Display systemd journal since system boot.

Get a unique kernel boot id (a 128-bit hexadecimal string).

$ cat  /proc/sys/kernel/random/boot_id

234ed12f-d1d0-4bbc-813c-96a0478dbcb9

Remove dash character.

$ cat /proc/sys/kernel/random/boot_id | tr -d '-'

234ed12fd1d04bbc813c96a0478dbcb9

Use boot id to display related journal entries.

$ sudo journalctl _BOOT_ID=$(cat /proc/sys/kernel/random/boot_id | tr -d '-')

-- Journal begins at Sun 2021-05-16 20:24:12 CEST, ends at Tue 2021-10-12 00:00:03 CEST. --
Sep 18 00:33:20 desktop kernel: microcode: microcode updated early to revision 0x28, date = 2019-11-12
Sep 18 00:33:20 desktop kernel: Linux version 5.11.0-34-generic (buildd@lgw01-amd64-059) (gcc (Ubuntu 10.3.0-1ubuntu1) 10.3.0, GNU ld (GNU Binutils for Ubuntu) 2.36.1) #36-Ubuntu SMP Thu Aug 26 1>
Sep 18 00:33:20 desktop kernel: Command line: BOOT_IMAGE=/vmlinuz-5.11.0-34-generic root=/dev/mapper/vgubuntu-root ro quiet splash vt.handoff=7
Sep 18 00:33:20 desktop kernel: KERNEL supported cpus:
Sep 18 00:33:20 desktop kernel:   Intel GenuineIntel
Sep 18 00:33:20 desktop kernel:   AMD AuthenticAMD
Sep 18 00:33:20 desktop kernel:   Hygon HygonGenuine
Sep 18 00:33:20 desktop kernel:   Centaur CentaurHauls
Sep 18 00:33:20 desktop kernel:   zhaoxin   Shanghai  
Sep 18 00:33:20 desktop kernel: x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
Sep 18 00:33:20 desktop kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
Sep 18 00:33:20 desktop kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
Sep 18 00:33:20 desktop kernel: x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
Sep 18 00:33:20 desktop kernel: x86/fpu: Enabled xstate features 0x7, context size is 832 bytes, using 'standard' format.
Sep 18 00:33:20 desktop kernel: BIOS-provided physical RAM map:
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009d7ff] usable
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x000000000009d800-0x000000000009ffff] reserved
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x0000000000100000-0x00000000d234efff] usable
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x00000000d234f000-0x00000000d2355fff] ACPI NVS
Sep 18 00:33:20 desktop kernel: BIOS-e820: [mem 0x00000000d2356000-0x00000000d2783fff] usable
[...]
Oct 12 00:00:03 desktop systemd[1]: var-lib-containers-storage-overlay.mount: Succeeded.
Oct 12 00:00:03 desktop systemd[2351]: var-lib-containers-storage-overlay.mount: Succeeded.
Oct 12 00:00:03 desktop systemd[1]: podman-auto-update.service: Succeeded.
Oct 12 00:00:03 desktop systemd[1]: Finished Podman auto-update service.

You can list previous boot ids using journalctl command.

$ journalctl --list-boots 

-21 43342c2ca7124871b2dd0c53767ef343 Sun 2021-05-16 20:24:12 CEST—Sun 2021-05-23 23:22:01 CEST
-20 bfae7188dcc34ac5bdd1b151924fe81b Sun 2021-05-23 23:22:25 CEST—Sun 2021-05-23 23:27:33 CEST
-19 3f6b464322074d08bc56c5f79bd9e253 Sun 2021-05-23 23:27:57 CEST—Fri 2021-06-11 10:56:52 CEST
-18 2361b3f5600c45a4959382ae527a9072 Fri 2021-06-11 10:57:58 CEST—Sat 2021-06-19 10:42:57 CEST
-17 383ece182704405cb55e007d8b13f449 Sat 2021-06-19 10:44:45 CEST—Sat 2021-06-19 10:56:55 CEST
-16 62ac9023b4024e7caa3cb8fcb3ac47b4 Sat 2021-06-19 10:57:21 CEST—Thu 2021-06-24 23:25:56 CEST
-15 44941d11b9a24b21b7f80a0cabb34667 Thu 2021-06-24 23:26:14 CEST—Fri 2021-07-02 15:26:59 CEST
-14 60b24306402f43f694e19fd6035c753b Fri 2021-07-02 15:27:18 CEST—Fri 2021-07-02 16:32:30 CEST
-13 b505c367886f4d9d8c47f0da9714ee6a Fri 2021-07-02 16:32:50 CEST—Fri 2021-07-02 16:37:08 CEST
-12 c53e061fa9b8485586416822763a6ba8 Fri 2021-07-02 16:37:33 CEST—Fri 2021-07-02 16:40:07 CEST
-11 e8a0b6f8015e4e22ba0946f6db444aac Fri 2021-07-02 16:40:35 CEST—Fri 2021-07-02 16:47:08 CEST
-10 a16c3f55dc1b41f6be0853043c8e4a8b Fri 2021-07-02 16:49:33 CEST—Fri 2021-07-02 16:53:45 CEST
 -9 0a54fde10dee4152be1c74f7c75b445f Fri 2021-07-02 16:56:05 CEST—Fri 2021-07-02 17:02:49 CEST
 -8 be19bf9fdd744e0fb741bf0b021aa6d9 Fri 2021-07-02 17:03:07 CEST—Sun 2021-07-18 14:06:41 CEST
 -7 dfcb6b23f5e4486e891064b640c4c985 Sun 2021-07-18 14:11:49 CEST—Sun 2021-07-18 14:15:56 CEST
 -6 f4246d3d8bcc4c7f8fc33594aaa0c8be Sun 2021-07-18 14:16:24 CEST—Thu 2021-07-29 17:52:24 CEST
 -5 010ec25579b543e7b162071eadc401d0 Thu 2021-07-29 17:59:36 CEST—Mon 2021-09-06 21:30:01 CEST
 -4 fef5b5c5eff14097ab09187b2fbc8d97 Mon 2021-09-06 21:37:43 CEST—Tue 2021-09-07 18:26:42 CEST
 -3 45201a4ba0de4a3eacb1ae72d92aec77 Tue 2021-09-07 20:24:34 CEST—Wed 2021-09-08 14:17:01 CEST
 -2 341eee9909e84beeb8547530e62a6f85 Thu 2021-09-09 00:17:57 CEST—Fri 2021-09-10 13:30:26 CEST
 -1 060f8143d38f4ace8e6cec5f0329920f Fri 2021-09-10 23:59:27 CEST—Sat 2021-09-18 00:33:00 CEST
  0 234ed12fd1d04bbc813c96a0478dbcb9 Sat 2021-09-18 00:33:20 CEST—Tue 2021-10-12 00:23:11 CEST

Interesting.