Introduction to the Linux Command Line

Every Linux program interacts with three core data streams:

• stdin: Standard input, usually from the keyboard
• stdout: Standard output, sent to the screen
• stderr: Error messages, separate from regular output

Redirecting Standard Output (stdout)

ls -l /usr/bin > ls-output.txt

This redirects output to a file instead of displaying it in the terminal.

Redirecting Standard Error (stderr)

ls -l /bin/usr 2> ls-error.txt

Error messages are saved to a separate file, while standard output remains visible.

Capturing Both stdout and stderr

Classic Method:

ls -l /bin/usr > ls-output.txt 2>&1

Bash Simplified Syntax:

ls -l /bin/usr &> ls-output.txt

Appending Instead of Overwriting

ls -l /usr/bin >> ls-output.txt

Appends new data to the existing file content.

Discarding Output with /dev/null

ls -l /bin/usr 2> /dev/null

Redirects error messages to the "bit bucket" where they are ignored.

Using cat with stdin

cat filename

Creating a File from Input:

cat > lazy_dog.txt

Ends input with Ctrl+D.

Redirecting Input from a File:

cat < lazy_dog.txt

Piping Commands with |

ls -l /usr/bin | less

Passes output of one command as input to another.

Filtering and Processing Pipelines

ls /bin /usr/bin | sort | uniq | less

Showing Only Duplicates:

ls /bin /usr/bin | sort | uniq -d | less

Counting with wc

ls /bin /usr/bin | sort | uniq | wc -l

Searching with grep

ls /bin /usr/bin | sort | uniq | grep zip

Options: -i (ignore case), -v (invert match)

Using head and tail

head -n 5 ls-output.txt
tail -n 5 ls-output.txt

Monitoring with tail -f:

tail -f /var/log/messages

Saving Output Mid-Pipeline with tee

ls /usr/bin | tee ls.txt | grep zip

Saves full list to file while allowing filtered output downstream.

Conclusion

By mastering Linux I/O streams and pipeline mechanics, users can redirect output, control errors, filter results, and structure complex data-processing workflows. Tools like sort, wc, grep, tee, and /dev/null empower users to manage data like pros within the shell environment.