# System and processes

_Peios / Using Peios / Peiosutils / System and processes_

> The commands that report on the system, run other commands in a managed way, signal processes, and work with time, terminals, and storage.

This topic gathers the commands that deal with the **running system** rather than with files: reading the time, asking what hardware and system you are on, launching another command under some constraint, signalling a process, working with the terminal.

## The commands

**Time**

| Command | Purpose |
|---|---|
| [`date`](/peios/using-peios/peiosutils/system-and-processes/date.md) | Print — or set — the system date and time. |
| [`sleep`](/peios/using-peios/peiosutils/system-and-processes/sleep.md) | Pause for a given length of time. |
| [`uptime`](/peios/using-peios/peiosutils/system-and-processes/uptime.md) | Show how long the system has been running. |

**Running a command under a constraint**

| Command | Purpose |
|---|---|
| [`timeout`](/peios/using-peios/peiosutils/system-and-processes/timeout.md) | Run a command, and kill it if it runs too long. |
| [`nohup`](/peios/using-peios/peiosutils/system-and-processes/nohup.md) | Run a command so it survives the terminal closing. |
| [`nice`](/peios/using-peios/peiosutils/system-and-processes/nice.md) | Run a command at an adjusted scheduling priority. |
| [`stdbuf`](/peios/using-peios/peiosutils/system-and-processes/stdbuf.md) | Run a command with altered stream buffering. |
| [`chroot`](/peios/using-peios/peiosutils/system-and-processes/chroot.md) | Run a command with a different directory as its root. |

**Signalling processes**

| Command | Purpose |
|---|---|
| [`kill`](/peios/using-peios/peiosutils/system-and-processes/kill.md) | Send a signal to a process. |

**System identity and capacity**

| Command | Purpose |
|---|---|
| [`uname`](/peios/using-peios/peiosutils/system-and-processes/uname.md) | Print system information — the OS, the kernel, the machine. |
| [`arch`](/peios/using-peios/peiosutils/system-and-processes/arch.md) | Print the machine's hardware architecture. |
| [`hostname`](/peios/using-peios/peiosutils/system-and-processes/hostname.md) | Display — or set — the system's host name. |
| [`hostid`](/peios/using-peios/peiosutils/system-and-processes/hostid.md) | Print the host's numeric identifier. |
| [`nproc`](/peios/using-peios/peiosutils/system-and-processes/nproc.md) | Print how many processor cores are available. |

**Who you are**

| Command | Purpose |
|---|---|
| [`whoami`](/peios/using-peios/peiosutils/system-and-processes/whoami.md) | Print the name of the user a process is running as. |
| [`id`](/peios/using-peios/peiosutils/system-and-processes/id.md) | Print the user and group identity of a process, or of a named user. |
| [`groups`](/peios/using-peios/peiosutils/system-and-processes/groups.md) | Print the groups a user belongs to. |
| [`logname`](/peios/using-peios/peiosutils/system-and-processes/logname.md) | Print the login name of the user a process is running as. |

These report the **projection** of a token onto POSIX user and group numbers, which is what Linux programs see. The numbers grant nothing on their own — [`id`](/peios/using-peios/peiosutils/system-and-processes/id.md) explains what they can and cannot say, and [`token`](/peios/security-fundamentals/tokens/token-command.md) shows the identity underneath them.

**Storage and terminal**

| Command | Purpose |
|---|---|
| [`sync`](/peios/using-peios/peiosutils/system-and-processes/sync.md) | Flush cached writes out to persistent storage. |
| [`tty`](/peios/using-peios/peiosutils/system-and-processes/tty.md) | Print the name of the terminal on standard input. |
| [`stty`](/peios/using-peios/peiosutils/system-and-processes/stty.md) | Display or change terminal settings. |

## A note on changing the system

Most commands here only *report*. A few can *change* the running system — `date` can set the clock, `hostname` can set the host name. Changing system-wide state is a privileged operation: it succeeds only for a caller whose token holds the right to do it, and is refused otherwise. The pages for those commands say so where it applies, and [Privileges](/peios/security-fundamentals/privileges/overview.md) is the full picture.

## Where to start

[`uname`](/peios/using-peios/peiosutils/system-and-processes/uname.md) answers "what am I running on?". [`kill`](/peios/using-peios/peiosutils/system-and-processes/kill.md) is the one to read carefully — sending the wrong signal to the wrong process is a quick way to lose work.

Related content:

- [uname](/peios/using-peios/peiosutils/system-and-processes/uname.md)
- [kill](/peios/using-peios/peiosutils/system-and-processes/kill.md)