1. Linux Shell Programming Tutorial 3 ENGR 3950U / CSCI 3020U Operating Systems Instructor: Dr. Kamran Sartipi

2. 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  …

3. 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, …

4. 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

5. “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 ?

6. 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  …

7. 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

8. 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

9. 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?

10. Local Variables #!/bin/bashHELLO=Hello function hello { local HELLO=World echo $HELLO } hello  Similar to internal function variables in C

11. Conditional Sentences #!/bin/bashT1="foo"T2="bar" if [ "$T1" = "$T2" ]; then echo expression evaluated as true else echo expression evaluated as false fi

12. 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’ ?

13. 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

14. Input arguments #!/bin/bash #!/bin/bash if [ -z "$1" ]; then if [ -z "$1" ]; then echo usage: $0 directory echo usage: $0 directory exit exit fi fi ls –la $1 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

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

16. Return Value #!/bin/bash #!/bin/bash cd /dada &> /dev/null cd /dada &> /dev/null echo rv: $? echo rv: $? cd $(pwd) &> /dev/null cd $(pwd) &> /dev/null echo rv: $? echo rv: $?  A program return value is stored in $?  Remember in C: return 0;

17. Reference  This tutorial is based on: “BASH Programming - Introduction HOW-TO” http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html#toc10