8. BASH Shell Scripting
The primary benefit of BASH scripting is that you can use everything that is available in the BASH shell within a BASH script. Because your Linux distribution has hundreds (maybe even thousands) of commands, each of which can be used in a BASH script, BASH shell scripting can be a very powerful tool.
This chapter focuses on providing you a firm understanding of how to write basic BASH scripts as well as introducing you to some of the more advanced features.
Basics of BASH Scripting
To an extent, you already know many of the basics of BASH scripting because you have already learned many features of the BASH shell in this book. For example, you learned about shell variables in Chapter 4, “Essential Commands.” Shell variables are used in BASH scripting to store values.
To start a BASH script, enter the following as the first line of the script file:
#!/bin/bash
This special sequence is called the sha-bang, and it tells the system to execute this code as a BASH script.
The Story Behind sha-bang
To understand the origin of the term sha-bang, you first need to know two other slang terms. In Linux, a # character is called a hash and a ! character is called a bang. Put them together and you get hash-bang or sha-bang for short.
Some documentation referred to this as a she-bang, but because of the origin of the term, I think sha-bang makes more sense. In any case, you now know why #! is called sha-bang!
Comments in a BASH script start with a # character and extend to the end of the line. For example:
echo "hello world" #prints "hello" to the screen
As shown in the preceding example, you can use the echo command to display information to the user who is running a program. The arguments to the echo command can contain any text data and can also include the value of a variable:
echo "The answer is $result"
After creating your BASH script and saving it, you then make it executable:
[student@OCS ~]$ more hello.sh
#!/bin/bash
#hello.sh
echo "hello world!"
[student@OCS ~]$ chmod a+x hello.sh
Now it can be run as a program by using the following syntax:
[student@OCS ~]$ ./hello.sh
hello world!
Note the need to place ./ before the name of the command. This is because the command might not be in one of the directories specified by the $PATH variable:
[student@OCS ~]$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
To avoid the need to include ./ whenever you want to run your script, you can modify the $PATH variable to include the directory in which your script is stored. Typically, regular users create a “bin” directory in their home directory and place scripts in this location:
[student@OCS ~]$ mkdir bin
[student@OCS ~]$ cp hello.sh bin
[student@OCS ~]$ PATH="$PATH:/home/student/bin"
[student@OCS ~]$ hello.sh
hello world!
In addition to the built-in variables that were discussed in Chapter 4, variables are available in BASH scripts that represent the arguments being passed into the script. For example, consider the following execution of a script called test.sh:
[student@OCS ~]$ test.sh Bob Sue Ted
The values “Bob,” “Sue,” and “Ted” are assigned to variables within the script. The first argument (“Bob”) is assigned to the $1 variable, the second argument is assigned to the $2 variable, and so on. Additionally, all arguments collectively are assigned to the $@ variable.
For additional details regarding these positional parameters variables, or anything related the BASH scripting, consult the man page for bash:
[student@OCS ~]$ man bash
Conditional Expressions
Several conditional statements are available for the BASH shell, including the if statement:
if [ cond ]
then
statements
elif [ cond ]
then
statement
else
statements
fi
Note the following:
An else if is spelled elif and is optional.
After the if and elif, you need a then statement. However, after an else, do not include a then statement.
End the if statement with the word if spelled backwards: fi
See Listing 8.1 for an example of an if statement.
Listing 8.1 Sample if statement
#!/bin/bash
#if.sh
color=$1
if [ "$color" = "blue" ]
then
echo "it is blue"
elif [ "$color" = "red" ]
then
echo "it is red"
else
echo "no idea what this color is"
fi
Listing 8.1 used the following conditional statement:
!![ "$color" = "blue" ]
Get in the habit of putting double quotes around your variables in BASH scripts. This is important in the event the variable hasn’t been assigned a value. For example, suppose the script in Listing 8.1 was execute with no arguments. The result would be that the color variable is unassigned and the resulting conditional statement would be if [ "" = "blue" ]
The result would be false, but without the quotes around $color, the result would be an error message and the script would exit immediately. This is because the resulting conditional statement would be missing one of its key components after the value of $color has been returned: if [ = "blue" ]
This syntax performs an implicit call of a BASH command named test that you can use to perform several comparison tests. This can include integer (numeric) comparisons, string comparisons, and file testing operations.1 For example, use the following syntax to test whether the string value that is stored in the $name1 variable does not equal the string stored in the $name2 variable:
[ "$name1" != "$name2" ]
Important Note
The spacing around the square brackets is very important. There should be a space before and after each square bracket. Without these, an error message occurs.
In addition to determining whether two strings are equal or not equal to each other, you might also find the -n option useful.2 This option determines whether a string is not empty, which is useful when testing user input. For example, the code in Listing 8.2 reads data from user input (the keyboard), assigns the input to the $name variable, and tests to make sure the user typed something for the name.
Listing 8.2 Testing user input
[student@OCS ~]$ more name.sh
#!/bin/bash
#name.sh
echo "Enter your name"
read name
if [ -n "$name" ]
then
echo "Thank you!"
else
echo "hey, you didn't give a name!"
fi
[student@OCS ~]$./name.sh
Enter your name
Bo
Thank you!
[student@OCS ~]$./name.sh
Enter your name
hey, you didn't give a name!
Integer Comparisons
If you want to perform integer (numeric) comparison operations, use the following:
-eq True if values are equal to each other.
-ne True if values are not equal to each other.
-gt True if first value is greater than second value.
-lt True if first value is less than second value.
-ge True if first value is greater than or equal to second value.
-le True if first value is less than or equal to second value.
File Test Comparisons
You can also perform test operations on files and directories to determine information about their status. These operations include:
-d True if “file” is a directory.
-f True if “file” is a regular file.
-r True if “file” exists and is readable by the user running the script.
-w True if “file” exists and is writable by the user running the script.
-x True if “file” exists and is executable by the user running the script.
-L True if “first” value is less than or equal to second value.
Flow Control Statements
In addition to if statements, the BASH scripting language has several other flow control statements:
The while loop—Executes a block of code repeatedly as long as the conditional statement is true.
The until loop—Executes a block of code repeatedly as long as the conditional statement is false. Essentially the opposite of a while loop.
The case statement—Similar to an if statement but provides an easier branching method for multiple situations.
The for loop—Executes a block of code for each item of a list of values.
The while loop
The following code segment prompts the user for a five-digit number. If the user complies, the program will continue because the condition of the while loop will be false. However, if the user provides incorrect data, the condition of the while loop will be true and the user will be prompted for the correct data again:
echo "Enter a five-digit ZIP code: "
read ZIP
while echo $ZIP | egrep -v "^[0-9]{5}$" > /dev/null 2>&1
do
echo "You must enter a valid ZIP code – five digits only!"
echo "Enter a five-digit ZIP code: "
read ZIP
done
echo "Thank you"
The egrep command from the previous example may be a bit tricky to understand. To begin with, the regular expression pattern is matching a value that is exactly five digits. The –v option is used to return a value if the pattern is not found. So, if $ZIP contains a valid five-digit number, then egrep returns a false result because it is trying to find lines that don’t contain a five-digit number. The egrep command returns a true result if the $ZIP contains something besides a five-digit number.
Why the > /dev/null 2>&1? Because we don’t want to display anything from the egrep command, just make use of its true/false return value. All OS commands return true or false3 when executed, and that is what is needed here. Any STDOUT or STDERR from the command is unnecessary and only serves to confuse matters if it is displayed to the user.
The for Loop
A for loop enables you to perform an operation on a set of items. For example, the following command, when run as the root user, creates five user accounts:
for person in bob ted sue nick fred
do
useradd $person
done
Loop Control
Like most languages, BASH scripting provides a way to prematurely exit a loop or to stop the current iteration of a loop and start a new iteration of a loop. Use the break command to immediately exit a while, until, or for loop. Use the continue command to stop the current iteration of a while, until, or for loop and start the next iteration.
The case Statement
A case statement is designed for when you want to perform multiple conditional checks. Although you could use an if statement with multiple elif statements, the syntax of if/elif/else is often more cumbersome than a case statement.
The syntax for a case statement is:
!!case var in
cond1) cmd
cmd;;
cond2) cmd
cmd;;
esac
For the preceding syntax example, var represents a variable’s value that you want to conditionally check. For example, consider the following code:
name="bob"
case $name in
ted) echo "it is ted";;
bob) echo "it is bob";;
*) echo "I have no idea who you are"
esac
The “condition” is a pattern that uses the same matching rules as file wildcards. An * matches zero or more of any character, a ? matches a single character, and you can use square brackets to match a single character of a specific range. You can also use a |character to represent “or.” For example, consider Listing 8.3, which is used to check a user’s answer to a question:
Listing 8.3 Example of the case statement
answer=yes
case $answer in
y|ye[sp]) echo "you said yes";;
n|no|nope) echo "you said no";;
*) echo "bad response";;
esac
User Interaction
The example in Listing 8.3 is a bit of a puzzle because it is intended to check user input. However, the variable is hard coded. Using actual user input, which the read statement can gather, would make more sense:
read answer
The read statement prompts the user to provide information and reads that data (technically from STDIN) into a variable that is specified as the argument to the read statement. See Listing 8.4 for an example.
Listing 8.4 Example of the read statement
read answer
case $answer in
y|ye[sp]) echo "you said yes";;
n|no|nope) echo "you said no";;
*) echo "bad response";;
esac
Additional Information
Do you want to learn more about creating BASH scripts? The following are good resources for additional information:
man bash—The man page for the BASH shell has a great deal of information about writing BASH scripts.
http://tldp.org—A website that is (sadly) mostly out of date.4 However, it has one gem of a document called the “Advanced Bash-Scripting Guide.” Click on the Guides link under the Documents section and scroll down until you see this guide. The author of this guide normally updates it on a regular basis. Because the guides are listed by publication date, this guide is almost always at the top of the list.
BASH Scripting Humor
This isn’t technically a script humor item, but because you will likely be editing files using the vim editor…
Open the vim editor by executing the command vim at the command line. Then type :help 42.
Summary
This book does not cover some additional features of BASH scripting. However, the goal of this chapter was to provide you with enough information to determine whether BASH scripting is a good language for you. If you liked the features and syntax of BASH scripting, consider exploring the documentation provided to learn more about this flexible language.
1 See the man page for test to learn more about its comparison operations: man test
2 A similar option, -z, returns true if the string contains zero characters.
3 Technically they return 0 for “true” and a positive number for “false.”
4 It has a few up-to-date documents, but most are very much outdated. Look at the publication date of the document and realize that anything older than a couple of years is probably no longer accurate (but still might provide you with some useful information).