Scripting and the shell in LINUX

Scripting and the Shell
Prepared By
Prof.Bhushan Pawar
MESCOE,Pune(Wadia Campus)
bhushan.pawar@mescoepune.org

SHELL BASICS
• All major shells in the sh(including Bourne
shell(Bash),ksh(KornShell).
• Command for editing :-
– emacs commands
– <Control-E> goes to the end of the line
– <Control-A> to the beginning.
– <Control-P> steps backward through recently
executed commands and recalls them for editing.
– <Control-R> searches incrementally through your
history to find old commands.
– http://www.computerhope.com/unix/uemacs.htm

SHELL BASICS (Continue…)
• If you like vi, put your shell’s command-line
editing into vi mode like this:
$ set -o vi
$ set -o emacs

Pipes and redirection
• Every process has at least three
communication channels available to it:
– “standard input” (STDIN)
– “standard output”(STDOUT)
– “standard error” (STDERR).

Pipes and redirection(Continue…)
• UNIX has a unified I/O model in which each
channel is named with a small integer called a
(File Descriptor).
• The exact number assigned to a channel is not
usually significant, but STDIN, STDOUT, and
STDERR are guaranteed to correspond to file
descriptors 0, 1, and 2, so it’s safe to refer to
these channels by number. In the context of an
interactive terminal window, STDIN normally
reads from the keyboard and both STDOUT and
STDERR write their output to the screen.

• The shell interprets the symbols <, >, and >>
as instructions to reroute a command’s input
or output to or from a file. A < symbol
connects the command’s STDIN to the
contents of an existing file. The > and >>
symbols redirect STDOUT.
• > replaces the file’s existing contents, and >>
appends to them.

• $ echo "This is a test message." > /tmp/mymessage
-it stores a single line in the file /tmp/mymessage,
creating the file if necessary.
• To connect the STDOUT of one command to the STDIN of
another, use the | symbol, commonly known as a pipe.
– E.g. $ ps -ef | grep httpd
(example runs ps to generate a list of processes and pipes it
through the grep command to select lines that contain the
word httpd.)

Variables and quoting
• Variable names are unmarked in assignments
but prefixed with a dollar sign ($)when their
values are referenced.
– E.g. $ etcdir='/etc‘
$ echo $etcdir

Common filter commands
• cut: separate lines into fields
– Syntax:- cut -c4 file.txt
– Explanation : The above cut command prints the
fourth character in each line of the file
– ( http://www.folkstalk.com/2012/02/cut-
command-in-unix-linux-examples.html )

sort command
• Sort: Sort command in unix or linux system is used
to order the elements or text. Sort command has the
capability of sorting numerical values and strings.
– Syntax :- sort [options] filename
– (http://www.folkstalk.com/2012/08/sort-
command-examples-in-unix-linux.html )

sort options
• -b : Ignores leading spaces in each line
• -d : Uses dictionary sort order. Conisders only spaces and
alphanumeric characters in sorting
• -f : Uses case insensitive sorting.
• -M : Sorts based on months. Considers only first 3 letters as month.
Eg: JAN, FEB
• -n : Uses numeric sorting
• -R : Sorts the input file randomly.
• -r : Reverse order sorting
• -k : Sorts file based on the data in the specified field positions.
• -u : Suppresses duplicate lines
• -t : input field separator

Uniq command
• Uniq command in unix or linux system is used
to suppress the duplicate lines from a file
• Syntax:- uniq [option] filename
– (http://www.folkstalk.com/search?q=uniq+comm
and+in+%3Blinux )

uniq options
• c : Count of occurrence of each line.
• d : Prints only duplicate lines.
• D : Print all duplicate lines
• f : Avoid comparing first N fields.
• i : Ignore case when comparing.
• s : Avoid comparing first N characters.
• u : Prints only unique lines.
• w : Compare no more than N characters in lines

wc command
• wc command in unix or linux is used to find
the number of lines, words and characters in a
file.
• Syntax :- wc [options] filenames
• ( http://www.folkstalk.com/2012/07/wc-
command-examples-in-unix-linux.html#more )

wc options
• -l : Prints the number of lines in a file.
• -w : prints the number of words in a file.
• -c : Displays the count of bytes in a file.
• -m : prints the count of characters from a file.
• -L : prints only the length of the longest line in
a file.

other commands
• tee
• head
• tail
• grep

tee command
• tee :- tee command writes to the STDOUT, and to
a file at a time .
• The following command writes the output only to
the file and not to the screen.
– $ ls > filename
• The following command (with the help of tee
command) writes the output both to the screen
(stdout) and to the file.
– $ ls | tee filename
(http://linux.101hacks.com/unix/tee-command-
examples/ )

head command
• The head command reads the first few lines of
any text given to it as an input and writes
them to standard output (which, by default, is
the display screen).
• head's basic syntax is:
– head [options] [file(s)]
( http://www.linfo.org/head.html )

Options of head command
• -c, --bytes=[-]N
– print the first N bytes of each file; with the leading '-', print all but the
last N bytes of each file
• -n, --lines=[-]N
– print the first N lines instead of the first 10; with the leading '-', print
all but the last N lines of each file
• -q, --quiet, --silent
– never print headers giving file names
• -v, --verbose
– always print headers giving file names

tail command
• tail :-output the last part of files
• Syntax :-
– tail [OPTION]... [FILE]...
(http://linux.about.com/library/cmd/blcmdl1_tail.ht
m )

Options of tail command
• -c, --bytes=N
– output the last N bytes
• -f, --follow[={name|descriptor}]
– output appended data as the file grows; -f, --
follow, and --follow=descriptor are equivalent
• -F
– same as --follow=name --retry
• -n, --lines=N
– output the last N lines, instead of the last 10

grep command
• grep :- The grep command allows you to
search one file or multiple files for lines that
contain a pattern. Exit status is 0 if matches
were found, 1 if no matches were found, and
2 if errors occurred.
• Syntax
– grep [options] pattern [files]
(http://www.techonthenet.com/unix/basic/grep.ph
p )

Bash Scripting
• Comment start with hash mark (#) and
continue to the end of line.
#!/bin/bash
echo "Hello, world!“
– The first line is known as the “shebang” statement
and declares the text file to be a script for
interpretation by /bin/bash.

Bash Scripting (Continue…)
• The kernel looks for this syntax when deciding
how to execute the file. From the perspective
of the shell spawned to execute the script, the
shebang line is just a comment.
• If bash were in a different location, we need to
adjust this line

Bash Scripting (Continue…)
• To prepare the file for running, just turn on its execute bit
$ chmod +x helloworld
$ ./helloworld
– Hello, world!
• You can also invoke the shell as an interpreter directly:
$ bash helloworld
– Hello, world!
$ source helloworld
– Hello, world!
• The first command runs helloworld in a new instance of
bash, and the second makes your existing login shell read
and execute the contents of the file

Regular Expression
• Regular expressions are powerful, but they
cannot recognize all possible grammars.
• They are so common that the name is usually
shortened to “regex.”

The matching process
• Code that evaluates a regular expression attempts to
match a single given text string to a single given
pattern. The “text string” to match can be very long
and can contain embedded newlines.
• For the matcher to declare success, the entire search
pattern must match a contiguous section of the search
text. However, the pattern can match at any position.
• After a successful match, the evaluator returns the text
of the match along with a list of matches for any
specially delimited subsections of the pattern.

Literal characters
• In general, characters in a regular expression
match themselves. So the pattern
I am the walrus
• Matches the string “I am the walrus” and that
string only. Since it can match anywhere in the
search text, the pattern can be successfully
matched to the string “I am the egg man. I am
the walrus. Koo koo ka-choo!” However, the
actual match is limited to the “I am the walrus”
portion. Matching is case sensitive.

Special characters
http://perldoc.perl.org/perlrecharclass.html

Example
I am the (walrus|egg man).
matches either “I am the walrus.” or “I am the egg man.”.
This example also demonstrates escaping of special
characters (here, the dot).
The pattern
(I am the (walrus|egg man). ?){1,2}
matches any of the following:
• I am the walrus.
• I am the egg man.
• I am the walrus. I am the egg man.
• I am the egg man. I am the walrus.

PERL PROGRAMMING
• Perl created by Larry Wall
• More power than bash
• Perl does not impose much stylistic discipline
on developers, so Perl code written without
regard for readability can be cryptic. Perl has
been accused of being a write-only language
• Perl’s catch phrase is that “there’s more than
one way to do it.”

PERL PROGRAMMING(Continue…)
• Perl statements are separated by semicolons.
Comments start with a hash mark (#) and
continue to the end of the line. Blocks of
statements are enclosed in curly braces.

Program
#!/usr/bin/perl
print "Hello, world!n";
• As with bash programs, you must either
chmod +x the executable file or invoke the
Perl interpreter directly.
$ ./helloworld
Hello, world!

Variables and arrays
• three fundamental data types
– Scalars (that is, unitary values such as numbers
and strings)
– Arrays
– Hashes (also known as associative Arrays)
• Scalar variables start with $, Array variables
start with @, and Hash variables start with %

Variables and arrays(Continue…)
• In Perl, the terms “list” and “array” are often
used interchangeably, but it’s more accurate to
say that a list is a series of values and an array is
a variable that can hold such a list.
• The individual elements of an array are scalars, so
like ordinary scalar variables, their names begin
with $. Array subscripting begins at zero, and the
index of the highest element in array
@a is $#a.
• The array @ARGV contains the script’s command
line arguments.

Program
#!/usr/bin/perl
@items = ("socks", "shoes", "shorts");
printf "There are %d articles of clothing.n",$#items + 1;
print "Put on ${items[2]} first, then ", join(" and ",
@items[0,1]), ".n";
• The output:
$ perl clothes
There are 3 articles of clothing.
Put on shorts first, then socks and shoes.

Explanation of the program
• Array and string literals
– Perl does not strictly require that all strings be
quoted so @items = (socks, shoes, shorts); is also
same
• Function calls
– Both print and printf accept an arbitrary number
of arguments, and the arguments are separated
by commas

• join receives three string arguments: “ and ”,
“socks”, and “shoes”. It concatenates its second
and subsequent arguments, inserting a copy of
the first argument between each pair. The result
is “socks and shoes”.

• Type conversions in expressions
The magic is in the + operator, which always
implies arithmetic. It converts its arguments to
numbers and produces a numeric result. Similarly,
the dot operator (.), which concatenates strings,
converts its operands as needed: "2" . (12 ** 2) yields
“2144”.

Regular expressions in Perl
• Regular expressions in Perl by “binding” strings to
regex operations with the
=~ operator.
e.g if ($text =~ m/ab+c/) { }
• checks to see whether the string stored in $text
matches the regular expression ab+c.
• To operate on the default string, $_ , you can simply
omit the variable name and binding operator.
• In fact, you can omit the m, too, since the operation
defaults to matching

Program
#!/usr/bin/perl
$names = "huey dewey louie";
$regex = '(w+)s+(w+)s+(w+)';
if ($names =~ m/$regex/)
{
print "1st name is $1.n2nd name is $2.n3rd name is$3.n";
$names =~ s/$regex/2 1/;
print "New names are "${names}".n";
}
else {
print qq{"$names" did not match "$regex".n};
}
(http://www.tutorialspoint.com/perl/perl_qq.htm)

Output
The output:
$ perl testregex
1st name is huey.
2nd name is dewey.
3rd name is louie.
New names are "dewey huey".

Input and output
• When you open a file for reading or writing,
you define a “filehandle” to identify the
channel.
• INFILE & OUTFILE are the filehandlers.
• The while loop condition is <INFILE>

Program
#!/usr/bin/perl
open(INFILE, "</etc/passwd") or die "Couldn’t open
/etc/passwd";
open(OUTFILE, ">/tmp/passwd") or die "Couldn’t open
/tmp/passwd";
while (<INFILE>)
{
($name, $pw, $uid, $gid, $gecos, $path, $sh) = split /:/;
print OUTFILE "$uidt$namen";
}

PYTHON SCRIPTING
• Python was created by Guido van Rossum
• It’s easier to code and more readable than Perl.
Python offers a simple-to-understand syntax that
is easy to follow even if you didn’t develop the
code. If you’re tired of remembering which
comparison operators to use, you’ll appreciate
Python’s unified approach.
• Python also offers additional data types that
some system administrators find useful.

Simple Program
#!/usr/bin/python
print "Hello, world!“
Invoke the python interpreter directly:
$ ./helloworld
Hello, world!

sys — (System-specific parameters
and functions)
• import sys
(https://docs.python.org/2/library/sys.html)

Program
#!/usr/bin/python
name = 'Gwen'
rating = 10
characters = [ 'SpongeBob', 'Patrick', 'Squidward' ]
elements = ( 'lithium', 'carbon', 'boron' )
print "name:t%snrating:t%d" % (name, rating)
print "characters:t%s" % characters
print "elements:t%s" % (elements, )
This example produces the following output:
$ python objects
name: Gwen
rating: 10
characters: ['SpongeBob', 'Patrick', 'Squidward']
elements: ('lithium', 'carbon', 'boron')

Loops
• The fragment below uses a for…in construct to
iterate through the range 1 to 10.
for counter in range(1, 10):
print counter,
Output:
1 2 3 4 5 6 7 8 9

Continue…
• Both for and while loops can have else clauses
at the end. The else clause is executed only if
the loop terminates normally, as opposed to
exiting through a break statement.
• This feature may initially seem
counterintuitive, but it handles certain use
cases quite elegantly.

Scripting and the shell in LINUX

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