1 / 17

Linux Shell Programming Tutorial 3

Linux Shell Programming Tutorial 3. ENGR 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 ….

astin
Download Presentation

Linux Shell Programming Tutorial 3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Linux Shell ProgrammingTutorial 3 ENGR 3950U / CSCI 3020UOperating 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: • #!/bin/bash • 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/bash HELLO=Hello function hello { local HELLO=World echo $HELLO } echo $HELLO hello echo $HELLO • Similar to internal function variables in C

  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

  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 • #!/bin/bash • function quit { • exit } • function e { • echo $1 } • e Hello • e World • quit • echo foo

  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

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

  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;

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

More Related