Linux Shell Programming

Tutorial 3 SOFE 3950U / CSCI 3020U

Operating Systems

Instructor: Dr. Kamran Sartipi

Linux Shells

§  Shells originally came with UNIX §  They are interactive environments which

let the user to access the computer resources

§  There are many shell §  Bash §  Tcsh § …

Programming vs. Scripting

§  Programming Langs. §  C++/C , Java, …

§  Scripting Langs. §  Bash, Perl, Tcl/tk, …

§  CPU instructions vs. executable files §  Scripting Langs are normally interpretive

§  Slower §  Easier to use and debug §  Suitable for text processing, repetitive jobs, …

BASH

§  Bourne again shell (BASH) §  The most widely used shell §  In Linux environment you are interacting with

BASH on a daily basis §  Operating system maintenance scripts are

generally bash scripts §  We will use examples

“Hello world” script

§  The traditional example: 1.  #!/bin/bash 2.  echo Hello World

§  To execute: §  chmod 755 hello.sh §  ./hello.sh

§  What if we omit the first line ?

Redirection

§  There are three standard file descriptors in Linux §  stdin, stdout, stderr

§  stdout and stderr are output devices, and stdin is an input device

§  We can §  Redirect stdin to a file, to stderr §  …

Redirection Examples

§  stdout to file §  ls -l > ls-l.txt

§  stderr to file §  grep da * 2> grep-errors.txt

§  stdout to stderr §  grep da * 1>&2

§  stderr and stdout to file §  rm -f $(find / -name core) &> /dev/null

Pipes

§  Using pipes you can feed the output of a program to another one as input

§  Example: §  ls -l | grep "\.txt$" §  Equal to: ls -l *.txt

Variables §  Environment variables

§  There are no data types §  No need to declare variables

§  Example: #!/bin/bash STR="Hello World!" echo $STR

§  Note that $ sign is used to dereference variables §  What if we omit it?

Local Variables #!/bin/bash HELLO=Hello function hello {

local HELLO=World echo $HELLO } echo $HELLO hello echo $HELLO

§  Similar to internal function variables in C

Conditional Sentences #!/bin/bash T1="foo" T2="bar" if [ "$T1" = "$T2" ]; then

echo expression evaluated as true else

echo expression evaluated as false fi

Loops: for §  #!/bin/bash

for i in $( ls ); do echo item:

$i done

§  #!/bin/bash for i in `seq 1 10`; do echo $i done

§  What is the meaning of ‘seq 1 10’ ?

Functions 1.  #!/bin/bash 2.  function quit { 3.  exit } 4.  function e { 5.  echo $1 } 6.  e Hello 7.  e World 8.  quit 9.  echo foo

Input arguments #!/bin/bash

if [ -z "$1" ]; then echo usage: $0 directory exit fi ls –la $1 §  If [ –z X] is used to check if X is has a length of zero

§  Can be used to check if something is defined

Reading Input #!/bin/bash echo Please, enter your firstname and lastname read FN LN echo "Hi! $LN, $FN !"

Return Value

#!/bin/bash cd /dada &> /dev/null echo rv: $? cd $(pwd) &> /dev/null echo rv: $?

§  A program return value is stored in $? §  Remember in C:

return 0;

Reference

§  This tutorial is based on: “BASH Programming - Introduction HOW-TO”

http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html#toc10