There are a few different Command Types In Linux. In fact, there are four command types in Linux. So just what are these commands? First up, there are executable programs or compiled binaries. For example if you run
ls /usr/bin, you will be greeted with a large collection of programs. These are most often C or C++ programs which have been compiled, or small scripts written in bash, Ruby, Python, or Perl. A second type of command is that which is a built in. Built in commands are part of the shell itself. For example, the convenient
pwd command is actually a shell builtin. Some other built ins are
getopts, and more. The third type is that of a shell function. These are small scripts that are integrated into the environment and are useful for scripting repetitive administration tasks. Last up we have the alias, which is a super way to define macros or shortcuts so to speak of existing commands to reach a desired result. Let’s take a closer look at command types in linux now.
Command Types In Linux
Check The Command Type From The Shell
So now that we have a basic overview of the four different Command Types In Linux, how can we know what we are dealing with? There are many sources of documentation online about this such as the bash command reference. We can also learn about these commands right in our own server by using the type command. Lets see a few now.
type is a shell builtin
vagrant@homestead:~$ `type which`
which is /usr/bin/which
vagrant@homestead:~$ `type man`
man is /usr/bin/man
vagrant@homestead:~$ `type apropos`
apropos is /usr/bin/apropos
vagrant@homestead:~$ `type info`
info is /usr/bin/info
vagrant@homestead:~$ `type whatis`
whatis is /usr/bin/whatis
vagrant@homestead:~$ `type alias`
alias is a shell builtin
Pretty Cool. We can see here that the type command gives us a way to figure out what type of command we are dealing with. We can see that in the case of executable commands, we are provided with the location of the binary. Another way to find this same information for executable programs is to use the
Getting Help With Linux Commands
There are a few ways to get help with commands in Linux. We have the help, info, and man commands to help get information about other commands. Some of them work differently than others, and they will return different information based on the type of command you apply them to. The best way to get a handle on this is to simply test it out.
Here we get
help with the
type: type [-afptP] name [name …]
Display information about command type.
For each NAME, indicate how it would be interpreted if used as a
-a display all locations containing an executable named NAME;
includes aliases, builtins, and functions, if and only if
the `-p` option is not also used
-f suppress shell function lookup
-P force a PATH search for each NAME, even if it is an alias,
builtin, or function, and returns the name of the disk file
that would be executed
-p returns either the name of the disk file that would be executed,
or nothing if `type -t NAME` would not return `file`.
-t output a single word which is one of `alias`, `keyword`,
`function`, `builtin`, `file` or “, if NAME is an alias, shell
reserved word, shell function, shell builtin, disk file, or not
NAME Command name to be interpreted.
Returns success if all of the NAMEs are found; fails if any are not found.
In this example we use
info to learn about the
File: *manpages*, Node: which, Up: (dir)
WHICH(1) General Commands Manual WHICH(1)
which – locate a command
which [-a] filename …
which returns the pathnames of the files (or links) which would be exe‐
cuted in the current environment, had its arguments been given as com‐
mands in a strictly POSIX-conformant shell. It does this by searching
the PATH for executable files matching the names of the arguments. It
does not follow symbolic links.
-a print all matching pathnames of each argument
0 if all specified commands are found and executable
1 if one or more specified commands is nonexistent or not exe‐
2 if an invalid option is specified
Debian 1 May 2009 WHICH(1)
We can also use
man to get help like so
PWD(1) User Commands PWD(1)
pwd – print name of current/working directory
Print the full filename of the current working directory.
use PWD from environment, even if it contains symlinks
avoid all symlinks
–help display this help and exit
output version information and exit
NOTE: your shell may have its own version of pwd, which usually supersedes the ver‐
sion described here. Please refer to your shell’s documentation for details about
the options it supports.
Written by Jim Meyering.
Report pwd bugs to firstname.lastname@example.org
GNU coreutils home page:
General help using GNU software:
Report pwd translation bugs to
Copyright © 2013 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or
This is free software: you are free to change and redistribute it. There is NO WAR‐
RANTY, to the extent permitted by law.
The full documentation for pwd is maintained as a Texinfo manual. If the info and
pwd programs are properly installed at your site, the command
info coreutils ‘pwd invocation’
should give you access to the complete manual.
GNU coreutils 8.21 March 2014 PWD(1)
Manual page pwd(1) line 1/56 (END) (press h for help or q to quit)
Note: When testing out the man command, you’ll often find that the information provided can be quite voluminous. This is why man makes use of the less utility in order to be able to view sections of the manual output in chunks which is much more user friendly. Something else to be aware of with manual pages, is that they are broken down into multiple sections. Eight is the number to be exact, and they are organized as follows.
programming interfaces used in system calls with the kernal
interfaces for use with the C Language
deals with drivers and device nodes
reserved for games
random use case
reserved for system administration
If you want to search a specific section, you can do so in the format of man number command, however most times leaving off the section number is quickest and easiest.
Finding Appropriate Commands and Brief Descriptions
The extensive manual information provided can at times be daunting. We can narrow this down by using something like
whatis. Suppose we have questions about working with disk related issues on the system. We can find a list of commands that may be appropriate, and then get more detailed information as we see fit. Let’s see how.
arm_sync_file_range (2) – sync a file segment with disk
cfdisk (8) – display or manipulate disk partition table
cgdisk (8) – Curses-based GUID partition table (GPT) manipulator
cryptdisks_start (8) – wrapper around cryptsetup that parses /etc/crypttab.
cryptdisks_stop (8) – wrapper around cryptsetup that parses /etc/crypttab.
df (1) – report file system disk space usage
eatmydata (1) – transparently disable fsync() and other data-to-disk synchronization …
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
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
partx (8) – tell the Linux kernel about the presence and numbering of on-disk par…
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
vagrant@homestead:~$ `whatis ram`
ram (4) – ram disk device
vagrant@homestead:~$ `whatis hd`
hd (1) – ASCII, decimal, hexadecimal, octal dump
hd (4) – MFM/IDE hard disk devices
vagrant@homestead:~$ `whatis df`
df (1) – report file system disk space usage
Using alias To Create Custom Commands
Linux has the very handy alias command which is a convenient way to string together a sequence of commands or options as one command. Think of it like a macro. With clever use of aliases you can speed up your daily workflow. Let’s try to create a few of our own now.
14:45:30 up 4 min, 1 user, load average: 0.01, 0.07, 0.05
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
vagrant pts/0 10.0.2.2 14:41 2.00s 0.08s 0.00s w
Linux homestead 3.13.0-30-generic #55-Ubuntu SMP Fri Jul 4 21:40:53 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Awesome! We were able to string together a few commands, and then simply type the one alias name to produce results. The only limit with this is your imagination. Try to think of all the useful combinations you might be able to come up with.
List all aliases in Linux
The concept of aliases brings up a good question. What are the list of aliases that already exist on your system? This will be different depending on the Linux distribution, but there is an easy way to see what you already using
compgen -a. Once you have your list of aliases, you can use the
type command to see exactly how they are constructed.
vagrant@homestead:/etc$ `type ..`
.. is aliased to ‘cd ..’
vagrant@homestead:/etc$ `type …`
… is aliased to ‘cd ../..’
vagrant@homestead:/etc$ `type alert`
alert is aliased to ‘notify-send –urgency=low -i “$([ $? = 0 ] && echo terminal || echo error)” “$(history|tail -n1|sed -e ‘s/^\s
vagrant@homestead:/etc$ `type c`
c is aliased to ‘clear’
vagrant@homestead:/etc$ `type egrep`
egrep is aliased to ‘egrep –color=auto’
vagrant@homestead:/etc$ `type fgrep`
fgrep is aliased to ‘fgrep –color=auto’
vagrant@homestead:/etc$ `type h`
h is aliased to ‘cd ~’
vagrant@homestead:/etc$ `type l`
l is aliased to ‘ls -CF’
vagrant@homestead:/etc$ `type la`
la is aliased to ‘ls -A’
vagrant@homestead:/etc$ `type ll`
ll is aliased to ‘ls -alF’
vagrant@homestead:/etc$ `type ls`
ls is aliased to ‘ls –color=auto’
Command Types In Linux Summary
In this episode we had a nice overview of the different Command Types In Linux. We saw how to check the command types from the shell using the
type command, which is certainly very useful. We also now have a good over view of how to get help in various ways for the different command types in Linux using tools like
man. We also found a way to find good commands for various use cases using the
apropos command, and finally we took a quick look at setting up your very own commands in Linux using the