bash.txt

uname --------- displays information about the operating system -----with no options — uname prints out the kernel name, in this case, Linux
!$ ------------ When you put !$ in a command in the bash, csh, or tcsh shell, the shell replaces the !$ with the last item, usually the last argument, of the previous command. For example: mkdir web_files ------------ cp *.html !$ # this is equelant to: cp *.html web_files
---------------------------------
file fil_name --------------- return discription of file.
history ---------- !3 # result of 3rd command listed in history ---------------- !p # result of cammand that starts with p
---------------------------------
most of these notes are from the book (The-linux-command-line-A-Complete-introduction)
---------------------------------
in 1991, Linus Torvalds wrote the first version of the Linux kernel
---------------------------------
Today, the world is very different. Computers are  verywhere, from tiny wristwatches to giant data centers to everything in between. In addition to ubiquitous computers, we also have a ubiquitous network connecting them together. This has created a wondrous new age of personal empowerment and creative freedom, but over the last couple of decades something else has been happening. A single giant corporation has been imposing its control over most of the world’s computers and deciding what you can and cannot do with them. Fortunately, people from all over the world are doing
something about it. They are fighting to maintain control of their computers by writing their own software. They are building Linux.
Many people speak of “freedom” with regard to Linux, but I don’t think most people know what this freedom really means. Freedom is the power to decide what your computer does, and the only way to have this freedom is to know what your computer is doing. Freedom is a computer that is without secrets, one where everything can be known if you care enough to find out.
---------------------------------
GUI = graphical user interface
CLI = command line interface
---------------------------------
Almost all Linux distributions supply a shell program from the GNU Project called bash.
The name bash is an acronym for Bourne Again Shell, a
reference to the fact that bash is an enhanced  eplacement for sh, the original Unix shell program written by Steve Bourne.
---------------------------------
When using a graphical user interface, we need another program called a terminal emulator to interact with the shell.
A number of terminal emulators are available for Linux, but they all do basically the same thing: give us access to the shell.
---------------------------------
If the last character of the prompt is a hash mark (#) rather than a dollar sign, the terminal session has  uperuser privileges. This means that either we are logged in as the root user or we’ve selected a terminal emulator that provides superuser (administrative) privileges
---------------------------------
ls   # multiple directories bhi pass kar sakty hen. eg: ls Desktop/ Downloads/
ls -lt #  the t option to sort the result by the file’s modification time
date # for date
cal  # for calender
df   # to know free spaces in disk drives
free # to know free space in RAM
pwd  # working directory
ls -a # for showing hiidden files also, Note: without <-a> you can\t obtain hidden files names.
less # View file contents, <cat> ki tarah hi kaam karta h, or kuch option zyada hen, asal faida ye h k ye <cat> k baraks poori file yakdam print nahi kardeta, balky 1 k baad 1 page print karta h.
---------------------------------
Linux has no concept of a “file extension” like some other operating systems. You may name files any way you like. The contents and/or purpose
of a file is determined by other means. Although Unix-like operating systems don’t use file extensions to determine the contents/purpose of files,
some application programs do
---------------------------------
aap file ko koi bhi extension dy do, linux uf file ko us k contant k hisab sy hi treat kary ga. let us say:
agar me py.csv me agar python ka code likh kar termial par <python py.csv> to agarcha is file ka extention <csv> h magar wo python ka code chala dy ga ---------------------------------
Commands are often followed by one or more options that modify their behavior and, further, by one or more arguments, the items upon which the command acts. So most commands look something like this:
command -options arguments
---------------------------------
In most Unix-like systems it is possible to
have a file referenced by multiple names. While the value of this may not
be obvious now, it is really a useful feature.
---------------------------------
wildcards:

* 						Any characters
? 						Any single character
[characters] 			Any character that is a member of the set characters
[!characters] 			Any character that is not a member of the set characters
[[:class:]] 			Any character that is a member of the specified class
---------------------------------
Commonly Used Character Classes:
[:alnum:] 	Any alphanumeric character
[:alpha:] 	Any alphabetic character
[:digit:] 	Any numeral
[:lower:] 	Any lowercase letter
[:upper:] 	Any uppercase letter
---------------------------------
Wildcard Examples:
* 							All files
g* 							Any file beginning with g
b*.txt 						Any file beginning with b followed by any characters and ending with .txt
Data???						Any file beginning with Data followed by exactly three characters
[abc]* 						Any file beginning with either a, b, or c
BACKUP.[0-9][0-9][0-9] 		Any file beginning with BACKUP. followed by exactly three numerals
[[:upper:]]* 				Any file beginning with an uppercase letter
[![:digit:]]* 				Any file not beginning with a numeral
*[[:lower:]123] 			Any file ending with a lowercase letter or the numerals 1, 2, or 3
---------------------------------
cp Options

-a, --archive 			Copy the files and directories and all of their attributes, including ownerships and 
						permissions. Normally, copies take on the default attributes of the user performing the copy.
-i, --interactive 		Before overwriting an existing file, prompt the user for confirmation. If this option is not 
						specified, cp will silently overwrite files.
-r, --recursive 		Recursively copy directories and their contents. This option (or the -a option) is required when 
						copying directories.
-u, --update 			When copying files from one directory to another, copy only files that either don’t exist or are
						newer than the existing corresponding files in the destination directory.
-v, --verbose 			Display informative messages as the copy is performed.
---------------------------------
cp Examples

cp file1 file2 			Copy file1 to file2. If file2 exists, it is overwritten with the contents of file1. If file2 does 
						not exist, it is created.
cp -i file1 file2 		Same as above, except that if file2 exists, the user is prompted before it is overwritten.
cp file1 file2 dir1 	Copy file1 and file2 into directory dir1. dir1 must already exist.
cp dir1/* dir2 			Using a wildcard, all the files in dir1 are copied into dir2. dir2 must already exist.
cp -r dir1 dir2 		Copy directory dir1 (and its contents) to directory dir2. If directory dir2 does not exist, it is 
						created and will contain the same contents as directory dir1.
---------------------------------
ls Options

-a 		--all 			List all files, even those with names that begin with a period, which are normally not listed
						(i.e., hidden).
-d 		--directory 	Ordinarily, if a directory is specified, ls will list the contents of the directory, not the
						directory itself. Use this option in conjunction with the -l option to see details about the
						directory rather than its contents.
-F 		--classify 		This option will append an indicator character to the end of each listed name (for example, a
						forward slash if the name is a directory).
-h 		--human-readable In long format listings, display file sizes in human-readable format rather than in bytes.
-l 						Display results in long format.
-r 		--reverse 		Display the results in reverse order. Normally, ls displays its results in ascending alphabetical 
						order.
-S 						Sort results by file size.
-t 						Sort by modification time.
---------------------------------
Be particularly careful with wildcards. Consider this classic example. Let’s say you want to delete just the HTML files in a directory. To do this, you type:
rm *.html
which is correct, but if you accidentally place a space between the * and the .html like so:
rm * .html
the rm command will delete all the files in the directory and then complain that there is no file called .html.
---------------------------------
rm Options:

-i, --interactive 		Before deleting an existing file, prompt the user for confirmation. If this option is not 
						specified, rm will silently delete files.
-r, --recursive 		Recursively delete directories. This means that if a directory being deleted has 
						subdirectories, delete them too. To delete a directory, this option must be specified.
-f, --force 			Ignore nonexistent files and do not prompt. This overrides the --interactive option.
-v, --verbose			 Display informative messages as the deletion is performed.
---------------------------------
rm Examples:

rm file1 			Delete file1 silently.
rm -i file1 		Before deleting file1, prompt the user for confirmation.
rm -r file1 dir1 	Delete file1 and dir1 and its contents.
rm -rf file1 dir1 	Same as above, except that if either file1 or dir1 does not exist, rm will continue silently.
---------------------------------
Links:
links (same as shortcuts in windows).

symbolic links ko ham hard links sy u identify kar sakty hen:
ls -li 
is sy jo list return ho gi us k first column me links or hardlinks ko 1 hi number assign ho ga.
---------------------------------
ln—Create Links: 
The ln command is used to create either hard or symbolic links. It is used in one of two ways:
	ln file link
to create a hard link and
	ln -s item link
to create a symbolic link where item is either a file or a directory.
---------------------------------
Links:

1- Hard Links
Hard links are the original Unix way of creating links; symbolic links are more modern. By default, every file has a single hard link that gives the file its name. When we create a hard link, we create an additional directory entry for a file. Hard links have two important limitations:
	A hard link cannot reference a file outside its own filesystem. This means a link cannot reference a file that is not on the same disk partition as the link itself.
	A hard link cannot reference a directory.
A hard link is indistinguishable from the file itself. Unlike a directory list containing a symbolic link, a directory list containing a hard link shows no special indication of the link. When a hard link is deleted, the link is removed, but the contents of the file itself continue to exist (that is, its space is not deallocated) until all links to the file are deleted.

It is important to be aware of hard links because you might encounter them from time to time, but modern practice prefers symbolic links, which we will cover next.


2- Symbolic Links
Symbolic links were created to overcome the limitations of hard links. Symbolic links work by creating a special type of file that contains a text pointer to the referenced file or directory. In this regard they operate in much the same way as a Windows shortcut, though of course they predate the Windows feature by many years. ;-)
A file pointed to by a symbolic link and the symbolic link itself are largely indistinguishable from one another. For example, if you write something to the symbolic link, the referenced file is also written to. However, when you delete a symbolic link, only the link is deleted, not the file itself.If the file is deleted before the symbolic link, the link will continue to exist but will point to nothing. In this case, the link is said to be broken. In many implementations, the ls command will display broken links in a distinguishing color, such as red, to reveal their presence.

The concept of links can seem confusing, but hang in there. We’re 	going to try all this stuff and it will, hopefully, become clear.
---------------------------------
hard link example:
cd 
mkdir playground
cd playground
mkdir folder1 folder2
cp /etc/passwd . 
# now we have 2 folders and 1 file
let us create a hard link for the <passwd>
ln passwd folder1/link_passwd    # yahan ham ny <passwd> ka 1 hard link folder1 k andar bana dya, ab data to 1 hi h is file ka magar us k 2 hard links hen (passwd) and (link_passwd), to ye (link_passwd) shortcut h (passwd) ka, ksi bhi link ko use karen data 1 hi h, yani k is data k ab 2 names ho gay, ksi bhi name sy call karen 1 hi data aay ga.
---------------------------------
Most Linux distributions configure ls to display broken links. On a Fedora box, broken links are displayed in blinking red text!
---------------------------------
type:
Display a Command’s Type

eg:
type cd
# cd is a shell builtin


type copyfrom
copyfrom is aliased to `xclip -sel cli <'
---------------------------------
which:
Display an Executable’s Location
works only for executable programs, not builtins or aliases that
are substitutes for actual executable programs.



eg:
which ls
/bin/ls
---------------------------------
help:
Get Help for Shell Builtins
help command == command --help

eg:
help cd
# cd: cd [-L|-P] [dir]
Change the current directory to DIR. The variable $HOME is the default DIR.
The variable CDPATH defines the search path for the directory containing DIR.
Alternative directory names in CDPATH are separated by a colon (:). A null
directory name is the same as the current directory, i.e. `.'. If DIR begins
with a slash (/), then CDPATH is not used. If the directory is not found, and
the shell option `cdable_vars' is set, then try the word as a variable name.
If that variable has a value, then cd to the value of that variable. The -P
option says to use the physical directory structure instead of following
symbolic links; the -L option forces symbolic links to be followed.


A note on notation: When square brackets appear in the description of
a command’s syntax, they indicate optional items. A vertical bar character
indicates mutually exclusive items. An example is the cd command above:
cd [-L|-P] [dir].
This notation says that the command cd may be followed optionally
by either a -L or a -P and further, optionally followed by the argument dir.
---------------------------------
whatis:
Display a Very Brief Description of a Command

The whatis program displays the name and a one-line description of a man page matching a specified keyword:

eg:
whatis ls
ls (1) - list directory contents
---------------------------------
info:
Display a Program’s Info Entry

The GNU Project provides an alternative to man pages called info pages. Info pages are displayed with a reader program named, appropriately enough, info. Info pages are hyperlinked much like web pages

eg:
info ls




The info program reads info files, which are tree-structured into individual nodes, each containing a single topic. Info files contain hyperlinks that can move you from node to node. A hyperlink can be identified by its leading asterisk and is activated by placing the cursor upon it and pressing the ENTER key.
To invoke info, enter info followed optionally by the name of a program. Table 5-2 lists commands used to control the reader while displaying an info page.


info Command:

? 						Display command help.
PAGE UP or BACKSPACE 	Display previous page.
PAGE DOWN or Spacebar 	Display next page.
n 						Next—Display the next node.
p 						Previous—Display the previous node.
u 						Up—Display the parent node of the currently displayed node, 
						usually a menu.
ENTER 					Follow the hyperlink at the cursor location.
q 						Quit
---------------------------------
Most of the command-line programs we have discussed so far are part of the GNU Project’s coreutils package, so you can find more information about them by typing:

info coreutils
---------------------------------
It’s possible to put more than one command on a line by separating each command with a semicolon character. It works like this:
command1; command2; command3...
---------------------------------
wc:
Print Line, Word, and Byte Counts.
eg:
wc file.txt
3 15 30 file.txt
# this file contain 3 lines, 15 words, and 30 Bytes.
---------------------------------
Filters:
Pipelines are often used to perform complex operations on data. It is possible to put several commands together into a pipeline. Frequently, the commands used this way are referred to as filters. Filters take input, change it
somehow, and then output it. The first one we will try is sort. Imagine we
want to make a combined list of all of the executable programs in /bin and
/usr/bin, put them in sorted order, and then view the list:
ls /bin /usr/bin | sort | less
Since we specified two directories (/bin and /usr/bin), the output of ls
would have consisted of two sorted lists, one for each directory. By including
sort in our pipeline, we changed the data to produce a single, sorted list.


uniq:
Report or Omit Repeated Lines
The uniq command is often used in conjunction with sort. uniq accepts a
sorted list of data from either standard input or a single filename argument
(see the uniq man page for details) and, by default, removes any duplicates
from the list. So, to make sure our list has no duplicates (that is, any programs of the same name that appear in both the /bin and /usr/bin directories) we will add uniq to our pipeline:
 ls /bin /usr/bin | sort | uniq | less
 In this example, we use uniq to remove any duplicates from the output
of the sort command. If we want to see the list of duplicates instead, we add
the -d option to uniq like so:
ls /bin /usr/bin | sort | uniq -d | less


grep:
Print Lines Matching a Pattern
grep is a powerful program used to find text patterns within files, like this:
grep pattern [file...]
When grep encounters a “pattern” in the file, it prints out the lines containing it. The patterns that grep can match can be very complex, but for
now we will concentrate on simple text matches. We’ll cover the advanced
patterns, called regular expressions, in Chapter 19.
Let’s say we want to find all the files in our list of programs that have the
word zip in the name. Such a search might give us an idea of which programs
on our system have something to do with file compression. We would do this:
ls /bin /usr/bin | sort | uniq | grep zip
# bunzip2
bzip2
gunzip
gzip
unzip
zip
zipcloak
zipgrep
zipinfo
zipnote
zipsplit
---------------------------------
head and tail:
# for text files, and also for ls, so:
ls /usr/bin | tail -n 5
# give us last 5 files names returned by (ls /usr/bin)
---------------------------------
tee:
Read from Stdin and Output to Stdout and Files
In keeping with our plumbing analogy, Linux provides a command called
tee which creates a “T” fitting on our pipe. The tee program reads standard
input and copies it to both standard output (allowing the data to continue
down the pipeline) and to one or more files. This is useful for capturing a
pipeline’s contents at an intermediate stage of processing. Here we repeat
one of our earlier examples, this time including tee to capture the entire
directory listing to the file ls.txt before grep filters the pipeline’s contents:
[me@linuxbox ~]$ ls /usr/bin | tee ls.txt | grep zip

# bunzip2
bzip2
gunzip
gzip
unzip
zip
zipcloak
zipgrep
zipinfo
zipnote
zipsplit
---------------------------------
Command Substitution
1- $
2- ``
Command substitution allows us to use the output of a command as an expansion:
[me@linuxbox ~]$ ls -l $(which cp)
# -rwxr-xr-x 1 root root 71516 2012-12-05 08:58 /bin/cp

Here we passed the results of which cp as an argument to the ls command, thereby getting the listing of the cp program without having to know its full pathname. We are not limited to just simple commands

There is an alternative syntax for command substitution in older shell programs that is also supported in bash. It uses back quotes instead of the dollar sign and parentheses:
[me@linuxbox ~]$ ls -l `which cp`
-rwxr-xr-x 1 root root 71516 2012-12-05 08:58 /bin/cp
---------------------------------
Double Quotes

If you place text
inside double quotes, all the special characters used by the shell lose their special meaning and are treated as ordinary characters. The exceptions are $ (dollar sign), \ (backslash), and ` (back tick). This means that word splitting, pathname expansion, tilde expansion, and brace expansion are suppressed, but parameter expansion,  rithmetic expansion, and command substitution are still carried out.

echo "$USER $((2+2)) $(cal)"

#me 4 February 2012
Su Mo Tu We Th Fr Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 
---------------------------------
The fact that newlines are considered delimiters by the word splitting
mechanism causes an interesting, albeit subtle, effect on command substitution. Consider the following:
[me@linuxbox ~]$ echo $(cal)
# February 2012 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

[me@linuxbox ~]$ echo "$(cal)"
# February 2012
Su Mo Tu We Th Fr Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29

In the first instance, the unquoted command substitution resulted in
a command line containing 38 arguments; in the second, the result was a
command line with 1 argument that includes the embedded spaces and
newlines.

---------------------------------
Single Quotes
If we need to suppress all expansions, we use single quotes. Here is a comparison of unquoted, double quotes, and single quotes:
[me@linuxbox ~]$ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
# text /home/me/ls-output.txt a b foo 4 me
[me@linuxbox ~]$ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER"
# text ~/*.txt {a,b} foo 4 me
[me@linuxbox ~]$ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'
# text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
---------------------------------
Escaping Characters
Sometimes we want to quote only a single character. To do this, we can precede a character with a backslash, which in this context is called the escape
character. Often this is done inside double quotes to selectively prevent an
expansion.
[me@linuxbox ~]$ echo "The balance for user $USER is: \$5.00"
# The balance for user me is: $5.00
It is also common to use escaping to eliminate the special meaning of a character in a filename. For example, it is possible to use characters in filenames that normally have special meaning to the shell. These would include
$, !, &, (a space), and others. To include a special character in a filename, you can do this:
[me@linuxbox ~]$ mv bad\&filename good_filename To allow a backslash character to appear, escape it by typing \\. Note that within single quotes, the backslash loses its special meaning and is treated as an ordinary character.
---------------------------------
keyboard commands that are used to edit characters on the command line.


# Cutting and Pasting (Killing and Yanking) Text
CTRL-A Move cursor to the beginning of the line.
CTRL-E Move cursor to the end of the line.
ALT-F Move cursor forward one word.
ALT-B Move cursor backward one word.
CTRL-A move cursor to beggining of line

# Text Editing Commands
CTRL-T Transpose (exchange) the character at the cursor location with  the one preceding it.
ALT-T Transpose the word at the cursor location with the one pre ceding it.
ALT-L Convert the characters from the cursor location to the end of the word to lowercase.
ALT-U Convert the characters from the cursor location to the end of the word to uppercase.

# Cut and Paste Commands
CTRL-K Kill text from the cursor location to the end of line.
CTRL-U Kill text from the cursor location to the beginning of the line.
ALT-D Kill text from the cursor location to the end of the current word.
ALT-BACKSPACE Kill text from the cursor location to the beginning of the cur rent word. If the cursor is at the beginning of a word, kill the previous word.
CTRL-Y Yank text from the kill-ring and insert it at the cursor location.
--------------------------------
history

bash maintains a history of commands that have been entered. This list of commands is kept in your home directory in a file called .bash_history. 


history me sy wo sari commands jin me (zip) word use hwa h
history | grep zip
---------------------------------
History Expansion Commands

!! Repeat the last command. It is probably easier to press the up arrow and ENTER.
!number Repeat history list item number.
!string Repeat last history list item starting with string.
!?string Repeat last history list item containing string
---------------------------------
Reading, Writing, and Executin

[me@linuxbox ~]$ > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-rw-r-- 1 me me 0 2012-03-06 14:52 foo.txt

The first 10 characters of the listing are the file attributes
The first of these characters is the file type.
The remaining nine characters of the
file attributes, called the file mode, represent the read, write, and execute permissions for the file’s owner, the file’s group owner, and everybody else. 

File Types
- 	A regular file.
d 	A directory.
l 	A symbolic link. Notice that with symbolic links, the remaining file attributes are always rwxrwxrwx and are dummy values. The real file attributes are those of the file the symbolic link points to.
c 	A character special file. This file type refers to a device that handles data as a stream of bytes, such as a terminal or modem.
b 	A block special file. This file type refers to a device that handles data in blocks, such as a hard drive or CD-ROM drive.


Permission Attributes

Attribute 	Files 												Directories
r 			Allows a file to be opened and read. 				Allows a directory’s contents to be listed if
																the execute attribute is also set.
w 			Allows a file to be written to or truncated; 
			however, this attribute does not allow files to 
			be renamed or deleted.
			The ability to delete or rename files is 
			determined by directory attributes.
																Allows files within a directory to be created,
																deleted, and renamed if the execute attribute is
																also set.
x 			Allows a file to be treated as a program and 
			executed. Program files written in scripting 
			languages must also be set as readable to be 
			executed.
																Allows a directory to
																be entered; e.g.,
																cd directory.



 Permission Attribute Examples
-rwx------ 	A regular file that is readable, writable, and executable by the file’s owner. No one 
			else has any access.
-rw------- 	A regular file that is readable and writable by the file’s owner. No one else has any access.
-rw-r--r-- 	A regular file that is readable and writable by the file’s owner. Members of the file’s 
			owner group may read the file. The file is world readable.
-rwxr-xr-x 	A regular file that is readable, writable, and executable by the file’s owner. The file 
			may be read and executed by everybody else.
-rw-rw---- 	A regular file that is readable and writable by the file’s owner and members of the file’s 
			owner group only.
lrwxrwxrwx 	A symbolic link. All symbolic links have “dummy” permissions. The real permissions are kept 
			with the actual file pointed to by the symbolic link.
drwxrwx--- 	A directory. The owner and the members of the owner group may enter the directory and 
			create, rename, and remove files within the directory.
drwxr-x--- 	A directory. The owner may enter the directory and create, rename, and delete files within 
			the directory. Members of the owner group may enter the directory but cannot create, delete, 
			or rename files.



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