Create linux_metacharacters.md

cheat_sheets/linux/linux_metacharacters.md

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+# 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.