Lecture 8. Introduction to Shell Programming. COP 3353 Introduction to UNIX

Transcription

1 Lecture 8 Introduction to Shell Programming COP 3353 Introduction to UNIX 1

2 What is a shell script? An executable file containing Unix shell commands Programming control constructs (if, then, while, until, case, for, break, continue, while, ) basic programming capabilities (assignments, variables, arguments, expressions, ) The file entries are the script A general convention is to use.sh extension for shell scripts The file is interpreted rather than compiled and executed The first line of the script indicates which shell is used to interpret the script 2

3 Simple script (egshell.sh) #!/bin/sh #this is the script in file egshell.sh cal date who grep liu exit The #! is used to indicate that what follows is the shell used to interpret the script The exit command immediately quits the shell script (by default it will also quit at the end of the file) 3

4 Simple Script to Generate User Information for Homework Assignments 3/15/2011 9:39:40 AM unix-lect11-shell-intro.ppt 4

5 Simple Script to Generate User Information for Homework Assignments 3/15/2011 9:40:58 AM unix-lect11-shell-intro.ppt 5

6 Shell Scripts cont. There are three ways to run a shell script bash myscript.sh # Use bash shell for interpretation source myscript.sh # Use the current shell for interpretation./myscript.sh # Use the shell specified by #! at the beginning of the file In the first two methods, the file needs to be readable In the third method, the file needs to be readable and executable In the second method, since the statements in the script are interpreted in the current shell, they can change the current working directory, shell and environment variables But in the first and third methods, the statements are interpreted in a separate shell program and they do not affect the current shell 3/15/2011 9:41:11 AM unix-lect11-shell-intro.ppt 6

7 Executing shell scripts sh myscript #uses Bourne shell tcsh myscript#uses t-cshell Note that the above explicitly invoke the appropriate shell with the file containing the commands as a parameter. The file does not need to be executable. You can also make the file executable and then simple run as a command chmod 755 myscript (or chmod +x myscript) myscript 7

8 Shell scripts Advantages Can quickly setup a sequence of commands to avoid a repetitive task Can make several programs work together Disadvantages Little support for large and complicated programming semantics Shell scripts need to be interpreted hence are slower programs Which shell to use? csh shell and tcsh shell are recommended for use at the command line sh (Bourne) shell and bash shell are recommended for writing shell scripts Examples will generally use the Bourne shell 8

9 A Typical Layout of a Shell Script The first line should be #!/bin/bash It needs to be changed when using a different shell other than /bin/bash In fact, it does not need to be a shell and any interpreter can be used That is the reason why python scripts start with #!/usr/bin/python Comments start with # (except #! at the beginning of the file) In other words, parts of a line after # are not interpreted If a line starts with #, the whole line is comments 3/15/2011 9:57:05 AM unix-lect11-shell-intro.ppt 9

10 Basic Ingredients of a Shell Script Variables Statements Including if and other control constructs Unix commands and shell built-in commands 3/15/2011 9:58:56 AM unix-lect11-shell-intro.ppt 10

11 Printing a line to standard output Use the echo command to print a line to stdout Form of command: echo <zero or more values> Examples echo Hello World echo hello world #two values echo hello #need not always use quotes echo please enter your name 11

12 Shell Environment Variables These are variables provided as part of the shell s operational environment They exist at startup but can be changed Examples are: USER, HOME, PATH, SHELL, HOSTNAME The setenv command (in tcsh) is used to set these, for example, by: setenv PATH $PATH:/home/here/bin (this sets the PATH variable so that it s current value is appended by :/home/here/bin) Note that setenv is how tcsh sets the environment variables 12

13 User defined variables You can also specify variables yourself and these can also be used inside a script In tcsh, the set command is used to set a variable to a string value Form: set <name> = <value> Examples: set alpha = any string set beta = 3 set mypath = /home/special/public_html Once a variable has been defined, it s value can be used by dereferencing it with $. ls al $mypath Note that using setenv or set without any parameters simply displays the current settings 13

14 Shell variables (Bourne shell) Note that for all shells, variables need not be declared explicitly, but simply used For the Bourne shell, the use is as follows (note that there should be no blanks before and after the equals sign and no need for the set command. Form: <name>=<value> Example alpha= hello world beta=45 echo $alpha $beta third argument Note that $alpha is the value of the variable alpha 14

15 Reading values into shell variables The read statement is used to read a line of standard input, split the line into fields of one or more strings, and assign those strings to shell variables. Any strings not assigned are assigned to the last variable. Form: read <var1> <var2> <varn> Examples read num read field1 field2 rest read field1 field2 < ifile.txt 15

16 Shell arguments Arguments on the command line can be passed to a shell script, just as you can pass command line arguments to a program $1, $2,, $9 are used to refer to up to nine command line arguments (similar to C s argv[1], argv[2],, argv[9]). Note that $0 contains the name of the script (argv[0]) Example: shprog.sh john 40 shprog.sh bob 45 new york 16

17 Example using shell arguments Script: #!/bin/sh #script name is greet.sh #friendly display of today s date echo Hello $1 $2 pleased to meet you echo The date is date exit Usage: greet.sh john smith 17