Linux Commands

Misc. Linux commands.

Account

Last Modify

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chage -d YYYY-MM-DD <user>

Password Never Expire

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chage -I -1 -m 0 -M 99999 -E -1 <user>

Lock/Unlock

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passwd -l <user>  # Lock
passwd -u <user>  # Unlock

usermod -L <user> # Lock
usermod -U <user> # Unlock

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pam_tally2 -r -u <user>

Sudo NOPASSWD

In /etc/sudoers, add NOPASSWD to sudo group.

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%sudo   ALL=(ALL:ALL) NOPASSWD:ALL

SSH

Remove Known Hosts

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ssh-keygen -R <hostname/ip>
ssh-keygen -f "~/.ssh/known_hosts" -R <hostname/ip>

ProxyCommand

host test
ProxyCommand ssh -W %h:%p jumpserver

ProxyJump

host test
ProxyJump jumpserver

Rotate Frame Buffer

Number can be 1, 2, 3

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echo 1 > /sys/class/graphics/fbcon/rotate_all
echo 1 > /sys/class/graphics/fbcon/rotate

File System

Resize Filesystem

Usually use after a partition / image resize

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resize2fs <device>
resize2fs /dev/sda1

Disable journal on ext4

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tune2fs -O ^has_journal /dev/<disk>

Create Sparse File

truncate -s <size> <filename>

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$ truncate -s 10G 10G.txt

$ ls -lah 10G.txt
-rw-r--r--    1 user user   10.0G Apr  1 00:00 10G.txt

$ du -sh 10G.txt
0       10G.txt

Ref: Sparse file wikipedia

File

Find

Find x-days before and action

In actuality, it should be (x-1).

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/bin/find <path> -maxdepth 1 -mtime +<x> -type f -name "<pattern>" -exec rm -f {} \;

Find x-minutes before and action

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/bin/find <path> -maxdepth 1 -mmin +<x> -type f -name "<pattern>" -exec rm -f {} \;

Detect Text File Charset/Encoding

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uchardet <file.txt>

Convert Text File Charset/Encoding

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iconv iconv -f <file charset> -t <output charset> <file.txt>
iconv iconv -f jis -t utf8 readme.txt

Content Type

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file <filename>

Image Info

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magick identify <filename>

Timeout

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timeout <duration> <command ...... >
timeout 1d tcpdump -n -i eth0 port 22 -s 65535 -w ssh_dump.cap &
nohup timeout 1d tcpdump -n -i eth0 port 22 -s 65535 -w ssh_dump.cap &

Rsync

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rsync -vahpt --size-only --stats --del <source> <target>

<source> is put/sync INTO <target> directory, not replacing <target>.

Network

Avahi/MDNS

Show All Entries

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avahi-browse -a
avahi-browse -a -d <domain> # specify domain other than .local

Lookup

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avahi-resolve -n4 door.local
avahi-resolve -n6 door.local

Ban IP

IPTables

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iptables -A INPUT -s <IP> -j DROP
ip6tables -A INPUT -s <IP> -j DROP

Fail2ban

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fail2ban-client status # show jail list
fail2ban-client -vvv set <jail from list> banip <ip>

Check NIC status

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ethtool <nic>

Check WiFi SSID

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iw dev <wifi nic> scan
iw dev wlan0 scan

Enable BBR

Create file /etc/sysctl.d/10-network-bbr.conf with following content and reboot:

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net.core.default_qdisc=fq
net.ipv4.tcp_congestion_control=bbr

Test Connection UDP

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nc -z -v -u [hostname/IP address] [port number]
nc -z -v -u 8.8.8.8 53

Curl

Skip certificate checking

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curl -k ... # Use -k to skip certificate check.

DNS Over HTTPS

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curl -sH 'accept: application/dns-json' 'https://dns.google/resolve?name=google.com' | jq .
curl -sH 'accept: application/dns-json' 'https://cloudflare-dns.com/dns-query?name=google.com' | jq .

Journalctl

List Field

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journalctl -N

List Identifier

Identifier value can be used in -t.

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journalctl -F SYSLOG_IDENTIFIE

John Siu

Update: 2023-09-28

Tag

[cheatsheet]

[command-line]

[linux]