Navigating the Shell: A Beginner's Guide to Redirections and Filters

If you have been following this series, you already know how to navigate the file system and work with files from the command line. In this post, we are going a step further: learning how to control where your data goes and how to filter through it efficiently. These are skills you will use constantly as a software engineer.

What is Shell Redirection?

Every command you run in the shell has three default data streams:

Standard input (stdin) is the data going into a command, usually from your keyboard.

Standard output (stdout) is the data a command produces, usually printed to your terminal.

Standard error (stderr) is where error messages go, also printed to your terminal by default.

Redirection lets you change where these streams come from or go to. Instead of output printing to your screen, you can send it to a file. Instead of typing input manually, you can read it from a file.

Redirecting Output

The > operator redirects the output of a command into a file. If the file does not exist, it will be created. If it does exist, it will be overwritten:

echo "Hello, World!" > output.txt

If you want to add to a file without overwriting what is already there, use >> instead:

echo "Appended text" >> output.txt

This is useful for logging or building up a file over time.

Redirecting Input

The < operator redirects input into a command from a file instead of the keyboard. For example, to sort a list of names stored in a file:

sort < names.txt

This reads the contents of names.txt and passes them to sort as input.

A Note on File Permissions

When using redirection, make sure you have the right permissions for the files involved. If you try to write to a file in a directory where you do not have write access, you will get a permission denied error. You can check file permissions with:

ls -l filename

The output shows who owns the file and what level of access different users have.

Common Filter Commands

Filters are commands that take input, process it in some way, and produce output. They are most useful when combined with redirection and pipes, which we will cover in Part 2.

head shows the first ten lines of a file by default. Useful for quickly checking what is in a file:

head filename.txt

tail shows the last ten lines. Particularly useful for checking log files:

tail filename.txt

sort arranges the lines in a file alphabetically or numerically:

sort names.txt

uniq removes duplicate lines from sorted data. It works best after running sort:

sort emails.txt | uniq

wc counts the number of lines, words, and characters in a file:

wc story.txt

grep searches for a specific pattern in a file and returns the matching lines:

grep "error" log.txt

tr translates or deletes characters. For example, to convert text to uppercase:

echo "hello" | tr 'a-z' 'A-Z'

find searches for files and directories in a given location:

find /documents -name "report.pdf"

cut extracts specific fields or characters from each line. For example, to extract the first and third columns from a CSV file:

cut -d',' -f1,3 data.csv

Final Thoughts

Redirection and filters are some of the most practical tools in the shell. Once you understand how data flows between commands and files, you start to see how powerful the command line really is. In Part 2, we will build on this by introducing pipes, special characters, and text manipulation tools that let you chain everything together.