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linux-general.md

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Linux

Fork/exec

  • When you run a command on the shell if first forks the process and then execs the command
  • exec does not fork
  • ps f shows processes as a tree
  • a child inherits stdin, stdout and stderr

bg/fg

  • Ctrl + z suspends the process
  • jobs shows jobs running
  • fg brings a process to the foreground (if there is only one)
  • fg %3 brings process no. 3 to the foreground
  • T means stopped, S means sleep
    • is last job and - is second to last

kill

  • ps ax show all processes
  • kill -l list of signals
  • kill -15 sigterm
  • kill sigterm, kills gracefully
  • kill -2 sigint
  • kill -1 sighup
  • kill -9 sigkill, brute force kill

Check open ports

# ss -lpntu
# ss -plunt

or

# lsof -i -P -n

Run process as user without login

su -s /bin/bash -c '/path/to/your/script' testuser

Check if something is already mounted

mountpoint /dev

Octal/numeric permissions

$ stat -c "%a %n" *

Convert from timestamp to date

$ date -d @1363292159.532
Thu Mar 14 21:15:59 CET 2013

Find file corresponding to an inode

$ find /sys -xdev -inum 30
/sys/devices/system/cpu/online

Find with multiple conditions and execs

find . \( $COND1 -exec $CMD1 \) , \( $COND2 -exec $CMD2 \)

Quotas

  • For XFS systems, on /etc/default/grub
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX rootflags=uquota,pquota"

Send to another process stdout

echo "test log1" >> /proc/$PID/fd/1

nsswitch.conf

  • files -> /etc/hosts
  • myhostname, mymachines -> systemd-container
  • resolve -> systemd-resolved
  • dns -> /etc/resolv.conf

https://news.ycombinator.com/item?id=19439722

Performance

  1. Disable cores:
echo 0 > /sys/devices/system/cpu/cpu<n>/online
  1. Check frequency range
cpupower frequency-info
  1. Adjust frequencies and governor
# set governor
cpupower frequency-set -g <governor>

# max freq
cpupower frequency-set -u <freq>

# min freq
cpupower frequency-set -d <freq>

Laptop not going to sleep

[Unit]
Description=Powertop tunings

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/sbin/powertop --auto-tune

[Install]
WantedBy=multi-user.target

Run init daemon as user

runuser -s /bin/bash <user> -c "$*"

setpriv is better for dropping privileges since it doesn't use PAM and it doesn't spawn an extra process

setpriv --no-new-privs --reuid=1000 --regid=1000 --init-groups --reset-env "$@"

ACLs

# default for new files
setfacl -d -R -m u:theUsersName:rwx

# actually set permission
setfacl -R -m u:theUsersName:rwx

Tar pipe

( cd "$src" && tar -cf - .) | (cd "$dst" && tar xvf - )

Routing tables

# show all rules
ip rule list

# create table
echo 200 custom >> /etc/iproute2/rt_tables

# from ip 
ip rule add from <source address> lookup <table name>

# from interface
ip rule add iif <interface> table isp2 priority 1000

# add default gw
ip route add default via 192.168.30.1 dev eth1 table custom

# show table 1
ip route show table 1