Command Operators

Command operators used in the terminal are used to combine, direct, and execute commands under certain conditions. Here are the basic command operators and their functions:

1. ‘;’ (Semicolon)

The semicolon operator is used to execute multiple commands one after another. The commands are executed independently of each other, the next command is executed when the previous command finishes. This means that the success or failure of one command does not affect the execution of other commands.
Usage
```
  comamnd1; command2; command3
```
command1, command2 and command3 are executed in order. Each command executes independently of the previous one.

Example

  echo "Hello, World!"; mkdir test; cd test

2. ‘&’ (Ampersand)

This operator is used to run a command in the background so that other operations can be performed in the terminal.
Usage
```
  command &
```
command is run in the background.
Example
```
  sleep 10 &
```
The fg command is used to bring a background job to the foreground.

3. ‘&&’ (AND)

This operator executes the next command if the first command succeeds. If the first command fails, the next command is not executed.
Usage
```
  command1 && command2
```
If command1 succeeds, command2 is executed.
Example
```
  pwd && ls
```

4. ‘||’ (OR)

This operator executes the next command if the first command fails. If the first command succeeds, the next command is not executed.
Usage
```
  command1 || command2
```
If command1 fails, command2 is executed.
Example
```
  pwd || ls
```

5. ‘|’ (PIPE)

The pipe operator is used to use the output of one command as input to another command. This operator is often used to create chains of data processing.
Usage
```
  command1 | command2
```
The output of command1 becomes the input of command2
Example
```
  ls -l | grep test
```

6. ‘!’

This command is used to re-run a command that was run in the past. Usage examples:

‘!number’
- Reruns a specific command number that was run in the past.
- Usage
```
  !number
```
- Example
```
  !12
```
‘!!’
- Reruns the last executed command.
- Usage
```
  !!
```
- Example
```
  ls
  !!
```
‘!string’
- Reruns the last command that started with string.
- Usage
```
  !string
```
- Example
```
  !echo
```
‘!$’
- Reuses the last argument of the last executed command.
- Usage
```
  command !$
```
- Example
```
  echo "Hello, World!"
  vi !$
```

7. ‘>’, ‘»’, ‘<’ (The Redirection Operators)

The Redirection Operators

‘>’
- This operator is used to redirect the output of a command to a file. If the file already exists, it overwrites the previous content.
- Usage
```
  command > output_file
```
  The output of command is written to the file output_file. If the file already exists, its contents are deleted and rewritten.
- Example
```
  echo "Hello, World!" > output_file
```
‘»’
- This operator appends the output of a command to a file. If the file does not exist, it is created; if it does, it is appended to the current content.
- Usage
```
  command >> output_file
```
  The output of command is appended to the end of the file output_file.
- Example
```
  echo "new line" >> output_file2
  echo "new line2" >> output_file2
```
‘<’
- This operator is used to get the input of a command from a file.
- Usage
```
  command < input_file
```
  The input of command is taken from the input_file file.
- Example
```
  grep a < alphabet.txt
```

8. ‘2>’ (Redirecting Standard Error)

This operator is used to redirect the error output of a command to a file. Unlike normal output, error messages are redirected with the 2> operator.
Usage
```
  command 2> error_file
```
The error output of command is written to the file error_file
Example
```
  ls -l output 2> /dev/null
```
The /dev/null file is specifically used to prevent unwanted output or error messages.