lec12 basic scripts

7/30/2019 Lec12 Basic Scripts

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CS 497C Introduction to UNIXLecture 32: - Shell Programming

Chin-Chih Chang
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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The shell is also a programming languagethat executes shell scripts in the interpretivemode one line at a time.Shell scripts run slower than compiledlanguages like , but for many jobs speed isno hurdle.

is more used as a language for the

system administration. An UNIX systemadministrator is supposed to be anaccomplished shell programmer.


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The shell of LinuxLinux has a variety of different shells:

Bourne shell (sh), C shell (csh), Korn shell (ksh), TCshell (tcsh), Bourne Again shell (bash).

Certainly the most popular shell is bash . Bash isthe shell that will appear in the GNU operating

system. Bash is an sh-compatible shell thatincorporates useful features from the Korn shell(ksh) and C shell (csh).It is intended to conform to the IEEE POSIXP1003.2/ISO 9945.2 Shell and Tools standard.It offers functional improvements over sh for bothprogramming and interactive use.


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Programming or Scripting ?bash is not only an excellent command line shell, but ascripting language in itself. Shell scripting allows us to usethe shell's abilities and to automate a lot of tasks that wouldotherwise require a lot of commands.Difference between programming and scripting languages:

Programming languages are generally a lot more powerful and a lot

faster than scripting languages. Programming languages generallystart from source code and are compiled into an executable. Thisexecutable is not easily ported into different operating systems.

A scripting language also starts from source code, but is notcompiled into an executable. Rather, an interpreter reads the

instructions in the source file and executes each instruction.Interpreted programs are generally slower than compiled programs.The main advantage is that you can easily port the source file to anyoperating system. bash is a scripting language. Other examples of scripting languages are Perl, Lisp, and Tcl.


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Different levels of scripting1. Simple commands listed in a filemany useful possibilities for administratorschanging passwords, setting up accounts

2. Using variables and positional parametersmany more possibilities process files (Project 2)

3. Using complex branches (if) and loops (while)

you can skip this or explore on your own4. Hugely powerful see Lec11 Advanced Scripts


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The first bash programWe must know how to use a text editor. There aretwo major text editors in Linux:

vi, emacs (or xemacs). So fire up a text editor; for example:

$ vi hello.sh &and type the following inside it:

#!/bin/bashecho Hello World

The first line tells Linux to use the bash interpreter to run this script. Now, make the script executable:

$ chmod 700 hello.sh$./hello.sh (if PATH is set properly, just say hello.sh)

Hello World


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The second bash programWe write a program that copies all files into a directory, andthen deletes the directory along with its contents. This can

be done with the following commands:$ mkdir trash$ cp * trash$ rm -rf trash$ mkdir trash

Instead of having to type all that interactively on the shell,write a shell program instead:

$ cat trash#!/bin/bash

# this script deletes some filesmkdir trashcp * trashrm -rf trashecho Deleted all files!


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A shell variable is assigned with the = symbolwithout using the $, but is evaluated byprefixing it with a $:

fname=profile; echo $fnameThe statement removes a variable fromthe shell.Variables are concatenated by placing them

side by side; no operators are needed:x=foo; y=.doc; z=$x$y; echo $z


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Shell variables can be evaluated by usingcurly braces around the variable name.

echo ${fname}

These braces also enable concatenation of avariable and a string.

echo ${fname}x or echo $ fnamex

These concatenation features are usefulwhen changing a files extension.


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VariablesWe can use variables as in any programming languages.Their values are always stored as strings, but there aremathematical operators in the shell language that willconvert variables to numbers for calculations.We have no need to declare a variable , just assigning avalue to its reference will create it.

Example#!/bin/bashSTR= Hello World! echo $STR

Line 2 creates a variable called STR and assigns the string"Hello World! " to it. Then the value of this variable isretrieved by putting the ' $' in at the beginning.


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Warning !

The shell programming language does not type-cast its variables. This means that a variable canhold number data or character data.

count=0count=Sunday

Switching the TYPE of a variable can lead toconfusion for the writer of the script or someonetrying to modify it, so it is recommended to use avariable for only a single TYPE of data in a script .

\ is the bash escape character and it preserves theliteral value of the next character that follows.$ ls \*ls: *: No such file or directory


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Single and Double QuoteWhen assigning character data containing spaces or special characters, the data must be enclosed in either

single or double quotes .Using double quotes to show a string of characters willallow any variables in the quotes to be resolved

$ var=test string

$ newvar=Value of var is $ var $ echo $newvarValue of var is test string

Using single quotes to show a string of characters will not

allow variable resolution$ var=test string $ newvar=Value of var is $ var $ echo $newvarValue of var is $var


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The export commandThe export command puts a variable into the environmentso it will be accessible to child processes. For instance:

$ x=hello$ bash # Run a child shell.

$ echo $x # Nothing in x.$ exit # Return to parent.

$ export x$ bash$ echo $xhello # It's there.

If the child modifies x, it will not modify the parents originalvalue. Verify this by changing x in the following way:

$ x=ciao$ exit$ echo $xhello


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Environmental VariablesThere are two types of variables:

Local variables

Environmental variables Environmental variables are set by the system and canusually be found by using the env command. Environmentalvariables hold special values. For instance,

$ echo $SHELL/bin/bash$ echo $PATH/usr/X11R6/bin:/usr/local/bin:/bin:/usr/bin

Environmental variables are defined in /etc/profile, /etc/profile.d/ and ~/.bash_profile . These files are theinitialization files and they are read when bash shell isinvoked. When a login shell exits, bash reads~/.bash_logout


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Environmental VariablesHOME: The default argument (home directory) for cd .PATH : The search path for commands. It is a colon-

separated list of directories that are searched when youtype a command.Usually, we type in the commands in the following way:

$ ./commandBy setting PATH=$PATH:. our working directory is includedin the search path for commands, and we simply type:

$ commandIf we type in

$ mkdir ~/bin

and we include the following lines in the ~/.bash_profile :PATH=$PATH:$HOME/binexport PATH

we obtain that the directory /home/rinaldi/bin is included inthe search path for commands.


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Environmental VariablesLOGNAME: contains the user nameHOSTNAME : contains the computer name.

PS1 : sequence of characters shown before the prompt\t hour \d date\w current directory

\W last part of the current directory\u user name\$ prompt character

Example

[rinaldi@homelinux rinaldi]$ PS1 =ciao \u * ciao rinaldi* _


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Read command

The read command allows you to prompt for input and storeit in a variable. Example

#!/bin/bashecho -n Enter name of file to delete: read file

echo Type 'y' to remove it, 'n' to change yourmind ... rm -i $fileecho "That was YOUR decision!"

Line 3 creates a variable called file and lets the user enter the filename by typing it. Then the value of this variable isretrieved in the rm command by putting the '$' in at thebeginning.


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Shell ParametersPositional parameters are assigned from the shells argument when it is invoked. Positional parameter N may

be referenced as ${N}, or as $N when N consists of asingle digit.Special parameters

$# is the number of parameters passed$0 returns the name of the shell script running as well as its location

in the filesystem$* gives a single word containing all the parameters passed to thescript$@ gives an array of words containing all the parameters passed tothe script

$ cat sparameters.sh#!/bin/bashecho $#; $0; $1; $2; $*; $ @ $ sparameters.sh alba chiara2; ./sparameters.sh; alba; chiara; alba chiara; alba

chiara


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Trash$ cat trash.sh#!/bin/bashif [ $# -eq 1 ];then

if [ ! d $ HOME/trash ];

thenmkdir $ HOME/trash

fimv $1 $ HOME/trash

elseecho Use : $0 filename exit 1

fi

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