# The Linux Command Line

The Linux command line is a way to reconfigure parts of the kernel without
having to fully recompile it.

## Editing the command line
sudo virsh net-start default 
You can view your current command line with the following command:

```bash
cat /proc/cmdline
```

*/proc/cmdline*

```
[root@arch blix]# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-linux root=UUID=2d53026e-e884-4578-89eb-0766c74f6e71 
rw zswap.enabled=0 rootfstype=ext4 loglevel=3 quiet
[root@arch blix]# 
```

The command line is made up of a series of parameters that are separated by a 
space. Some parameter have a value, and some are just flags.

### Updating the command line in GRUB

*In this manual we wont go over every single bootloader but for more information
you can look at this page 
[on the ArchWiki about editing the command line](https://wiki.archlinux.org/title/Kernel_parameters)*

#### Non-persistent changes

When in the GRUB menu, hover over the entry you want to edit and press `e` to 
edit it. This will bring up a simple text editor where you can edit the GRUB
configuration.

Look for the line that starts with `linux`, it will be quite long and you can 
press `end` on the `linux` line to jump to the end of it. Lets add the `splash`
parameter to the end of the line. To boot this configuration press `ctrl + x` or `F10`.

You should now either see a blank screen where you would normally see the boot 
messages, or you should see a splash screen if your distribution has one.

Now cat out the command line again and you should see that the `splash` parameter 
has been added to the command line.

#### Making changes persistent

In your booted system use `vim` or `nano` to edit the file `/etc/default/grub`.
Look for the line:

```
GRUB_CMDLINE_LINUX_DEFAULT=
```

In there you can append or edit kernel configuration parameters. To then install
this configuration use `grub-mkconfig`:

```bash
sudo grub-mkconfig -o /boot/grub/grub.cfg
```

### Updating the command line in systemd-boot

Editing command line parameters in systemd-boot is similar to GRUB but you will
need to edit the entry in the `/boot` folder.

#### Non-persistent changes

When in the systemd-boot menu, hover over the entry you want to edit and 
press `e` to edit it.

#### Making changes persistent

Edit the file `/boot/loader/entries/arch.conf` and add the parameters you want 
to the end of the `options` line. Then save the file and reboot.

## Hijacking the command line

Without using your bootloader, you can force the command line to be something
else. This is done by forcing the `/proc/cmdline` file to use a mounted file instead of the real command line.

```
cp /proc/cmdline /tmp/cmdline
# Then edit it as desired
mount --bind /tmp/cmdline /proc/cmdline
```

**Note**: This will only change the command line for the user space processes,
the kernel will still use the original command line that is in memory.

## Kernel boot parameters

Below is a short list of boot related parameters as a quick guide:

- `init`: This parameter specifies the initial program that the kernel should 
run after fully booting. By default, this is usually set to `/sbin/init`.
- `initrd`: This parameter specifies the initial RAM disk (initrd) image that
the kernel should use during the boot process.
- `cryptdevice`: This parameter is used to specify the encrypted block device
- `debug`: Enable kernel debugging messages during boot.
- `lsm`: Set the initialization order of the Linux Security Modules (LSMs).
- `maxcpus`: Limit the number of CPUs that the SMP kernel will use during boot.
- `mem`: Force a specific amount of memory to be used.
- `netdev`: Network devices configuration.
- `nomodeset`: Disable kernel mode setting.
- `panic`: Time before automatic panic reset.
- `quiet`: Hide kernel logs during boot.
- `splash`: Enable splash screen during boot.
- `resume`: Specify a swap device for hibernation resume.
- `ro`: Mount the root filesystem as read-only during boot.
- `rw`: Mount the root filesystem as read-write during boot.
- `root`: The root filesystem to use and mount on boot.
- `rootflags`: Additional flags for the root filesystem.
- `systemd.unit`: Boot to a specified systemd target instead of the default.
- `video`: Set the framebuffer configuration.

## A deep dive into the Kernel Command Line

*This section is a work in progress and may be incomplete*

### Kernel startup subsystem

### Audit

```
audit=<integer {0|1}>
```

Enable the kernel audit subsystem.

- `audit=1`: Enable auditing.
- `audit=0`: Disable auditing. *default*

### Audit backlog limit

```
audit_backlog_limit=<integer>
```

Set the queue limit for the kernel audit subsystem. Defaults to `64`.

### Boot delay

```
boot_delay=<integer>
```

Delays `printk` messages during the first seconds of boot. This can be useful
for debugging and slowing down the boot speed to see what is going on as each
hardware component is being initialized.

Defaults to `0` which means no delay.

### Boot memory debug

```
bootmem_debug
```

Enable boot memory debugging.