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technology/applications/cli/Shell.md
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obj: concept
arch-wiki: https://wiki.archlinux.org/title/Command-line_shell
wiki: https://en.wikipedia.org/wiki/Unix_shell
rev: 2024-03-07
rev: 2024-09-05
---
# Shell
The shell is a command-line interpreter that provides a user interface to an operating system's services. It allows users to interact with the system through text-based commands and scripts. Shell scripting refers to writing a series of commands in a script file to automate tasks and perform complex operations.
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Commands can be either a script, a binary or anything that can be executed. If you don't provide a full path to the file you want to run the shell will search in the locations defined in the `$PATH` environment variable.
### Keyboard Shortcuts
- `Ctrl + C`: Terminate the current running process or command.
- `Ctrl + Z`: Suspend the current process and push it to the background.
- `Ctrl + D`: Exit the terminal or send EOF (End Of File) if running a script or command.
- `Ctrl + L`: Clear the terminal screen (similar to the clear command).
- `Ctrl + A`: Move the cursor to the beginning of the line.
- `Ctrl + E`: Move the cursor to the end of the line.
- `Ctrl + U`: Delete everything from the cursor position to the beginning of the line.
- `Ctrl + K`: Delete everything from the cursor position to the end of the line.
- `Ctrl + W`: Delete the word before the cursor.
- `Ctrl + R`: Search command history backward interactively.
- `Ctrl + T`: Swap the current character with the previous one (transpose).
- `Ctrl + Y`: Paste (yank) the last killed text.
- `Ctrl + P`: Previous command in history (similar to the Up arrow key).
- `Ctrl + N`: Next command in history (similar to the Down arrow key).
- `Alt + F`: Move forward one word.
- `Alt + B`: Move backward one word.
- `Alt + D`: Delete the word after the cursor.
- `Shift + PageUp/PageDown`: Scroll the terminal output up or down.
- `Tab`: Auto-complete file or directory names.
### Process Management
- `Ctrl + Z`: Suspend a running process.
- `fg`: Resume the last suspended process in the foreground.
- `bg`: Resume the last suspended process in the background.
- `jobs`: List all jobs currently running or suspended.
### Arguments
Everything after the command will be provided to the command as arguments. Each argument is separated by a space character. To avoid that you could quote an argument or escape the space character.
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| * (Multiplication) | Multiplies values on either side of the operator | `expr $a \* $b` will give 200 |
| / (Division) | Divides left hand operand by right hand operand | `expr $b / $a` will give 2 |
| % (Modulus) | Divides left hand operand by right hand operand and returns remainder | `expr $b % $a` will give 0 |
| = (Assignment) | Assigns right operand in left operand | `a = $b` would assign value of b into a |
| = (Assignment) | Assigns right operand in left operand | `a = $b` would assign value of b into a |
| == (Equality) | Compares two numbers, if both are same then returns true. | `[$a == $b ]` would return false. |
| != (Not Equality) | Compares two numbers, if both are different then returns true. | `[ $a != $b ]` would return true. |
@ -250,8 +278,8 @@ Assume variable **a** holds 10 and variable **b** holds 20 then −
##### Boolean Operators
Assume variable **a** holds 10 and variable **b** holds 20 then −
| Operator | Description | Example |
| -------- | -------------------------------------------------------------------------------------------------------- | ------------------------------------- |
| Operator | Description | Example |
| -------- | -------------------------------------------------------------------------------------------------------- | --------------------------------------- |
| **!** | This is logical negation. This inverts a true condition into false and vice versa. | `[ ! false ]` is true. |
| **-o** | This is logical **OR**. If one of the operands is true, then the condition becomes true. | `[ $a -lt 20 -o $b -gt 100 ]` is true. |
| **-a** | This is logical **AND**. If both the operands are true, then the condition becomes true otherwise false. | `[ $a -lt 20 -a $b -gt 100 ]` is false. |