Difference between Linux shell command and System call

What is shell?

The shell is the part of OS that is most visible to the user. It receives and interprets commands entered by the user. Every activity initiated by the user involves a shell command either directly or indirectly. The request can be a utility or application program, and the shell forwards the corresponding request to the kernel.

A shell consists of 2 parts.

(1) An interpreter that reads the commands and works with the kernel to execute them.

(2) A programming capability to write the command script. It can be a terminal, or shell script.

Commands form the basis of all linux interactions. It is an action request given to the shell for execution. They are normally single-lined, and may invoke utilities or user applications.They apply some action on some input data and create some output data. The input may be keyboard, disk files or system files. The output can be monitor, printer, system files or disk files.

The syntax of a shell command is:

verb [options] [arguments]

Verb – Command name

Options – How the command is applied, defined by characters preceded by +

Argument – Additional information like object identifiers, file names, etc.

Command – cal
——————-

*The command displays the calendar for a specified month or year.

cal [[month] year]

Command – date
———————

The command displays the system date and time. The time shown will be the time where the system is physically located (if remote
access, the remote time is taken).

date [-u] [format]

Command – who
———————

Displays all users currently logged into the system

who [-u IdleTime –H Header] [am i]

u– Indicates idle time. If inactive for 24 hours or more, displays “old”

H– Adds header to the display

am I– only display about yourself

Command – passwd
————————

Change the password

passwd

No options and arguments.

Command – echo
———————-

Copies argument back to the terminal.

echo [argument]

If one word, it is displayed directly. If more than one word message, better enclose them in quotes.

Command – man
———————-

Looks up documentation for the command

man [-k search] [keyword]

The command looks up the manual for the given keyword. If you don’t know the keyword itself, you can search using –k.

Command – lpr
——————-

Utility for line printer. If no argument is given, keyboard is taken as input.

lpr [-Pprinter] [file-list]

If no printer is specified, it sends output to default printer. You can specify the printer name using –P, followed by printer name (no space between option and name).

Eg:

lpr file1 file2 file3

lpr –Plp0 file1

Command – tty
——————-

Display the name of terminal you are using.

tty

By default, the terminal is /dev/ttyq0 or simply ttyq0.

Command – clear
———————–

Clears the console and puts cursor at top.

clear

Command – stty
——————–

Set or unset terminal I/O options.

stty [-ag] [arguments]

Without any argument, it simply lists the current settings for the terminal. Neither option allows arguments. –a displays current settings, while –g displays selected settings in a format usable for other programs. When you use arguments, you set terminal properties.

Command – script
———————–

Record an interactive session.

script [-a] [filename]

To record the whole session, make it the first command of the session. It stops recording when you give the “exit” command. The record is automatically placed in a file typescript. If –a is used, the existing typescript file is appended with current data. If we
need a different file name, give it as the argument.

Command – uname
———————–

Display data about UNIX itself.

uname [-ansr]

a -all options

n -name only

s -operating system

r -release

Command – bc
——————-

A calculator-like language in started. It ends with ^d.

bc

Internally, the following is possible.

(1)Simple arithmetic – +,-,*,/,%,^

(2)Floating point – set number of decimal points with scale.

(3)Arithmetic base – Set input and output bases with ibase and obase (max. 16)

System call
==========

For performing any operation a user must have to request for a service call which is know as system call.

* They are typically written in a high-level language.

* There are two types modes in the operation of system which is user mode or system mode

1) User mode:-All user processed are executed.
2) System mode:- All privileged operations are executed

The user programs and kernal functions are being executed in their respective space alloted in the main memory partitions.User mode programs need to execute some privileged operations, Which are not permitted in user mode functions, user mode interface between user mode and kernal mode. This interface is called system call. So system calls is an interface between the user program and the OS.

System call generate an interrupt that causes the OS to gain control of the CPU. The OS then finds out the type of system call and the corresponding interrupt handler routine is executed to perform the operation.

Basically there are five broad categories of system calls:-

(1) Process control system calls

Process is a basic entity in the system. The processes in te system need to be created,deleted and aborted. Besides these many operations are required on the processes for their management. There are some example

*fork():- Create a process
*exit():- Termiante a process
*kill():- Terminate a process

(2) File management system calls

Creation, deletion,opening,closing,reading and writing are some general operatio on files, there is a directory system and thereby ayatem calls for managing them.

*create ():- To create a new file
*open() :- To open a file
*close() :- To close a file
*read() :- To read a file
*write() :- To write a file

(3) Device management system calls

The general commands are:-

(1) Request of device
(2) Release of device
(3) Read & write operation and so on

(4) Information maintenance system calls

Many system calls exist simply for the purpose of transferring information between the user program and the OS.

==> return current time and date.
==> no:of current users.
==> Amount of free memory

Ex:gettime(),setdate(),getprocess(),getdate(),getsystemdate(),setprocess()

(5) Communication system calls

Communication amoung the process in the system

General operations
—————————

==> opening and closing the connection
==> sending & receving messages
==> Reading & writing messages and so on ..

Msgsnd() Sending a message
Msgrcv() Receving a message

These system calls may be related to the communication between process either on the same machine or between processes on different nodes of a network. Thus interprocess communication is provided by the OS through these communication-related system calls.

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