07. Functions (Part 2)

Variable Scope – Local Scope

• Global variables can be dangerous when used in a library function, which typically is called by many different scripts

• For example, we write a library function called fA:

  • fA ()  { temp=10; }

  • Since fA is a library function, it can be called from many different scripts that need fA’s functionality

• Supposed someone has a script called scriptB that contains these lines of code:

temp=‘/etc/passwd’

fA

echo $temp

• • When scriptB runs, the value being printed will be: 10 and not /etc/passwd

  • This can make us unpopular with the callers of our fA function!

  • In this case, we want temp in fA to have local scope. This means that temp exists only when fA runs, and it is a different variable than any other temp variable that might be in use

• To make a variable have local scope, use the local declaration:

local  varName

• Examples:

fA ()  { local temp=10; } # temp is only for fA

OR

readID( )

{

    local id   # id can only be used by readID

    read –p “Enter ID: ” id

}

Function Usage

• There are 3 common ways that functions are used

  • The function block appears at the begining of a script

  • The function can be called as many time as needed in the script, but the function is not available outside of the script

  • The function block is put in the ~/.bashrc file

    • When a shell first starts running, it reads the .bashrc file, and any function defined in this file is then ‘known’ to the shell

    • Any function defined in ~/.bashrc can be run on the command line

    • For example, in the .bashrc file is the function

      • goHome ( ) {  cd;  echo  At `pwd`;  }

      • On the command line:  goHome

      • will bring the user to the home directory, and the user’s current directory (home) path will be printed

• The  function block is put in a text file

• Before calling the function, the text file needs to be loaded into memory with the . operator

• Then the function can be called on the command line

• Example: 

$ cat fileA fileA has the function sayHi

sayHi ()

{

    echo Hi $USER

}

$ . fileA activate fileA

$ sayHi calling function sayHi

Hi testUser

Library of Functions

• A library of functions is created by storing multiple function blocks in a text file

• The text file becomes the library file

• Once the library file is loaded into memory with the . operator, all functions in the library can be called individually on the command line

• In a similar way, a script can also use the . operator to load the library file into memory and call functions in the library file

• Example:

  • Assume fileA from the previous slide, with the function sayHi, we can have a scriptB:

$ cat scriptB

. fileA # activate fileA

sayHi # call function sayHi

• • Now run scriptB:

$ scriptB # run scriptB

Hi testUser # output of sayHi

The Here Document Introduction

• There are times when the script needs to print multiple lines of text, such as instructions, to the user

• There are 2 ways to do this

  • Use multiple echo commands in the script, one echo per line of text

  • Put all the lines of text into a text file, then use the cat command in the script to print the text file

• A more convenient way is to merge the 2 ways above and use the here document. The here document is called this way because the text is here in the script itself

• The here document is like a mini text file within the script

• This mini text file is sent to the user through any command that can accept multiple input lines of text

• For example

  • Use cat if you want to print the here document

  • Use mail if you want to email the here document 

  • Use grep if you want to select certain lines of the here document

• Format:

command  <<  token

    text

    …

    text

token

• • command is any utility that can accept multiple input lines of text

  • token is any symbol or string of your choice, but the 2 tokens at the start and the end must match

  • The << operator works like the input redirection operator < 

  • The start token must be on the same line as <<

  • The end token must be on a line by itself, and it must be at the beginning of the line

  • The shell sends everything between the start and end tokens to the command, and the command processes every line of text in the here document as if they are from a text file

The Here Document Examples

• In a script, print instructions to the user

cat  << menu

    d: delete file

    p: print file

    q: quit

menu

• From a script, email a short message to the user

mail $USER << msg

    start of script $0

    on $date by $USER

msg

• From a script, select certain lines of the document based on user command line argument

grep $1 << +

    doctor 555-1234

    school 555-3456

    work 555-6789

+