Linux Shell Programming Tutorial 3

1. Linux Shell ProgrammingTutorial 3 Amin Farbod CS3SH3: Operating Systems Concepts, Winter 2005

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 • … CS3SH3: Operating Systems Concepts, Winter 2005

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, … CS3SH3: Operating Systems Concepts, Winter 2005

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 CS3SH3: Operating Systems Concepts, Winter 2005

5. “Hello world” script • The traditional example: • #!/bin/bash • echo Hello World • To execute: • chmod 755 hello.sh • ./hello.sh • What if we omit the first line ? CS3SH3: Operating Systems Concepts, Winter 2005

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 • … CS3SH3: Operating Systems Concepts, Winter 2005

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 CS3SH3: Operating Systems Concepts, Winter 2005

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 CS3SH3: Operating Systems Concepts, Winter 2005

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? CS3SH3: Operating Systems Concepts, Winter 2005

10. 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 CS3SH3: Operating Systems Concepts, Winter 2005

11. Conditional Sentences #!/bin/bash T1="foo" T2="bar" if [ "$T1" = "$T2" ]; then echo expression evaluated as true else echo expression evaluated as false fi CS3SH3: Operating Systems Concepts, Winter 2005

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’ ? CS3SH3: Operating Systems Concepts, Winter 2005

13. Functions • #!/bin/bash • function quit { • exit } • function e { • echo $1 } • e Hello • e World • quit • echo foo CS3SH3: Operating Systems Concepts, Winter 2005

14. 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 CS3SH3: Operating Systems Concepts, Winter 2005

15. Reading Input #!/bin/bash echo Please, enter your firstname and lastname read FN LN echo "Hi! $LN, $FN !" CS3SH3: Operating Systems Concepts, Winter 2005

16. 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; CS3SH3: Operating Systems Concepts, Winter 2005

17. Reference • This tutorial is based on: “BASH Programming - Introduction HOW-TO” http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html#toc10 CS3SH3: Operating Systems Concepts, Winter 2005