Linux metacharacters

1. ; : Separates commands.

   command1 ; command2 # Run command1, then run command2 regardless of whether command1 succeeded.
   

2. & : Background execution.

   command & # Runs "command" in the background.
   

3. && : AND operator.

   command1 && command2 # Run command1, then run command2 only if command1 succeeded.
   

4. || : OR operator.

   command1 || command2 # Run command1, then run command2 only if command1 failed.
   

5. | : Pipe operator.

   command1 | command2 # Output of command1 is passed as input to command2.
   

6. () : Command group.

   (command1; command2) # Group commands into a subshell.
   

7. {} : Command block.

   { command1; command2; } # Group commands in the current shell.
   

8. $() : Command substitution.

   echo $(command) # Runs "command" and substitutes its output in place.
   

9. `` (Backticks): Another way of command substitution.

   echo `command` # Same as above, but this syntax can be harder to spot.
   

10. > : Output redirection.

    command > file # Redirect the output of command to a file, overwriting the file.
    

11. >> : Append output.

    command >> file # Append the output of command to a file.
    

12. < : Input redirection.

    command < file # Use "file" as input for command.
    

13. 2> : Error redirection.

    command 2> file # Redirect the error output of command to a file, overwriting the file.
    

14. 2>> : Append error output.

    command 2>> file # Append the error output of command to a file.
    

15. &> : Redirect all output (stdout and stderr).

    command &> file # Redirect all output of command to a file, overwriting the file.
    

16. * : Wildcard.

    ls *.txt # List all .txt files.
    

17. ? : Single character wildcard.

    ls ?.txt # List all .txt files with a single character name.
    

18. [] : Character class.

    ls [ab]*.txt # List all .txt files starting with 'a' or 'b'.
    

19. ! : Negation.

    command1; ! command1 # Execute command1, then execute command1 again only if the first execution failed.
    

20. # : Comment.

    # This is a comment in Bash.
    

21. \$ : Escape character.
    

    echo \$HOME # prints $HOME, not the value of the variable.
    

22. \" : Escape character for quotes.

    echo "This is a \"quote\"" # prints This is a "quote".
    

Be careful, especially when using redirections, as they can overwrite your files without warning if you're not careful.