Kinds of Shells
---------------
bash
- Bourne Again
SHell
- based on
earlier version of sh
- commonly
default shell in Linux
sh
- Bourne shell
- this is were
BASH was based
- not often
used in Linux but usually a pointer to /bin/bash
tcsh
- based on
earlier C shell (csh)
- not popular
in Linux
csh
- the original
C Shell
- not commonly
used in Linux
- very similar
to tcsh
ksh
- combination
of features of csh and sh
zsh
- Z Shell
- evolution of
ksh
How to start a shell?
--------------------
There are several
ways and here are the most common ways.
1. Once you logged in
to a linux server (either by SSH or via console), you
automatically starts your own shell
2. In xterm or GUI,
you can right click on the desktop and open a terminal. In
that way, you are also starting a shell
Parts of a shell
----------------
Understanding every
part of the shell is very important specially for linux
administrators
because it is the one you interact with everytime you deal with
Linux.
As an example, when
you logged as a regular, the shell will provide you a prompt
where you can type
your commands.
$ uname
Linux
$
The "$"
represents something. In Linux by default, it means that you are logged
in as regular user.
You may customized it by changing the PS1 environment
variable.
A root's prompt
starts with "#" so its important to clearly distinguished a
regular user's prompt
from a root's prompt. That can avoid accidentally running
destructive command
like "rm -fr *" on /. The command used in the example above
is "uname"
and its output is "Linux". Depending on the command you type, it may
return an output or
nor. Or it may even return an error.
Types of commands you can use
-----------------------------
2 types of commands
are "Internal" and "External" commands. Internal are the
ones built-in in
linux while External are the ones came from outside sources
(e.g came from
installing a package). To see what type
of command, execute
the following:
$ type cd
cd is a shell builtin
$ type date
date is /bin/date
$
$ type -a pwd
pwd is a shell
builtin
pwd is /bin/pwd
$
The first example
tells us that "cd" is an Internal command while the second one
indicates that
"date" is an external command. There are some commands you may
encounter that has
both Internal and External versions.
So to see it, add
"-a" on type command that shown in the 3rd example above.
Keeping Track of Command History
--------------------------------
- bash history
displays previous commands (usually last 500)
- this doesn't record
the commands you enter inside of a prompt as:
[root@localhost ~] passwd # this will appear in bash history
Changing password for user root.
Changing password for root.
Current password: # anything you type here will not
history # displays typically last 500 lines
history -c # clears current history
~/.bash_history # this is your history file (be aware that
this might be
readable by other
users)
Ways of executing a program
---------------------------
1. Using
"exec"
- run by: exec /path/to/program
- exec doesn't create a new process
- exec replaces the current shell
- when the process terminate, your shell also
terminates
2. The most common
way
- to run a script this way: ./myscript.sh
- or this way: /path/to/myscript.sh
- the script must have executes bit enabled
(chmod +x myscript.sh)
Shell Tricks
------------
Typing in linux
terminal is a major part in using a Linux based system so
here are some shell
tricks and shortcuts that makes typing easier and more
efficient.
tab completion
- you may hit TAB at the end of each command
to auto complete
- if the amount of string entered doesn't
allow bash to determine uniqueness,
you may
hit another TAB to display possible results
(so it would be TAB..TAB)
- further improved by installing
bash-completion
examining previous
commands entered
- typing "history" will show past
entries accompanied with a number
- given the history number, you may type
! to re-execute a specific
entry which
is helpful in re-typing long commands such
as "for i in blah blah..; do
blah ... blah
- another efficient way of searching is by
using "reverse search"; that is by
pressing CTRL + R then you may start
searching any keyword at the start,
middle, or end of the command
fast moving
- pressing ALT + F (or CTRL + right arrow in
some) will move the cursor
position 1 word forward while ALT + B to
move 1 word backward (CTRL + left
for some)
- to quickly move at the end of the line, you
may press CTRL + E (or just hit
END button) on your keyboard while moving
at the start is CTRL + A (or just
HOME button)
delete multiple words
at a time
- by default pressing backspace (or CTRL + H)
will delete 1 character at a
time
- to delete 1 word at a time, press CTRL + W
- pressing CTRL + U will delete all words
before the current cursor position
- CTRL + K will delete all words after the
current cursor positi
getting the last word
of previous command (one of my favorite)
- often we need to use the last word of the
previous command we entered
- as an example, see the series of commands
below:
touch /tmp/test
chmod 600 /tmp/test
vi /tmp/test
- to quickly reuse /tmp/test instead of
re-typing it or copying it, you may
press ESC followed by . (period)
Terminating the shell
---------------------
Two commands can be
used to terminate a shell session:
exit - terminates any
shell
logout - same as
above but terminates only login shells (e.g logins from ssh)
Getting help
------------
Since we are dealing
with command line, we often need more information on a
particular command we
are using. There are several ways on how to display
command information.
1. Using
"man" - perhaps the most important help utility in Linux
man
man -k
# searches the man database for commands (or description)
matching the specified
string
man -K
# similar as above but do a full search (takes
significantly
longer to complete)
man
manual section (see discussion
below)
Man contains sections
which groups the type of information you can see on a
particular command.
Not all command have the same manual sections. Here are the
example of manual
sections:
Section Number
1 -- user
commands (contains traditional command usage and options)
2 -- system
calls provided by the kernel (not commonly used by sys ads)
3 -- library
calls
4 -- device
files
5 -- file
formats (these are config files like "man 5 fstab")
6 -- games
7 --
miscellaneous
8 -- system
administration commands (usually ones that must be run as root)
9 -- kernel
routines
On the 9 manual
sections, only 1, 5, and 8 are the most commonly used by system
administrators.
2. Using pinfo/info -
not all commands support this
info coreutils
'ls invocation' # as an example, you can see this command
under "SEE ALSO" of ls (1)
3. Exploring
/usr/share/doc
- some
packages includes README files under this directory
- you can see
examples here
- sometimes
there are also sample configurations
Environment Variables
---------------------
These are global
values read by the shell and inherited by the user (which can
also be overriden)
during startup. To see all environment variables, type "env".
Here are the common
environment variables in Linux.
PS1 - this is your
prompt (e.g [user@host] $)
HOME - contains path
to your home directory
HOSTNAME - the
hostname of current machine where you are logged in
LOGNAME - the
username used to login to the machine
To display the
values, use the following command:
echo $HOME
sources:
PS1 code generator | http://bashrcgenerator.com/
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