Create 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.