# Why would I need a shell script?

There are instances when you need to do the same operation many times for slightly different input parameters. Now, imagine you were changing one parameter in an input file many times, and half way through, you made a mistake and spend another 5 hours tracking this down. Yes, this happened to me. So, why not instead use computers to do this quickly and accurately? There is no reason, and the way to accomplish this is through a shell script.

# Scripting Basics

Let’s edit a file called my_first_shell_script.sh. The .sh extension generally indicates that the file you are working with is a bash script. In this file, let’s write two lines using the editor of your choice:

#!/bin/bash
echo Hello World


The first line is the so-called “shebang”, or “hashbang” ( or any other variety of variables ) and tells the kernel which program to use to run the script. The second is a simple print statement.

Make sure to save the file, now change the file permissions to make this executable.

$chmod +x my_first_shell_script.sh  Now, let’s try running the script: $ ./my_first_shell_script.sh
Hello World


You’ve written your first shell script.

# Setting Variables

Often times in a script you’d like to set a variable, be that a directory, a file name, or whatever. The way to do this in bash is by using the = sign. Let’s copy my_first_shell_script.sh to my_second_shell_script.sh, and do the same thing using variables. Modify the file to be the following:

#!/bin/bash
MYSTR="Hello World"
echo $MYSTR  Here, we’ve set the variable MYSTR in the second line, and asked for its value in the third, by putting $ in front of the variable name. Make sure the file permissions are correct and run the script. What happens if you forget the $? (NB: There are no spaces between the variable name and the = sign!) We can set variable to anything we want. Let’s make a third script called use_variables_script.sh. #!/bin/bash mydir=$(pwd)
echo "You are in " $mydir echo "Goodbye!"  Again, check the file permissions and run. What have we done? We’ve effectively redirected the output of pwd to mydir by creating a subshell with $()

# Loops.

Loops are your friend. There are three different types of loops.

Loop type Meaning
for iterate over a series
while while some condition is true, do stuff, exit when false
until same as while, but evaluates while control is false

## for Loop example

Edit a new file to contain the following:

#!/bin/bash
mydir=$(pwd) echo$mydir "contains"
for file in $( ls$mydir ); do
echo $file done  Note, the spacing is important! Let’s break this script down. First, we set mydir to be the current directory. Next, we print out the variable to check it. Then, for each file in the directory mydir, do print the file. Finally, we’re done. Remember that any variable must have the $ before it.

## while Loop example

NB: Go see this site for the following examples.

Edit another new file, and have it have the lines:

#!/bin/bash
COUNTER=0
while [  $COUNTER -lt 10 ]; do echo The counter is$COUNTER
let COUNTER=COUNTER+1
done


Breaking this down, we set the variable COUNTER, and while the counter is less than 10, we print out the counter value. Finally we let the counter iterate its value by one. Something we haven’t seen before is the [ ] construction. This allows for a conditional test, i.e. less than.

## until Loop example

The final example we will cover is the until construction. Edit this file and say:

COUNTER=20
until [  $COUNTER -lt 10 ]; do echo COUNTER$COUNTER
let COUNTER-=1
done


Now go ahead and run it. Again, let’s break down the logic.

The first step is to set COUNTER to 20. Next, we say that until COUNTER is less that 10, do print out counter. Then we finally let the counter subtract 1 from its value.