Chapter 7 Replicating the `head` command

7.1 Learning objectives

How to replicate the head bash command in Python
Understand how to read a file and print the first few lines
Learn how to use a for loop to limit the number of lines printed

7.2 Create a longer file

Let’s create a simple text file called sample2.txt using the Jupyter text editor

The file should contain the following lines:
```
apple
banana
cherry
date
elderberry
fig
grape
peach
kiwi
lemon
```

7.3 Practice the `head` command

This exercise should be done in the Jupyter terminal.

The head command in Bash prints the first few lines of a file. By default, it shows the first 10 lines, but you can specify a different number with the -n option.

In the terminal, run the command:

head sample2.txt

This will print all the 10 lines of sample2.txt

apple
banana
cherry
date
elderberry
fig
grape
peach
kiwi
lemon

Now try running it with the -n option to print only the first 5 lines
```
head -n 5 sample2.txt
```
- You should see the following output
```
apple
banana
cherry
date
elderberry
```

7.4 Designing the `head` algorithm

We can replicate the head command by adding some logic to the for loop we introduced in Ch 6. Parsing text files.

Starting with a for loop is great for iterating through items in a collection, like lines in a file. We can add a counter variable and a conditional if statement to limit how many lines we print.

i = 0
for my_line in my_file:
    if i >= max_lines:
        break
    my_line = my_line.rstrip("\n")
    print( my_line )
    i = i + 1

Key points:

We can use a counter variable i to keep track of how many lines we’ve printed. i = i + 1 increments the counter by 1 each time we print a line.
The if statement checks if the number of lines printed exceeds the maximum specified by the user
The max_lines variable is set to 10 by default, but can be changed by providing a second command line argument

7.5 Coding step by step

Create a new python script called 04-head.py and add what we introduced in Ch 6. Parsing text files

#!/usr/bin/env python3

import sys

my_file = open( sys.argv[1] )

for my_line in my_file:
    my_line = my_line.rstrip("\n")
    print( my_line )

my_file.close()

Now add code after the open() to set a maximum number of lines to print or defaulting to 10 if not specified
```
max_lines = 10
if len(sys.argv) > 2:
    max_lines = int(sys.argv[2])
```

Finally, add the for loop to limit the number of lines printed

i = 0
for my_line in my_file:
    if i >= max_lines:
        break
    my_line = my_line.rstrip("\n")
    print( my_line )
    i = i + 1

The complete script should look like this:

#!/usr/bin/env python3

import sys

my_file = open( sys.argv[1] )

max_lines = 10
if len(sys.argv) > 2:
    max_lines = int(sys.argv[2])

i = 0
for my_line in my_file:
    if i >= max_lines:
        break
    my_line = my_line.rstrip("\n")
    print( my_line )
    i = i + 1

my_file.close()

Save the file and make it executable
```
chmod +x 04-head.py
```
Run the script with the file name and number of lines as arguments
```
./04-head.py sample2.txt 5
```
This will print the first 5 lines of sample2.txt
```
apple
banana
cherry
date
elderberry
```

7.6 Summary

Congratulations! You have just:

Created a Python script that replicates the head command
Used command line arguments to specify the file and number of lines to print

Chapter 7 Replicating the head command

7.1 Learning objectives

7.2 Create a longer file

7.3 Practice the head command

7.4 Designing the head algorithm

7.5 Coding step by step

7.6 Summary

Chapter 7 Replicating the `head` command

7.3 Practice the `head` command

7.4 Designing the `head` algorithm