This section contains examples of Scripting Basics 1. Most of the answers are at the end of the section, with some comments. Some of them require you to actually test the solutions on your own. These examples are not meant to replace the Exercises. Do the exercise sets!

1. Given two integers xxx and yyy, write a command to 

2. (this question was moved to module 3)

3. You have the following shell script, named ss1:

e. Would there have been any difference in the results if ss1 had the line export answer as its last line?

5. Alexi found the following line in the beginning of a shell script:

TESTDIR=${TESTDIR:-$PWD}

Explain to him what it means.

Solutions

1a. There are several ways to do this:

• expr $num1 - $num2
• echo $((num1 - num2))  # the $ inside (( )) are optional, except with positional parameters.

• use (( )) as a command, where the assignment is inside it:  (( sum = num1 + num2))
• use $(( )) as a substitution, where the assignment is outside it: sum=$((num1 + num2))

3a. The description 'shell script' or 'commands file' refers to how you run the file of commands, not to the file itself, although a file of commands that is meant to be source-d (using the dot operator) is usually referred to as a commands file, and a file of commands that is meant to be executed directly is usually referred to as a shell script.

3b. Only read permission is necessary to source the file using the dot operator.

3c. Read and execute permission is necessary to run the file as a shell script.

3d. Type in the file and run it each of the four ways. Then  read the notes below and they should make sense:

• when you run a file of commands as a commands file, the commands in it are executed as if you typed them in. This means:
• any commands affect you
• any variables that are set affect you
• there is no difference between a local and an enviornment variable
• when you run a file of commands as a shell script, the commands in it are executed in a different shell. This means:
• only variables that you have exported (environment variables) are copied to the shell script, and any changes the shell script makes to these variables do not affect you (because they are copied).
• commands executed in the shell script do not affect you.

3e. no. the export command only affects local variables. Once a variable is in the environment, it is in the environment for that process and all of its child (and its child's children ... ) proecesses. Since a variable that is exported is copied to a child process (such as a shell script), it can only be used to initialize the variables of a child process. A child's variables can never affect the parent. (Note that the terminology 'child' only affects files run as shell scripts, where there are two sets of variables: one for the parent and one for the child. Files run using the dot operator (sourced) run their commands in the current shell, so there is only one set of variables.) 

4. 4run num.ber comma,s and techo-crat  are illegal. A variable name may include letters, numbers and underscores, but it cannot start with a number.

5. The shell script is ensuring that the variable TESTDIR has a value. If it was set and exported, the shell script will use the value. If it was not set (or not exported), the shell script will set TESTDIR to the current directory. Note that, in this case the syntax TESTDIR=${TESTDIR:=$PWD} is no different.

Copyright 2011 Greg Boyd - All Rights Reserved.