bash_multi

There will come a time when using bash that typing in specific commands in the command line will become cumbersome. This is where shell scripts come in. A user can write a series of commands in a shell script that the computer will then execute sequencially. It is important to know that commands that work on the command line will work the same way when implemented in a script. Likewise if a series of commands are implemented in a script, the same result will occur if the user inputs the commands manually in the command line. The following section will introduce the concept of shell scripts, how to write them, and how to exectue them.

#!/bin/bash
echo This script lists the files in the directory
ls

Lets break down the script and how its works. In the first line you’ll see #!/bin/bash is written. Every shell script should start with this exact line, this tells Linux which interpreter to use for this file. The second command is the echo command which prints everything that follows it. And finally we have the ls command which just lists the directory. If a user were to execute this series of command manually it would look like this:

$ echo This script lists the files in the directory
This script lists the files in the directory
$ ls
bin data home usr

Remember that Linux is an extensionless system therefore when writing a shell script you don’t have to append the .sh extension, however doing this will help you identify which files are shell scripts and which are not.

In a shell script lines that start with # are considered comments and will not be interpreted. There are some exeptions however, including the first line of every script.

You can also declare variables within a shell script. This can be done in the following way:

#!/bin/bash
# Declare variable
str='Hello World!'
echo $str

Notice that when we are defining the variable there are no spaces on either side of the = sign. Addtionally when you want to refer to the varible you have to place $ sign right infront of the variable to signify that you are refereing to a variable. The previous script will output:

$ ./script.sh
Hello World!

To run the shell script we typed ./ and then the filename. Remember a few sections back that . specifies the current directory. When executing a shell script you are essentially telling the computer that the script you are attempting to execute is in the current directory. An absolute path would also work. If the script in is the directory above your current directory you could use ../ to run the script.

It may be the case that when you write a shell script your system does not yet know it’s an executable script. To change this adjust the permissions on the file by typing:

$ chmod +x script.sh

As in any other progamming language you can also implement if,else,fi statments, for loops, and while loops. For example you can write the following in a script:

#!/bin/bash
count=100
if [ $count -eq 100 ]
then 
    echo "Count is 100"
else 
    echo "Count is not 100"
fi

This script defines the variable count and executes the echo command if the variable count equals a certain value. Notice that the statement ends with fi. Sometimes it is useful to run a command until a certain requirement is met. For example, lets consider the following while loop:

#!/bin/bash
count=1
while(($n <= 5))
do 
    echo "This loop has run $n times"
    n=$((n+1))
done

This loop has run 1 times
This loop has run 2 times
This loop has run 3 times
This loop has run 4 times
This loop has run 5 times

Take a look at the structure of the while loop. The basic structure of the while loop can be broken down to:

while <condition>
do
    <commands>
done

Similar to the while loop, the for loop will execute a command a command a number of times. However instead of executing the command untl a condition is met a for loop will only execute the command for a pre-specified number of times. For example:

#!/bin/bash
for value in {1..5}
do 
    echo "This loop has run $n times"
done

This loop will produce the exact same result as the while loop. However you are not limited to just numerical values. You can also execute a command based on the items within a variable. This could be

names='Name1 Name2 Name3'

More advanced loop could include the break and continue commands. The break command tells bash to exit the loop when a condition is met. The continue command tells bash to skip the current iteration and move on to the next iteration within the loop. For example:

#!/bin/bash
for value in {1..10}
do 
    if [ $value -eq 5 ]
    then
        echo The current count is $value
        continue
    fi
    if [ $value -eq 7 ]
    then
        echo The count is $value
        echo Now exiting loop
        break
    fi
done

You can also prompt the user to select from a list of choices using the select,do,done sequence. For example the following script will prompt the user to run or quit:

#!/bin/bash
options='Continue Quit'
PS3='Choose 1) or 2) to Continue or Quit: '
select option in $options
do
    if [ $option == 'Continue' ]
    then
        continue
    fi
    if [ $option == 'Quit' ]
    then
        break
    fi
done

Shell Scripts

Do Exercise #5