With our previous post, we knew that UNIX is an operating system which was developed in and Bells Labs back in 1970 and after that many versions of UNIX have become available and some UNIX Like Operating Systems like Linux has came into picture. In the time line Linux has also gained popularity and most of the people today use Linux whether they know it or not.
We have taken look on the big picture. Now, we'll be looking into the basics of the Operating System.
UNIX/Linux has the following programs at its bare minimum,
We have taken look on the big picture. Now, we'll be looking into the basics of the Operating System.
UNIX/Linux has the following programs at its bare minimum,
Kernel
The programs that deals with the hardware directly. This is the main building block of the OS. These basically allocates time and memory of other programs. Also deals with low level file structure and communication related works.
Shell
Shell can be thought of a bridge between the Kernel and the user. It is a Comman Line Interpreter (CLI). When user types in any command in the Shell, the shell finds the exact program that deals with the command and invokes the kernel.
There are many versions of shell available. Each comes with its own features.
To name a few different Unix/Linux shells,
There are many versions of shell available. Each comes with its own features.
To name a few different Unix/Linux shells,
- B Shell
- C Shell
- K Shell
- Bash Shell
- tcsh
- Zsh
File and Processes
Well, in UNIX everything can be categorized in either of these types, File/Process.
A file is a collection of data. As I mention data, it may be anything, a document, a C program, a media file, an executable. Files are arranged in directories.
Each file has a name associated with it which works as an unique identifier for the file within a directory. Similar goes for a directory, within a single parent directory, two sub-directories cannot have same name.
Now coming to process, a process is a program that is being executed in the hardware. A process has also a unique identifier for it and is known as PID (Proces Identifier). We can perform many operations by our own with this PID.
Directories
As we've known that files are arranged into directories and in the similar fashion, directories can be again arranged into directories. The hierarchy starts usually with root directory and is usually marked with front slash (/). If we think this a graph, then directories are the node where files are the leaf nodes of the graph.
Let's look at a directory hierarchy,
Let's look at a directory hierarchy,
In this hierarchy, we can see that root directory has two directories bin and home(There are many more directories present in the root. This is just for demonstration purpose. Don't get confused.). So, bin and home are sibling. In home directory, we see another two directories palash and Anyone. Similarly, in palash we've git and within that we've me and within me we find a file pom.xml. So, except pom.xml all others are directories and bin & home are sibling while palash & Anyone are sibling.
Now, let's look into the running processes. To see all the running processes, issue the following command in the shell, To open a shell press Ctrl+Alt+T. This is the default key combination for opening a terminal.
ps -ef
You will see the output similar to the following,
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 Feb23 ? 00:00:19 /sbin/init
root 2 0 0 Feb23 ? 00:00:00 [kthreadd]
root 3 2 0 Feb23 ? 00:00:04 [ksoftirqd/0]
root 4 2 0 Feb23 ? 00:00:00 [kworker/0:0]
root 5 2 0 Feb23 ? 00:00:00 [kworker/0:0H]
root 7 2 0 Feb23 ? 00:00:32 [rcu_sched]
root 8 2 0 Feb23 ? 00:00:00 [rcu_bh]
root 9 2 0 Feb23 ? 00:00:28 [rcuos/0]
root 10 2 0 Feb23 ? 00:00:00 [rcuob/0]
root 11 2 0 Feb23 ? 00:00:01 [migration/0]
root 12 2 0 Feb23 ? 00:00:00 [watchdog/0]
root 13 2 0 Feb23 ? 00:00:00 [watchdog/1]
root 14 2 0 Feb23 ? 00:00:00 [migration/1]
root 15 2 0 Feb23 ? 00:00:03 [ksoftirqd/1]
root 17 2 0 Feb23 ? 00:00:00 [kworker/1:0H]
root 18 2 0 Feb23 ? 00:00:06 [rcuos/1]
root 19 2 0 Feb23 ? 00:00:00 [rcuob/1]
root 20 2 0 Feb23 ? 00:00:00 [khelper]
root 21 2 0 Feb23 ? 00:00:00 [kdevtmpfs]
root 22 2 0 Feb23 ? 00:00:00 [netns]
root 23 2 0 Feb23 ? 00:00:00 [perf]
root 24 2 0 Feb23 ? 00:00:00 [khungtaskd]
root 25 2 0 Feb23 ? 00:00:00 [writeback]
root 26 2 0 Feb23 ? 00:00:00 [ksmd]
root 27 2 0 Feb23 ? 00:00:19 [khugepaged]
root 28 2 0 Feb23 ? 00:00:00 [crypto]
root 29 2 0 Feb23 ? 00:00:00 [kintegrityd]
root 30 2 0 Feb23 ? 00:00:00 [bioset]
root 31 2 0 Feb23 ? 00:00:00 [kblockd]
root 32 2 0 Feb23 ? 00:00:00 [ata_sff]
root 33 2 0 Feb23 ? 00:00:00 [md]
root 34 2 0 Feb23 ? 00:00:00 [devfreq_wq]
root 36 2 0 Feb23 ? 00:00:07 [kworker/1:1]
root 38 2 0 Feb23 ? 00:00:00 [kswapd0]
root 39 2 0 Feb23 ? 00:00:00 [fsnotify_mark]
root 40 2 0 Feb23 ? 00:00:00 [ecryptfs-kthrea]
root 52 2 0 Feb23 ? 00:00:00 [kthrotld]
root 53 2 0 Feb23 ? 00:00:00 [acpi_thermal_pm]
root 54 2 0 Feb23 ? 00:00:00 [scsi_eh_0]
root 55 2 0 Feb23 ? 00:00:00 [scsi_tmf_0]
root 56 2 0 Feb23 ? 00:00:00 [scsi_eh_1]
root 57 2 0 Feb23 ? 00:00:00 [scsi_tmf_1]
root 59 2 0 Feb23 ? 00:00:29 [kworker/0:1]
root 63 2 0 Feb23 ? 00:00:00 [ipv6_addrconf]
root 84 2 0 Feb23 ? 00:00:00 [deferwq]
root 85 2 0 Feb23 ? 00:00:00 [charger_manager]
root 139 2 0 Feb23 ? 00:00:00 [mpt_poll_0]
root 140 2 0 Feb23 ? 00:00:00 [mpt/0]
root 141 2 0 Feb23 ? 00:00:00 [kpsmoused]
root 142 2 0 Feb23 ? 00:00:00 [kworker/1:2]
root 143 2 0 Feb23 ? 00:00:00 [scsi_eh_2]
root 144 2 0 Feb23 ? 00:00:00 [scsi_tmf_2]
root 146 2 0 Feb23 ? 00:00:00 [scsi_eh_3]
root 147 2 0 Feb23 ? 00:00:00 [scsi_tmf_3]
root 148 2 0 Feb23 ? 00:00:00 [scsi_eh_4]
root 149 2 0 Feb23 ? 00:00:00 [scsi_tmf_4]
root 150 2 0 Feb23 ? 00:00:00 [scsi_eh_5]
Well, these are some processes running in my system. This list may vary from system to system. Also the output may vary depending on the system you are working.
For example, this output was taken from Ubuntu 14.04 while in FreeBSD you will get some different output. But one thing will be common in all the output, PID. This shows the system generated identifier for the process.
That's all what I can tell you for now about the very much basics of UNIX/Linux.
Happy learning...
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