Linux Tips and Tricks

These tips and tricks shall be promoted to its own post when the topic has accumulated enough contents.

David Bombal
Linux for Hackers Tutorial (And Free Courses) - YouTube
Linux for Hackers Tutorial with OTW! - YouTube
Linux Networking that you need to know (Episode 3) - YouTube

Check kernel config

zcat /proc/config.gz

Check system install time

How do I find how long ago a Linux system was installed? - Unix & Linux Stack Exchange
How To Find Exact Installation Date And Time Of Your Linux OS

We need several tools:

df to look up the device path of /
tune2fs to query the file system's info
GNU tools for command line magic ;-)

# commands to query file system
$ df /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb3       184G   18G  164G  10% /
$ sudo tune2fs -l /dev/sdb3
...
<fs info>
Filesystem created:       Tue Aug 25 18:15:09 2015
<fs info>
...

# apply command line magic
$ sudo tune2fs -l $(df / | sed -n '2 p' | cut -d' ' -f1) | grep 'Filesystem created:'
Filesystem created:       Tue Aug 25 18:15:09 2015

Alternatively, you can use lshw:

$ sudo lshw -C volume
...
  *-volume:1
       description: EXT4 volume
       vendor: Linux
       physical id: 2
       bus info: nvme@0:1,2
       logical name: /dev/nvme0n1p2
       logical name: /
       version: 1.0
       serial: ee2e31d0-9a88-4c14-8d99-4a4e3c149734
       size: 459GiB
       capacity: 459GiB
       capabilities: journaled extended_attributes large_files huge_files dir_nlink recover 64bit extents ext4 ext2 initialized
       configuration: created=2021-01-02 09:17:28 filesystem=ext4 lastmountpoint=/ modified=2023-03-18 16:51:17 mount.fstype=ext4 mount.options=rw,noatime mounted=2023-03-18 16:51:17 state=mounted
...
  *-volume:2
       description: Windows NTFS volume
       vendor: Windows
       physical id: 3
       bus info: nvme@2:1,3
       logical name: /dev/nvme2n1p3
       version: 3.1
       serial: 2a0a5da8-5504-324d-92e8-d634ad2ce2b8
       size: 476GiB
       capacity: 476GiB
       capabilities: ntfs initialized
       configuration: clustersize=4096 created=2020-08-15 19:16:55 filesystem=ntfs name=Basic data partition state=clean
...

Check file system

How to force fsck to check filesystem after system reboot on Linux - LinuxConfig.org
Modify PASS in /etc/fstab and set "Maximum mount count".

sudo tune2fs -l /dev/sdX1 | grep Last\ c
sudo tune2fs -c 10 /dev/sdX1

boot - Automatically force fsck -fy when encountering "UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY." - Ask Ubuntu
Add "fsck.mode=force fsck.repair=yes" to boot param

Check process info

ls -l /proc/$pid/exe
ls -l /proc/$pid/cwd

ps aux | grep $pid

CPU

Linux 下如何開啟或關閉 Intel CPU 的 Turbo Boost 功能？ | MagicLen

Freezes

hakavlad/nohang: A sophisticated low memory handler for Linux

Bug #159356 “System freeze on high memory usage” : Bugs : linux package : Ubuntu
Some suggested adding this to /etc/rc.local:

sysctl vm.vfs_cache_pressure=100000

drop caches

free -m
echo 3 > /proc/sys/vm/drop_caches
# check cached column
free -m

Mounting storage

fstab: static information about the filesystems | util-linux File Formats | Man Pages | ManKier
blkid/lsblk -f to check partition id
The second last field (dump) should always be 0
The last field is filesystem check sequence at boot, 1 for rfs, 2 for other file system, 0 to skip

systemd时代的/etc/fstab - Systemd系列文章

Swap

sudo su
SWAPFILE=/swapfile
dd if=/dev/zero of=${SWAPFILE} bs=1G count=16 status=progress
chmod 600 ${SWAPFILE}
mkswap ${SWAPFILE}
# test
swapoff -a; swapon ${SWAPFILE}
free -h
# commit to fstab
printf "\n${SWAPFILE} swap swap sw 0 0\n" >> /etc/fstab

Linux Add a Swap File – Howto
hakavlad/nohang: A sophisticated low memory handler for Linux

swappiness

Tales from responsivenessland: why Linux feels slow, and how to fix that — Rudd-O.com in English
All about Linux swap space | Linux.com | The source for Linux information

"the swap partition is really just a fall back, where i dont care about performance, i just care about not crashing."

/etc/sysctl.d/sysctl.conf:

# decrease swappiness, can be higher for swapping on SSD
vm.swappiness = 30

# prefer file system (inode/dentry) cache
vm.vfs_cache_pressure = 70

zswap

Zswap vs zram in 2023, what's the actual practical difference? : r/linux
Zswap or Zram: at this time, which one is more efficient? : r/archlinux
kernel - zram vs zswap vs zcache Ultimate guide: when to use which one - Ask Ubuntu

cat /sys/module/zswap/parameters/enabled

zcat /proc/config.gz | grep -i zswap

Network buffer

This modify the buffer sizes temporarily. Change sysctl.conf to make the change permanent.

echo 1048576 > /proc/sys/net/core/rmem_max
echo 1048576 > /proc/sys/net/core/rmem_default

Kernel Modules

What is the Linux Kernel and What Does It Do?

# show available modules and drivers (`uname -r` show kernel release)
ls /lib/modules/$(uname -r)
ls /lib/modules/$(uname -r)/kernel/drivers/

# show loaded modules
lsmod

# load moduleX
modprobe moduleX
# unload moduleX
modprobe -r moduleX

mount `.iso` without root

Script: mountiso « IgnorantGuru's Blog

HOW TO: Allow Mounting Of ISO Files By A Regular User « IgnorantGuru's Blog

Enterprise Hum

Emulate the Enterprise Hum... : LinuxActionShow

requires sox

play -n -c1 synth whitenoise lowpass -1 120 lowpass -1 120 lowpass -1 120 gain +14

Logging

journalctl
How To Use Journalctl to View and Manipulate Systemd Logs | DigitalOcean

Generating and Applying Patch

diff -u oldfile newfile > patch.diff
patch < patch.diff  # `patch` will lookup `oldfile`

# diff whole tree
diff -Naur /usr/src/openvpn-2.3.2 /usr/src/openvpn-2.3.4 > openvpn.patch
# patch whole tree, striping 3 slashes with `-p`
patch -p3 < /root/openvpn.patch

7 Patch Command Examples to Apply Diff Patch Files in Linux
Unified Diff Format

Linux Tips and Tricks

Check kernel config

Check system install time

Check file system

Check process info

CPU

Freezes

drop caches

Mounting storage

Swap

swappiness

zswap

Network buffer

Kernel Modules

mount .iso without root

Enterprise Hum

Logging

auditd

audit.rules

Analyzing audit.log

Articles

go-audit

logrotate

Generating and Applying Patch

mount `.iso` without root

Analyzing `audit.log`