Update Linux command reference post

Author: a-gavin

Diff for: content/blog/linux-cmds.md

@@ -9,6 +9,12 @@ date = "2023-12-26"
 <!-- General commands -->
 <!-- TODO: grep w/ regex -->
 
+## Overview
+
+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.
+
+Some command explanation is included with each, but Linux and command line experience is assumed.
+
 ## General Commands
 
 These commands aren't necessarily Linux machine-specific, but they may come in handy when using one.
@@ -84,6 +90,7 @@ $ kill 921
 
 ```Bash
 # Run with '-w' to follow in real time (like 'tail -f')
+# The 'journalctl -k' command will also show kernel logs and is generally more flexible
 $ sudo dmesg
 ```
 
@@ -117,6 +124,9 @@ Mar 17 12:29:11 meadowlark systemd[1]: Finished Network Manager Wait Online.
 
 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`.
 
+Active (enabled) systemd unit files are located in `/etc/systemd/system/` and `/etc/systemd/user/` directories.
+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.
+
 ```Bash
 # Start the unit
 $ sudo systemctl start run_periodically.timer
@@ -130,7 +140,7 @@ $ sudo systemctl restart run_periodically.timer
 # Enable the unit (start automatically)
 $ sudo systemctl enable run_periodically.timer
 
-# Disable the unit (do not start automatically)
+# Disable the unit (only starts manually)
 $ sudo systemctl disable run_periodically.timer
 
 # Mask the unit
@@ -154,6 +164,8 @@ $ sudo systemctl daemon-reload
 
 #### View Service (Daemon) Logs
 
+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).
+
 Just as Systemd `systemctl` commands support both system-level and user-level units, `journalctl` does as well using the same `--user` argument.
 
 ```Bash
@@ -164,20 +176,34 @@ $ journalctl
 $ journalctl -n 100
 
 # Follow logs in real time (similar to 'tail -f')
+# Can also substitute '--follow' for '-f'
 $ journalctl -f
 
+# Show logs in reverse order (most recent to least recent)
+# Cannot use this option with '-f'
+$ journalctl -r
+
+# Show all logs that contain the provided pattern, here 'wlan0'
+# Can also substitute the '--grep' option for '-g'
+$ journalctl -g wlan0
+
 # Only show logs for a specific ID, here 'test_program'.
 # For example, the following would create a log message and
 # tag it with the 'test_program' tag:
-# logger -t test_program "This is a log message"
+#   echo "This is a warning message" | systemd-cat -t test_program -p warning
 #
 # Note that this is different from the '-u' option
 $ journalctl -t test_program
 
 # Only show logs for a specific service, here, 'wpa_supplicant.service'
+# Can also provide full Systemd unit name, e.g. here 'wpa_supplicant.service'
 $ journalctl -u wpa_supplicant
-# or
-$ journalctl -u wpa_supplicant.service
+
+# Show kernel logs (same output as 'dmesg' command)
+# Can also substitute the '--dmesg' option for '-k'
+#
+# This is the same as 'journalctl -t kernel'
+$ journalctl -k
 
 # Only show logs for this boot (can use '-b' option instead of '--boot')
 # When using the '--boot' and '-b' arguments, '0' and '-0' function the same (this boot)
@@ -324,10 +350,11 @@ $ du -h
 You probably want to use Network Manager to configure your networking instead.
 See the [next section](#managing-networking-network-manager) for more details.
 
-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.
+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.
+
+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.
 
-It is possible to configure networking with variations of these commands. Unless you know
-what you're doing, though, you're better off just using Network Manager.
+For more information, see this [very detailed guide](https://axil.gitlab.io/iproute2/).
 
 #### Show Network Interface Link-Layer Info</h3>
 
@@ -346,7 +373,7 @@ wlan0            UP             xx:xx:xx:xx:xx:xx <BROADCAST,MULTICAST,UP,LOWER_
 
 #### Show Network Interface IP-Layer Info
 
-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).
+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).
 
 ```Bash
 # Shorthand shown. Full command would be 'ip addr show', but the 'show' is optional.
@@ -526,7 +553,11 @@ $ nmcli c m SSID_NAME \
 
 **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.
 
-#### Show WiFi Interface General Info
+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).
+
+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.
+
+#### Show WiFi Interface General Information
 
 Includes STA MAC, SSID, phy device, channel, frequency, transmit power.
 
@@ -548,6 +579,8 @@ Interface wlan0
 
 #### Show WiFi Interface Link Information
 
+**NOTE:** This command will only show meaningful output when the WiFi interface is connected (associated).
+
 Includes AP MAC (if station), SSID, frequency, bandwidth, RSSI (if station), and phy rate (MCS),
 among other things. Phy rate may or may not include NSS.
 
@@ -569,7 +602,7 @@ Connected to xx:xx:xx:xx:xx:xx (on wlan0)
         beacon int:     100
 ```
 
-#### Show WiFi Radio Info
+#### Show WiFi Radio/Phy Info
 
 Includes channels and bands in current regulatory domain, ciphers, MCS rates, and antennas, among other things.
 
@@ -601,16 +634,138 @@ $ iw phy0 info
 
 #### Show Wireless Regulatory Domain Info
 
+You may see a combination of 'global' and per-phy regulatory configuration. The following shows both.
+
 ```Bash
+# Notice the inclusion of sub-1GHz (WiFi HaLow, 802.11ah) and 60GHz (WiGig) spectrum
 $ iw reg get
 global
+country US: DFS-FCC
+        (902 - 904 @ 2), (N/A, 30), (N/A)
+        (904 - 920 @ 16), (N/A, 30), (N/A)
+        (920 - 928 @ 8), (N/A, 30), (N/A)
+        (2400 - 2472 @ 40), (N/A, 30), (N/A)
+        (5150 - 5250 @ 80), (N/A, 23), (N/A), AUTO-BW
+        (5250 - 5350 @ 80), (N/A, 24), (0 ms), DFS, AUTO-BW
+        (5470 - 5730 @ 160), (N/A, 24), (0 ms), DFS
+        (5730 - 5850 @ 80), (N/A, 30), (N/A), AUTO-BW
+        (5850 - 5895 @ 40), (N/A, 27), (N/A), NO-OUTDOOR, AUTO-BW
+        (5925 - 7125 @ 320), (N/A, 12), (N/A), NO-OUTDOOR
+        (57240 - 71000 @ 2160), (N/A, 40), (N/A)
+
+phy#0 (self-managed)
 country 00: DFS-UNSET
-        (2402 - 2472 @ 40), (6, 20), (N/A)
-        (2457 - 2482 @ 20), (6, 20), (N/A), AUTO-BW, PASSIVE-SCAN
-        (2474 - 2494 @ 20), (6, 20), (N/A), NO-OFDM, PASSIVE-SCAN
-        (5170 - 5250 @ 80), (6, 20), (N/A), AUTO-BW, PASSIVE-SCAN
-        (5250 - 5330 @ 80), (6, 20), (0 ms), DFS, AUTO-BW, PASSIVE-SCAN
-        (5490 - 5730 @ 160), (6, 20), (0 ms), DFS, PASSIVE-SCAN
-        (5735 - 5835 @ 80), (6, 20), (N/A), PASSIVE-SCAN
-        (57240 - 63720 @ 2160), (N/A, 0), (N/A)
+        (2402 - 2437 @ 40), (6, 22), (N/A), AUTO-BW, NO-HT40MINUS, NO-80MHZ, NO-160MHZ
+        (2422 - 2462 @ 40), (6, 22), (N/A), AUTO-BW, NO-80MHZ, NO-160MHZ
+        (2447 - 2482 @ 40), (6, 22), (N/A), AUTO-BW, NO-HT40PLUS, NO-80MHZ, NO-160MHZ
+        (5170 - 5190 @ 160), (6, 22), (N/A), NO-OUTDOOR, AUTO-BW, IR-CONCURRENT, NO-HT40MINUS, PASSIVE-SCAN
+        (5190 - 5210 @ 160), (6, 22), (N/A), NO-OUTDOOR, AUTO-BW, IR-CONCURRENT, NO-HT40PLUS, PASSIVE-SCAN
+        (5210 - 5230 @ 160), (6, 22), (N/A), NO-OUTDOOR, AUTO-BW, IR-CONCURRENT, NO-HT40MINUS, PASSIVE-SCAN
+        (5230 - 5250 @ 160), (6, 22), (N/A), NO-OUTDOOR, AUTO-BW, IR-CONCURRENT, NO-HT40PLUS, PASSIVE-SCAN
+        (5250 - 5270 @ 160), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40MINUS, PASSIVE-SCAN
+        (5270 - 5290 @ 160), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40PLUS, PASSIVE-SCAN
+        ...
+        (5590 - 5610 @ 160), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40PLUS, PASSIVE-SCAN
+        (5610 - 5630 @ 160), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40MINUS, PASSIVE-SCAN
+        (5650 - 5670 @ 80), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40MINUS, NO-160MHZ, PASSIVE-SCAN
+        (5670 - 5690 @ 80), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40PLUS, NO-160MHZ, PASSIVE-SCAN
+        (5690 - 5710 @ 80), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40MINUS, NO-160MHZ, PASSIVE-SCAN
+        (5710 - 5730 @ 80), (6, 22), (0 ms), DFS, AUTO-BW, NO-HT40PLUS, NO-160MHZ, PASSIVE-SCAN
+        (5735 - 5755 @ 80), (6, 22), (N/A), AUTO-BW, IR-CONCURRENT, NO-HT40MINUS, NO-160MHZ, PASSIVE-SCAN
+        (5755 - 5775 @ 80), (6, 22), (N/A), AUTO-BW, IR-CONCURRENT, NO-HT40PLUS, NO-160MHZ, PASSIVE-SCAN
+        (5775 - 5795 @ 80), (6, 22), (N/A), AUTO-BW, IR-CONCURRENT, NO-HT40MINUS, NO-160MHZ, PASSIVE-SCAN
+        (5795 - 5815 @ 80), (6, 22), (N/A), AUTO-BW, IR-CONCURRENT, NO-HT40PLUS, NO-160MHZ, PASSIVE-SCAN
+        (5815 - 5835 @ 20), (6, 22), (N/A), AUTO-BW, IR-CONCURRENT, NO-HT40MINUS, NO-HT40PLUS, NO-80MHZ,NO-160MHZ, PASSIVE-SCAN
+```
+
+<!-- Querying DBus Information -->
+
+## Querying DBus Information
+
+#### List DBus Peers
+
+```Bash
+# The same output is displayed when just 'busctl' is run
+$ busctl list
+NAME    PID PROCESS         USER            CONNECTION    UNIT                     SESSION DESCRIPTION
+:1.0    719 systemd-resolve systemd-resolve :1.0          systemd-resolved.service -       -
+:1.1    718 systemd-oomd    systemd-oom     :1.1          systemd-oomd.service     -       -
+:1.10   821 NetworkManager  root            :1.10         NetworkManager.service   -       -
+:1.103 4099 busctl          lanforge        :1.103        session-3.scope          3       -
+:1.15  1023 fwupd           root            :1.15         fwupd.service            -       -
+...
+```
+
+#### Show DBus Peer Status
+
+```Bash
+# Can specify using either the 'UniqueName' identifier
+# or the more human-readable DBus peer name
+#
+# Run just 'busctl status' to view main DBus socket status
+$ busctl status org.freedesktop.ModemManager1
+PID=805
+...
+Comm=ModemManager
+CommandLine=/usr/sbin/ModemManager --debug
+CGroup=/system.slice/ModemManager.service
+Unit=ModemManager.service
+Slice=system.slice
+...
+UniqueName=:1.7
+EffectiveCapabilities=cap_net_admin cap_sys_admin
+PermittedCapabilities=cap_net_admin cap_sys_admin
+InheritableCapabilities=cap_sys_admin
+BoundingCapabilities=cap_net_admin cap_sys_admin
+```
+
+#### Show DBus Peer Object Tree
+
+```Bash
+# Identifier is same as that used by 'busctl status'.
+# See above for more information.
+#
+# Can specify more than one peer to list. For example,
+# show trees for both the ':1.7' and ':1.10' peers:
+#   busctl tree :1.7 :1.10
+#
+# Run just 'busctl' to view DBus tree for each DBus peer
+# active on the system (may take some time to fully complete)
+$ busctl tree org.freedesktop.ModemManager1
+└─ /org
+  └─ /org/freedesktop
+    └─ /org/freedesktop/ModemManager1
+      ├─ /org/freedesktop/ModemManager1/Modem
+      │ └─ /org/freedesktop/ModemManager1/Modem/0
+      └─ /org/freedesktop/ModemManager1/SIM
+        └─ /org/freedesktop/ModemManager1/SIM/0
+```
+
+#### Introspect DBus Peer Object
+
+```Bash
+# Shows very verbose information on components of DBus peer object
+#
+# Here the object is '/org/freedesktop/ModemManager1/Modem/0', a cellular modem
+# configured by the 'org.freedesktop.ModemManager1' DBus peer
+$ busctl introspect org.freedesktop.ModemManager1 /org/freedesktop/ModemManager1/Modem/0
+NAME                                TYPE      SIGNATURE RESULT/VALUE                             FLAGS
+...
+org.freedesktop.ModemManager1.Modem interface -         -                                        -
+.Command                            method    su        s                                        -
+.CreateBearer                       method    a{sv}     o                                        -
+.DeleteBearer                       method    o         -                                        -
+.Enable                             method    b         -                                        -
+...
+.CurrentBands                       property  au        57 5 6 7 8 9 10 12 31 32 33 34 35 37 38… emits-change
+.CurrentCapabilities                property  u         204                                      emits-change
+.CurrentModes                       property  (uu)      28 16                                    emits-change
+...
+.Drivers                            property  as        2 "option" "qmi_wwan"                    emits-change
+.EquipmentIdentifier                property  s         "XXXXXXXXXXXXXXX"                        emits-change
+.HardwareRevision                   property  s         "XXXXX"                                  emits-change
+.Manufacturer                       property  s         "Quectel"                                emits-change
+...
+.MaxBearers                         property  u         1                                        emits-change
+...
 ```