unix
sync (2)             - commit buffer cache to disk
sync (8)             - synchronize data on disk with memory
sync_file_range (2)  - sync a file segment with disk
sync_file_range2 (2) - sync a file segment with disk
syncfs (2)           - commit buffer cache to disk
udisks (8)           - Disk Manager
udisksctl (1)        - The udisks command line tool
udisksd (8)          - The udisks system daemon
usb-creator-gtk (8)  - Ubuntu startup disk creation tool for Gtk+
wodim (1)            - write data to optical disk media

As for the location of the system commands, most command are stored in the following directories:

/bin/
/usr/bin
/usr/sbin
/sbin

You can use the ls command to list the specific commands stored in each of these directories.

For further information:

• Manpage for apropos

Update:

You can use echo $PATH, to find all the paths presently specified for executable:

e.g:

$ echo $PATH

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

Then you can use ls on each of the individual folders(each folder is separated by a : ) to find the executable commands present in that path.

PATH is an environment variable on Unix-like operating systems, DOS, OS/2, and Microsoft Windows, specifying a set of directories where executable programs are located. In general, each executing process or user session has its own PATH setting.

Courtesy: http://en.wikipedia.org/wiki/PATH_%28variable%29

The number in the brackets of the apropos output refers to the man section number. The man pages have been categorized into various sections, namely:

1. Commands (Programs)

   Those commands that can be executed by the user from within a shell.

2. System calls

   Those functions which must be performed by the kernel.

3. Library calls

   Most of the libc functions.

4. Special files (devices)

   Files found in /dev.

5. File formats and conventions

   The format for /etc/passwd and other human-readable files.

6. Games

7. Overview, conventions, and miscellaneous

   Overviews of various topics, conventions and protocols, character set standards, and miscellaneous other things.

8. System management commands

   Commands like mount(8), many of which only root can execute. System administration commands (usually only for root)

9. Kernel routines [Non standard]

Courtesy: http://manpages.ubuntu.com/manpages/trusty/man7/man-pages.7.html

Answer 2

You can also use man -k <keyword> to search for any command based on the particular keyword. apropos mentioned in other answer actually uses the database generated by mandb. So, both of the following will produce same output:

man -k disk
apropos disk

Both of the above provides regular expressions based pattern search by default.

See the manpages of man and apropos for details.