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Copy file name to clipboardExpand all lines: content/blog/linux-cmds.md
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<!-- General commands -->
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<!-- TODO: grep w/ regex -->
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## Overview
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This post serves to document useful Linux commands I use regularly or find useful. I intend to update it periodically as I find more and more useful commands.
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Some command explanation is included with each, but Linux and command line experience is assumed.
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## General Commands
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These commands aren't necessarily Linux machine-specific, but they may come in handy when using one.
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```Bash
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# Run with '-w' to follow in real time (like 'tail -f')
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# The 'journalctl -k' command will also show kernel logs and is generally more flexible
The following assumes an example _system-level_ Systemd unit called `run_periodically.timer`. You can use the same commands for _user-level_ Systemd units like `pulseaudio.service`, e.g. `systemctl status --user pulseaudio.service`.
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Active (enabled) systemd unit files are located in `/etc/systemd/system/` and `/etc/systemd/user/` directories.
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When modifying systemd units, modify files in these directories. Other unit files exist on Linux systems, primarily in a `/usr/lib/systemd/` directory (depends on distribution). However, generally do not modify these as they are installed by your package manager and are meant as defaults.
The following is a list of `journalctl` commands, each with separate options. Many of these may be combined to perform a specific task. For example, `journalctl -k -b 50 -r` will show the last 50 lines of kernel logs in reverse order (most recent to least recent).
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Just as Systemd `systemctl` commands support both system-level and user-level units, `journalctl` does as well using the same `--user` argument.
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```Bash
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$ journalctl -n 100
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# Follow logs in real time (similar to 'tail -f')
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# Can also substitute '--follow' for '-f'
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$ journalctl -f
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# Show logs in reverse order (most recent to least recent)
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# Cannot use this option with '-f'
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$ journalctl -r
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# Show all logs that contain the provided pattern, here 'wlan0'
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# Can also substitute the '--grep' option for '-g'
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$ journalctl -g wlan0
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# Only show logs for a specific ID, here 'test_program'.
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# For example, the following would create a log message and
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# tag it with the 'test_program' tag:
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#logger -t test_program "This is a log message"
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# echo "This is a warning message" | systemd-cat -t test_program -p warning
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#
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# Note that this is different from the '-u' option
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$ journalctl -t test_program
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# Only show logs for a specific service, here, 'wpa_supplicant.service'
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# Can also provide full Systemd unit name, e.g. here 'wpa_supplicant.service'
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$ journalctl -u wpa_supplicant
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# or
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$ journalctl -u wpa_supplicant.service
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# Show kernel logs (same output as 'dmesg' command)
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# Can also substitute the '--dmesg' option for '-k'
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#
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# This is the same as 'journalctl -t kernel'
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$ journalctl -k
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# Only show logs for this boot (can use '-b' option instead of '--boot')
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# When using the '--boot' and '-b' arguments, '0' and '-0' function the same (this boot)
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You probably want to use Network Manager to configure your networking instead.
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See the [next section](#managing-networking-network-manager) for more details.
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Most Linux distributions run Network Manager to configure and manage networking nowadays. This is a common point of confusion, as there is still material available online which assumes older, network-scripts based network management.
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Most Linux distributions run Network Manager to configure and manage networking nowadays. This is a common point of confusion, as many guides online reference older, network-scripts based network management and the deprecated `ifconfig` command.
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It is possible to configure networking with variations of these commands. Unless you know
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what you're doing, though, you're better off just using Network Manager.
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It is possible to configure networking with variations of these commands. Generally, though, you'll almost always be better off using a tool like `NetworkManager` or `systemd-networkd`, unless you have a very specific use case.
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For more information, see this [very detailed guide](https://axil.gitlab.io/iproute2/).
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#### Show Network Interface Link-Layer Info</h3>
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#### Show Network Interface IP-Layer Info
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Displays IPv4 and IPv6 information in addition to interface status, MAC address, MTU, and interface routing table, among other things (some of which require `-d` option). More detailed output is possible by using the `-d` option (without the `-br` option).
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These commands display IPv4 and IPv6 information in addition to other detailed information including interface status, MAC address, MTU, and interface routing table, among other things (some of which require `-d` option). More detailed output is possible by using the `-d` option (without the `-br` option).
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```Bash
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# Shorthand shown. Full command would be 'ip addr show', but the 'show' is optional.
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**NOTE:** Most Linux systems use Network Manager to manage and configure networking, including WiFi. See the [Managing Networking](#managing-networking-network-manager) section for more details.
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#### Show WiFi Interface General Info
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On Linux WiFi interfaces are created using a parent radio device, referred to as 'phys'. These radios come in a variety of form factors, including single radio, single phy and single radio, multi-phy. To view all system phys, run `ls /sys/class/ieee80211/`, which lists all `ieee80211` devices (WiFi phys). Supported interfaces, combinations, and settings depend on the radio firmware and associated Linux device driver. By default, a single WiFi interface is created per phy on system boot in 'managed' mode (WiFi station).
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While most will be content with `NetworkManager` managing their WiFi interface settings, a more advanced user may find the `wpa_supplicant` and `hostapd` programs of interest. The program `wpa_supplicant` configures WiFi clients, whereas `hostapd` configures WiFi access points (APs, what most people refer to as a router). Both live under the `hostap` project and are widely used, including within tools like `NetworkManager` itself and within commercial APs as well. The configuration syntax is notoriously somewhat difficult, especially if you don't know the details of WiFi well.
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#### Show WiFi Interface General Information
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Includes STA MAC, SSID, phy device, channel, frequency, transmit power.
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#### Show WiFi Interface Link Information
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**NOTE:** This command will only show meaningful output when the WiFi interface is connected (associated).
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Includes AP MAC (if station), SSID, frequency, bandwidth, RSSI (if station), and phy rate (MCS),
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among other things. Phy rate may or may not include NSS.
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beacon int: 100
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```
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#### Show WiFi Radio Info
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#### Show Channels Supported by WiFi Radio/Phy
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```Bash
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$ iw phy0 channels
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Band 1:
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* 2412 MHz [1]
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Maximum TX power: 30.0 dBm
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Channel widths: 20MHz HT40+
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...
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* 2467 MHz [12] (disabled)
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* 2472 MHz [13] (disabled)
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* 2484 MHz [14] (disabled)
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Band 2:
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* 5180 MHz [36]
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Maximum TX power: 23.0 dBm
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Channel widths: 20MHz HT40+ VHT80
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...
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```
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Includes channels and bands in current regulatory domain, ciphers, MCS rates, and antennas, among other things.
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#### Show All WiFi Radio/Phy Info
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This information is very verbose and includes channels and bands in current regulatory domain, ciphers, MCS rates, and antennas, and more.
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```Bash
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# Can also run 'iw phy phy0 info', but 'phy' is optional
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#### Show Wireless Regulatory Domain Info
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You may see a combination of 'global' and per-phy regulatory configuration. The following shows both.
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```Bash
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# Notice the inclusion of sub-1GHz (WiFi HaLow, 802.11ah) and 60GHz (WiGig) spectrum
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