OS-Module 2 Linux Programming Important topics

Linux Shell Programming
UNIT 2- Syllabus

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Introduction
■ Shell is an environment in which we can run our commands, programs, and
shell scripts.
■ It gathers input from you and executes programs based on that input. When
a program finishes executing, it displays that program's output.
■ - kernal = core of a computer’s operating system
■ - terminal = text input/output environment
■ - shell = command line interpreter
■ - shell scripts = commands written in a file

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Shells Available in Unix
- C shell (csh)
- TC shell (tcsh)
- Bourne shell (sh)
- Korn shell (ksh)
- Bourne Again Shell (bash)
Bourne Again Shell (bash)
Bash is the default shell in most Linux distributions.Bash offers functional
improvements over sh for both programming and interactive use.

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Special Characters
Comments-Lines beginning with a # (with the exception of #!) will not be executed.
# This line is a comment.
; Command separator. Permits putting two or more commands on the same line.
echo hello ; echo world
;; Terminator in a case option
case "$variable" in
abc) echo "$variable = abc" ;;
xyz) echo "$variable = xyz" ;;
esac
` command substitution. The `command` construct makes available the output of
command for assignment to a variable

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Special Characters
$ Variable substitution (contents of a variable)
$(variable)
“ “ Weak quotes
echo “hello world”
‘ ‘ Strong quotes
echo ‘hello ‘world’ ’
Single character quote

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Getting help
The help command provides information on built-in commands.
simply entering ‘help’ at the terminal prompt will show a complete list of the built-in
commands available.
enter ‘help’ followed by the command you wish to learn more about.
help pwd
With the ‘-d’ option, the help command will only return a short description of the specified
command.
help -d pwd
With the -s option, help will return a short usage synopsis for the specified command.
help -s pwd
The -m option formats the help command output as a pseudo-manpage.
help -m pwd

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man pages
The man command shows detailed manuals for each command. These are referred to as “man
pages.”
It provides a detailed view of the command which includes NAME, SYNOPSIS, DESCRIPTION,
OPTIONS, EXIT STATUS, RETURN VALUES, ERRORS, FILES, VERSIONS, EXAMPLES, AUTHORS ..etc
man pwd

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Linux file system structure

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Linux home page structure

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Commands for navigating the Linux file system
★ pwd - prints the complete path of the current working directory.
pwd -L : Prints the symbolic path.
pwd -P : Prints the actual path.
★ cd - change the current working directory.
cd [directory]
★ ls - list of the names of all files in the current working directory / specified directory.
ls
ls /etc
★ file - displays the type of a file.
file [option] [filename]
file -b filename : used to display just file type in brief mode.
file directoryname/* : used to display all files filetypes in particular directory.

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★ cat – read and concatenate data from the file and gives their content as output.
cat filename read single filename
cat file1 file2 show contents of file1 and file2
cat -n file show contents of fille with preceding line number
cat -s file suppress repeated empty lines in output
★ cp - copy fiiles or directories from source to destination
cp Src_file Dest_file copy the contents of src_file to the dest_file
cp Src_file1 Src_file2 Src_file3 Dest_directory copies each source file to the destination directory
cp -R Src_directory Dest_directory copies all files of the source directory to the destination directory, creating any files or directories
needed
★ mv - move fiiles or directories from source to destination
mv Src_file Dest_file move the file src_file to the dest_file
mv -i Src_file Dest_directory ask the user for confirmation before moving a file that would overwrite an existing file,
mv -f Src_directory Dest_directory overwrite the destination file forcefully and delete the source file.

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★ mkdir - create multiple directories at once as well as set the permissions for the directories
mkdir foldername
mkdir -m 777 foldername used to set the file modes
★ rmdir - remove empty directories from the filesystem
rmdir -p directory remove directory if it is empty
rmdir -p directory remove all child and parent empty directories
rmdir directorylist remove all directories
★ whereis - locates source/binary and manuals sections for specified files.
whereis perl List the directories where the perl source files, documentation, and binaries are stored.
whereis -b perl List only perl binaries are stored.
whereis -s perl List only perl sources are stored.
whereis -m perl List only perl manuals are stored.

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Piping and Redirection
Every program we run on the command line automatically has three data streams connected
to it.
STDIN (0) - Standard input (data fed into the program)
STDOUT (1) - Standard output (data printed by the program, defaults to the terminal)
STDERR (2) - Standard error (for error messages, also defaults to the terminal)
Piping and redirection is the means by which we may connect these streams between
programs and files to direct data in interesting and useful ways.

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Piping
Pipes allow you to funnel the output from one command into another where it will be used as
the input.
Pipes are unidirectional
The | operator feeds the output from the program on the left to the program on the right.
This will sort the given file and print the unique values only.
sort record.txt | uniq

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Redirection
By default, stdout and stderr are printed to your terminal.
But we can redirect that output to a file using the > operator:
echo hello > new-file
echo didn’t print anything to the terminal because we redirected its output to a file named new-file.
it replaces new-file’s contents with the new contents.
If you want to append to the file, you can use the >> operator
echo hello again >> new-file

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Informal Commands
★ ps - to see which processes you're currently running on your Linux system.
Using the -f option for ps you can gain additional useful information on each process in the
listing.
★ w - command-line utility that displays information about currently logged in users and what
each user is doing. It also gives information about how long the system has been running, the
current time, and the system load average.
★ id - used to find out user and group names and numeric ID’s (UID or group ID) of the
current user or any other user in the server.
★ free - displays the total amount of free space available along with the amount of memory
used and swap memory in the system, and also the buffers used by the kernel.
★ clear - used to remove all previous commands and output fromterminal windows

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Informal Commands
★ echo - display a line of text/string on standard output or a file.
echo [option(s)] [string(s)]
eg:-
echo Hello World
echo The value of variable x = $x
echo -e "Hello nWorld"
★ more - used to view the text files in the command prompt, displaying one screen at a time in case the file is large. The
more command also allows the user do scroll up and down through the page.
more [-options] [-num] [+/pattern] [+linenum] [file_name]
[-options]: any option that you want to use in order to change the way the file is displayed. (-d, -l, -f, -p, -c, -s, -u)
[-num]: type the number of lines that you want to display per screen.
[+/pattern]: replace the pattern with any string that you want to find in the text file.
[+linenum]: use the line number from where you want to start displaying the text content.
[file_name]: name of the file containing the text that you want to display on the screen.

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File Permissions
Ownership of Linux files
Every file and directory on your Unix/Linux system is assigned 3 types of owner,
User : owner of the file. By default, the person who created a file becomes its owner.
Group : group can contain multiple users. All users belonging to a group will have the same access
permissions to the file.
Other : This person has neither created the file, nor he belongs to a usergroup who could own the
file. Practically, it means everybody else.
Permissions
Every file and directory in your UNIX/Linux system has following 3 permissions.
Read : authority to open and read a file.
Write: authority to modify the contents of a file.
Execute : In Unix/Linux, you cannot run a program unless the execute permission is set.

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File Permissions
using chmod command, we can set permissions (read, write, execute) on a file/directory for
the owner, group and the world.
chmod permission file
There are 2 ways to use the command
★ Absolute mode
★ Symbolic mode

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File Permissions
Absolute Mode
file permissions are not represented as characters but a three-digit octal number.
chmod 764 filename
‘764' absolute code says the following:
- Owner can read, write and execute
- Usergroup can read and write
- World can only read
This is shown as
chmod -rwxrw-r- filename
Number Permission Type Symbol
0 No permission ---
1 Execute --x
2 Write -w-
3 Write + Execute -wx
4 Read r--
5 Read+Execute r-x
6 Read+Write rw-
7 Read+Write+Execute rwx

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File Permissions
Symbolic Mode
n the symbolic mode, you can modify permissions of a specific owner.
It makes use of mathematical symbols to modify the file permissions..
chmod o=rwx filename
setting others read+write+execute permissions
chmod g+x filename
adding execute permission to group
chmod u-r filename
removing read permission of user
User Description
u user
g group
o other
Operator Description
+ No permission
- Execute
= Write

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Comments
Lines beginning with a # (with the exception of #!) are comments and will not be
executed.
# This line is a comment
Comments may also occur following the end of a command.
echo “a comment will follow” # this is a comment
Comments may also follow whitespace at the beginning of a line.
# This is also a comment
a quoted or an escaped # in an echo statement does not begin a comment.
echo “hello #This is not a comment”

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Variables
A variable is a name assigned to a location or set of locations in computer memory
holding an item of data.
If variable1 is the name of a variable, then $variable1 is a reference to its value.
The only times a variable appears without the $ prefix is when declared or assigned.
variable1=23
echo $variable1
Essentially, Bash variables are character strings, but, depending on context, Bash
permits arithmetic operations and comparisons on variables. The determining factor
is whether the value of a variable contains only digits.

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Operators
Assignment Operator
= is the all-purpose assignment operator, which works for both arithmetic and string
assignments.
var=27
category=minerals # No spaces allowed after the "="
Logical Operators
! NOT
&& AND
|| OR

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Operators
Arithmetic Operators
+ plus
- minus
* multiplication
/ division
** exponentiation
% mod (returns the remainder of an integer division operation)
+= plus-equal (increment variable by a constant) [38]
-= minus-equal (decrement variable by a constant)
*= times-equal (multiply variable by a constant)
/= slash-equal (divide variable by a constant)
%= mod-equal (remainder of dividing variable by a constant)

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Operators
Relational Operators
-eq equal to
-nt not equal to
-gt greater than
-lt lessthan
-ge greater than or equal to
-le less than or equal to
< is less than (within double parentheses) (("$a" < "$b"))
<= is less than or equal to (within double parentheses) (("$a" <= "$b"))
> is greater than (within double parentheses) (("$a" > "$b"))
>= is greater than or equal to (within double parentheses) (("$a" >= "$b"))

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Quoting
When referencing a variable, it is generally advisable to enclose its name in
double quotes. This prevents reinterpretation of all special characters within
the quoted string except $, ` (backquote), and (escape).
Use double quotes to prevent word splitting.An argument enclosed in double
quotes presents itself as a single word, even if it contains whitespace
separators.
Within single quotes, every special character except ' gets interpreted.
Consider single quotes ("full quoting") to be a stricter method of quoting than
double quotes ("partial quoting").
The escape () preceding a character tells the shell to interpret that character
literally.

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read
Reads the value of a variable from stdin, that is, interactively fetches input from the
keyboard.
echo "Enter the value of variable 'var1': "
read var1
echo
prints (to stdout) an expression or variable
echo Hello
echo $a
An echo requires the -e option to print escaped characters.
Normally, each echo command prints a terminal newline, but the -n option suppresses
this.

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Conditional Commands
if can test any command.Reads the value of a variable from stdin, that is, interactively fetches input from the keyboard.
if [ condition ]
then
command1
else
command2
fi
elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.
if [ condition1 ]
then
command1
elif [ condition2 ]
then
command4
else
default-command
fi

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Conditional Commands
The case construct is the shell scripting analog to switch in C/C++. It permits
branching to one of a number of code blocks, depending on condition tests.
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;

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iterative commands
A loop is a block of code that iterates [52] a list of commands as long as the loop
control condition is true.
for arg in [list]
do
command(s)...
done
This construct tests for a condition at the top of a loop, and keeps looping as long as
that condition is true (returns a 0 exit status)..
while [ condition ]
do
command(s)...
done

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break and continue
break command terminates the loop (breaks out of it)
for innerloop in 1 2 3 4 5
do
if [ "$innerloop" -eq 3 ]
then
break # Try break 2 to see what happens.
fi
done
continue causes a jump to the next iteration of the loop, skipping all the remaining
commands in that particular loop cycle.
for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [[ "$inner" -eq 7 && "$outer" = "III" ]]
then
continue 2 # Continue at loop on 2nd level, that is "outer loop".
fi
done

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expr
All-purpose expression evaluator: Concatenates and evaluates the arguments
according to the operation given (arguments must be separated by spaces).
a=`expr 5 + 3`
-

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bc
Bash can't handle floating point calculations, and it lacks operators for certain
important mathematical functions.
echo "scale=2; 2/3" | bc
.66

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grep
searches a file for a particular pattern of characters and displays all lines that contain
that pattern.
case insensitive search
grep -i “Word” file.txt
display count of matches
grep -c “Word” file.txt
checking whole word only
grep -w “Word” file.txt

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arrays
Bash arrays can hold multiple values at the same time.
arrayname[index]=value

OS-Module 2 Linux Programming Important topics

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