Programming in a UNIX Environment: Essential Commands

Understanding Basic UNIX Commands

UNIX provides a powerful command-line environment where users interact with the system via a shell. Here, we explore some fundamental commands and concepts that every UNIX user should know.

User Identification Commands

who am i

This command displays the current user’s name along with the terminal (TTY), date, and time of login:

$ who am i
john_doe    tty1    Mar 19 10:15

whoami

This command prints only the username of the currently logged-in user:

$ whoami
john_doe

Directory Navigation

In UNIX systems:

• The parent directory is denoted by ..
• The current directory is denoted by .

Removing a Directory

• rmdir <directory_name> removes an empty directory. To remove non-empty directories, use rm -r <directory_name>.

About the Shell

The shell is an ordinary program that interprets commands and executes them. It processes wildcards (like * and ?) before passing arguments to commands.

Using Wildcards in Commands

Wildcards allow flexible pattern matching in filenames.

cat Command and Wildcards

• cat c* prints the contents of all files starting with ‘c’.
• The * wildcard represents any string of characters.

echo Command

The echo command prints arguments to the terminal:

$ echo Hello, UNIX!
Hello, UNIX!

Wildcards with echo:

$ echo ch*
changelog checklists chapter1.txt

$ echo *
file1.txt file2.txt script.sh test.c

rm (Remove Files)

• rm * deletes all files in the current directory (use with caution!).
• rm *.txt deletes all .txt files.
• rm te[a-z]* removes all files starting with te followed by a lowercase letter.

Advanced Wildcard Usage

The [ ] brackets specify a range of characters:

$ pr r[1234]*

Prints all files starting with ‘r’ followed by 1, 2, 3, or 4.

The ? wildcard matches exactly one character:

$ ls ?.txt

Lists files that have a single-character name followed by .txt.

Input and Output Redirection

UNIX allows redirecting command outputs using > and <.

Redirecting Output

• ls > filelist saves the list of files into filelist.
• cat file1.c file2.c > file3.c merges file1.c and file2.c into file3.c.
• cat file4.c >> file3.c appends file4.c to file3.c.

Redirecting Input

• pr -3 < filelist prints the contents of filelist in three-column format.
• grep main < source.c searches for occurrences of ‘main’ in source.c.

Conclusion

Understanding these essential UNIX commands enhances productivity and efficiency when working in a UNIX-based environment. Wildcards, redirection, and basic command utilities provide a powerful toolkit for managing files, directories, and data. Master these, and you’ll navigate UNIX with ease!

Author: Abhilash

Hi, I’m Abhilash! A seasoned web developer with 15 years of experience specializing in Ruby and Ruby on Rails. Since 2010, I’ve built scalable, robust web applications and worked with frameworks like Angular, Sinatra, Laravel, Node.js, Vue and React. Passionate about clean, maintainable code and continuous learning, I share insights, tutorials, and experiences here. Let’s explore the ever-evolving world of web development together! View all posts by Abhilash