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Linux Shell Scripting Bootcamp

Chapter 1. Getting Started with Shell Scripting

This chapter is a brief introduction to shell scripting. It will assume the reader is mostly familiar with script basics and will serve as a refresher.

The topics covered in this chapter are as follows:

The general format of a script.
How to make a file executable.
Creating a good Usage message and handling return codes.
Show how to pass parameters from the command line.
Show how to validate parameters by using conditional statements.
Explain how to determine the attributes of files.

Demonstrating the use of scripts

Here is an example of a very simple script. It might not look like much but this is the basis for every script:

Chapter 1 - Script 1

#!/bin/sh
#
#  03/27/2017
#
exit 0

Note

By convention, in this book the script lines will usually be numbered. This is for teaching purposes only, in an actual script the lines are not numbered.

Here is the same script with the lines numbered:

1  #!/bin/sh
2  #
3  # 03/27/2017
4  #
5  exit 0
6

Here is an explanation for each line:

Line 1 tells the operating system which shell interpreter to use. Note that on some distributions /bin/sh is actually a symbolic link to the interpreter.
Lines that begin with a # are comments. Also, anything after a # is also treated as a comment.
It is good practice to include a date in your scripts, either here in the comments section and/or in the Usage section (covered next).
Line 5 is the return code from this script. This is optional but highly recommended.
Line 6 is a blank line and is the last line of the script.

Using your favorite text editor, edit a new file named script1 and copy the preceding script without the line numbers into it. Save the file.

To make the file into an executable script run this:

$ chmod 755 script1

Now run the script:

$ script1

If you did not prepend a . to your path as mentioned in the introduction then run:

$ ./script1

Now check the return code:

$ echo $?
0

Here is a script that does something a little more useful:

Chapter 1 - Script 2

#!/bin/sh
#
# 3/26/2017
#
ping -c 1 google.com        # ping google.com just 1 time
echo Return code: $?

The ping command returns a zero on success and non-zero on failure. As you can see, echoing $? shows the return value of the command preceding it. More on this later.

Now let's pass a parameter and include a Usage statement:

Chapter 1 - Script 3

  1  #!/bin/sh
  2  #
  3  # 6/13/2017
  4  #
  5  if [ $# -ne 1 ] ; then
  6   echo "Usage: script3 file"
  7   echo " Will determine if the file exists."
  8   exit 255
  9  fi
 10  
 11  if [ -f $1 ] ; then
 12   echo File $1 exists.
 13   exit 0
 14  else
 15   echo File $1 does not exist.
 16   exit 1
 17  fi
 18

Here is an explanation for each line:

Line 5 checks to see if a parameter was given. If not, lines 6 through 9 are executed. Note that is it usually a good idea to include an informative Usage statement in your script. It is also good to provide a meaningful return code.
Line 11 checks to see if the file exists and if so lines 12-13 are executed. Otherwise lines 14-17 are run.
A word about return codes: It is standard practice under Linux/UNIX to return zero if the command was successful, and non-zero if not. In this way the code returned can mean something useful, not only to humans, but to other scripts and programs as well. However, it is not mandatory to do this. If you want your script to return codes that are not errors but indicate some other condition by all means do so.

This next script expands on this topic:

Chapter 1 - Script 4

  1  #!/bin/sh
  2  #
  3  # 6/13/2017
  4  #
  5  if [ $# -ne 1 ] ; then
  6   echo "Usage: script4 filename"
  7   echo " Will show various attributes of the file given."
  8   exit 255
  9  fi
 10  
 11  echo -n "$1 "                # Stay on the line
 12  
 13  if [ ! -e $1 ] ; then
 14   echo does not exist.
 15   exit 1                      # Leave script now
 16  fi
 17  
 18  if [ -f $1 ] ; then
 19   echo is a file.
 20  elif [ -d $1 ] ; then
 21   echo is a directory.
 22  fi
 23  
 24  if [ -x $1 ] ; then
 25   echo Is executable.
 26  fi
 27  
 28  if [ -r $1 ] ; then
 29   echo Is readable.
 30  else
 31   echo Is not readable.
 32  fi
 33  
 34  if [ -w $1 ] ; then
 35   echo Is writable.
 36  fi
 37  
 38  if [ -s $1 ] ; then
 39   echo Is not empty.
 40  else
 41   echo Is empty.
 42  fi
 43  
 44  exit 0                       # No error
 45

Here is an explanation for each line:

Lines 5-9: If the script is not run with a parameter display the Usage message and exit with a return code of 255.
Line 11 shows how to echo a string of text but still stay on the line (no linefeed).
Line 13 shows how to determine if the parameter given is an existing file.
Line 15 leaves the script as there is no reason to continue if the file doesn't exist.

The meaning of the remaining lines can be determined by the script itself. Note that there are many other checks that can be performed on a file, these are just a few.

Here are some examples of running script4 on my system:

guest1 $ script4
Usage: script4 filename
 Will show various attributes of the file given.

guest1 $ script4 /tmp
/tmp is a directory.
Is executable.
Is readable.
Is writable.
Is not empty.

guest1 $ script4 script4.numbered
script4.numbered is a file.
Is readable.
Is not empty.

guest1 $ script4 /usr
/usr is a directory.
Is executable.
Is readable.
Is not empty.

guest1 $ script4 empty1
empty1 is a file.
Is readable.
Is writable.
Is empty.

guest1 $ script4 empty-noread
empty-noread is a file.
Is not readable.
Is empty.

This next script shows how to determine the number of parameters that were passed to it:

Chapter 1 - Script 5

#!/bin/sh
#
# 3/27/2017
#
echo The number of parameters is: $#
exit 0

Let's try a few examples:

guest1 $ script5
The number of parameters is: 0

guest1 $ script5 parm1
The number of parameters is: 1

guest1 $ script5 parm1 Hello
The number of parameters is: 2

guest1 $ script5 parm1 Hello 15
The number of parameters is: 3

guest1 $ script5 parm1 Hello 15 "A string"
The number of parameters is: 4

guest1 $ script5 parm1 Hello 15 "A string" lastone
The number of parameters is: 5

Note

Remember that a quoted string is counted as 1 parameter. This is a way to pass strings that contain blank characters.

This next script shows how to handle multiple parameters in more detail:

Chapter 1 - Script 6

#!/bin/sh
#
# 3/27/2017
#

if [ $# -ne 3 ] ; then
 echo "Usage: script6 parm1 parm2 parm3"
 echo " Please enter 3 parameters."

 exit 255
fi

echo Parameter 1: $1
echo Parameter 2: $2
echo Parameter 3: $3

exit 0

The lines of this script were not numbered as it is rather simple. The $# contains the number of parameters that were passed to the script.

Description

Linux Shell Scripting Bootcamp is all about learning the essentials of script creation, validating parameters, and checking for the existence of files and other items needed by the script. We will use scripts to explore iterative operations using loops and learn different types of loop statements, with their differences. Along with this, we will also create a numbered backup script for backup files. Further, you will get well-versed with how variables work on a Linux system and how they relate to scripts. You’ll also learn how to create and call subroutines in a script and create interactive scripts. The most important archive commands, zip and tar, are also discussed for performing backups. Later, you will dive deeper by understanding the use of wget and curl scripts and the use of checksum and file encryption in further chapters. Finally, you will learn how to debug scripts and scripting best practices that will enable you to write a great code every time! By the end of the book, you will be able to write shell scripts that can dig data from the web and process it efficiently.

What you will learn

[*]Get acquainted with the basics of a shell script to serve as a refresher for more advanced topics [*]Learn different ways to create and run a script [*]Discuss the passing and verification of parameters, along with the verification of other items. [*]Understand the different forms of conditions and loops, and go over the sleep command in detail [*]Learn about different ways to handle the reporting of return codes [*]Create an interactive script by reading the keyboard and use subroutines and interrupts [*]Create scripts to perform backups and go over the use of encryption tools and checksums [*]Use wget and curl in scripts to get data directly from the Internet

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