# Basic Commands

• touch: Creates a new empty file.

  • Example: touch newfile.txt creates a new empty file named newfile.txt.

• mkdir: Creates a new directory.

  • Example: mkdir newdir creates a new directory named newdir.

• mv: Moves or renames files and directories.

  • Example: mv oldfile.txt newfile.txt renames oldfile.txt to newfile.txt.

• cp: Copies files and directories.

  • Example: cp file1.txt file2.txt copies file1.txt to file2.txt.

• rm: Removes files.

  • Example: rm file.txt deletes file.txt.

• rmdir: Removes empty directories.

  • Example: rmdir emptydir deletes an empty directory named emptydir.

# Pattern Matching and Globbing

• Globbing with *: Matches any number of characters.
  • Example: rm *.txt removes all files ending with .txt.

# Advanced Globbing with Wildcards

• [0-9]: Matches any single digit.

  • Example: ls file[0-9].txt lists files like file1.txt, file2.txt, etc.

• [a-z]: Matches any single lowercase letter.

• [A-Z]: Matches any single uppercase letter.

• ?: Matches any single character.

• **: Used for recursive globbing in newer versions of Bash.

# Viewing File Content

• cat: Displays the entire content of a file.

  • Example: cat file.txt displays the contents of file.txt.

• head: Displays the first few lines of a file.

  • Example: head -n 5 file.txt shows the first 5 lines of file.txt.

• tail: Displays the last few lines of a file.

  • Example: tail -n 5 file.txt shows the last 5 lines of file.txt.

# Counting Words and Disk Usage

• wc: Counts lines, words, and characters in a file.

  • Example: wc file.txt displays the line, word, and character count of file.txt.

• du: Shows the disk usage of files and directories.

  • Example: du -sh * shows the size of all files and directories in the current directory in human-readable format.

# Writing and Appending to Text Files

• Redirection Operator >: Writes output to a file, overwriting it.

  • Example: echo "Hello" > file.txt writes “Hello” to file.txt, overwriting its contents.

• Appending Operator >>: Appends output to a file.

  • Example: echo "World" >> file.txt appends “World” to file.txt.

# The Standard Streams

• stdin (Standard Input): Usually the keyboard.
• stdout (Standard Output): The screen or a file.
• stderr (Standard Error): Error messages output stream.

# Dual Output with tee

• tee: Reads from stdin and writes to stdout and files.
  • Example: echo "Hello" | tee file.txt writes “Hello” to both file.txt and the screen.

# Pipe: |

• Used to pass the output of one command as input to another.
  • Example: cat file.txt | grep "Hello" passes the contents of file.txt to grep to search for “Hello”.

# Sorting and Removing Duplicates

• sort: Sorts lines in text files.

  • Example: sort file.txt sorts the lines in file.txt.

• uniq: Removes duplicate lines from sorted data.

  • Example: sort file.txt | uniq sorts file.txt and removes duplicate lines.

# Searching for Patterns with grep

• grep: Searches for patterns in files.
  • Example: grep "Hello" file.txt searches for the word “Hello” in file.txt.

# Character Replacement and Reversal

• tr: Translates or deletes characters.

  • Example: echo "hello" | tr 'a-z' 'A-Z' translates lowercase to uppercase, outputting “HELLO”.

• rev: Reverses lines character-wise.

  • Example: echo "hello" | rev outputs “olleh”.

# The cut Program

• cut: Removes sections from each line of files.
  • Example: cut -d':' -f1 file.txt cuts out the first field from each line in file.txt, with fields delimited by ‘:’.

# Text Substitution with sed

• sed: A stream editor for filtering and transforming text.
  • Example: sed 's/old/new/g' file.txt replaces all occurrences of “old” with “new” in file.txt.