Linux - Bag of Tricks

Introduction

This document has many useful command.

Linux Set Time Examples

You can also simplify format using following syntax:

date +%Y%m%d -s "20081128"

To set time use the following syntax:

date +%T -s "10:13:13"

Use the following syntax to set new data and time:

date --set="STRING"

For example, set new data to 2 Oct 2006 18:00:00, type the following command as root user:

date -s "2 OCT 2006 18:00:00"

date --set="2 OCT 2006 18:00:00"

Rsync Copy Examples

This is to move files from one server to another

Ending the folder WITHOUT a “/” slash means copy that folder everything in that folder

Ending the folder WITH a “/” slash means copy everything within that folder

Example for “remote to local” location

rsync -chavzP --stats --progress -e ssh user@remote_host:/remote_folder/dir1/ /local_folder/dir1/

Example for “local to remote” location

rsync -chavzP --stats --progress -e ssh /local_folder/dir1/ user@remote_host:/remote_folder/dir1/

Example to add a Rsync key on the remote server

ssh-keygen -t rsa

If it already exists simply hit "n"

Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
/root/.ssh/id_rsa already exists.
Overwrite (y/n)?

If not then simply hit enter through all options

Example: of using ROOT

Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa
Your public key has been saved in /root/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:JoMN/cxvsqZWBHws4eyrU5Q0F0qRe//44qdrrjiQmbU root@DSS-US-TMAP-XXX
The key's randomart image is:
+---[RSA 3072]----+
| .+=.. |
| =B.+ |
| ..=O |
| ==+o |
| = E=... |
| o... oo |
| ..o..++ o |
| .oo+o==*. |
+----[SHA256]-----+
You have mail in /var/spool/mail/root

Run the following to add the key to the remote server

ssh-copy-id -i ~/.ssh/id_rsa.pub remuser@sfl-lin-001

Example of using a USER you will have to enter yes and the USER password

/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/root/.ssh/id_rsa.pub"
The authenticity of host 'sfl-lin-020 (192.168.136.80)' can't be established.
ED25519 key fingerprint is SHA256:oZnvrgY+2Xpd2/huaffvzLMBAgI52AMPUmq/LPLIXbE.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
remuser@dss-us-map-020's password:
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified

Number of key(s) added: 1

Now try logging into the machine, with: "ssh 'remuser@sfl-lin-020'"
and check to make sure that only the key(s) you wanted were added.

Optional: If the command cannot be run above you can copy the key to the remote server manually into the “authorized_keys” file

cd
cd .ssh
vi authrized_keys

Optional: Change the permissions on the local server

chmod 600 ~/.ssh/*
chmod 711 ~/.ssh
chmod 711 ~

Synology Rsync

rsync -aXHmS --syno-acl /volum1/[xxx] /volume2/[xxx]

-a, --archive archive mode; equals -rlptgoD (no -H,-A,-X)
-p, --perms preserve permissions
-X, --xattrs preserve extended attributes
-o, --owner preserve owner (super-user only)
-g, --group preserve group
--syno-acl copy Synology ACL data

I use the following options myself:
rsync -avhxWog --stats --backup --suffix $OLDSUFFIX --exclude-from=$RSYEXCL --syno-pseudo-root

No idea why I list options "og" since they're implied by -a, but it works...

Regards, Arild
PS: "rsync --help" lists all available options for rsync

Find and Replace String with sed

There are several versions of sed, with some functional differences between them. macOS uses the BSD version, while most Linux distributions come with GNU sed pre-installed by default. We’ll use the GNU version.

The general form of searching and replacing text using sed takes the following form:

sed -i 's/SEARCH_REGEX/REPLACEMENT/g' INPUTFILE

Cop

-i - By default, sed writes its output to the standard output. This option tells sed to edit files in place. If an extension is supplied (ex -i.bak), a backup of the original file is created.
s - The substitute command, probably the most used command in sed.
/ / / - Delimiter character. It can be any character but usually the slash (/) character is used.
SEARCH_REGEX - Normal string or a regular expression to search for.
REPLACEMENT - The replacement string.
g - Global replacement flag. By default, sed reads the file line by line and changes only the first occurrence of the SEARCH_REGEX on a line. When the replacement flag is provided, all occurrences are replaced.
INPUTFILE - The name of the file on which you want to run the command.

Find and Replace String with sed within vi

This is to search and replace a file globally withing vi

:%s/search_string/replacement_string/g

Kill Users in Linux

This is to be used when trying to kill users using the connection, replace the ? with the number of the session.

pkill -KILL -t pts/?

Create a CERT

First, you need to generate the private key and the Certificate Signing Request (CSR). You can do this via the openssl command:

openssl req -nodes -newkey rsa:2048 -keyout privatekey.key -out mail.csr

Then, generate a signing request

openssl x509 -req -days 365 -in mail.csr -signkey privatekey.key -out secure.crt

Create a localhost cert on the server

openssl req -newkey rsa:2048 -nodes -keyout /etc/pki/tls/private/localhost.key -x509 -days 365 -out /etc/pki/tls/certs/localhost.crt

Mariadb Log Rotate

If log file is large, try if the logrotate

logrotate --force /etc/logrotate.d/mariadb

MySQL Fail to Start

If MySQL does not restart, it probably will not as the index of the log files will not be changed

cd /var/lib/mysql
mv ib_logfile0 ib_logfile0.old
mv ib_logfile1 ib_logfile1.old
systemctl restart mariadb

Configure Rsync

Useful for system migrations

Create a “/etc/rsyncd.conf” containing:

[root]

exclude = /dev /etc/fstab /proc /sys

path = /

read only = yes

list = yes

uid = root

gid = root

Enable and start:

systemctl enable rsyncd.service

systemctl start rsyncd.service

Change Run level

systemctl set-default multi-user.target

To switch from graphical to multi-user:

systemctl isolate multi-user.target;

Change Local settings

# timedatectl set-timezone Europe/London

# localectl set-locale LANG=en_GB.UTF-8

# localectl set-keymap uk

Temporary change

$ loadkeys us

Configure Alternate Authentication

authconfig-tui

SSD Considerations

Change the value of “issue_discards” option from 0 to 1 in “/etc/lvm/lvm.conf”

# systemctl enable fstrim.timer

Adjust “/etc/fstab”

/dev/mapper/xxx /XXX xfs defaults,noatime,discard 0 0

Optionally set /tmp in RAM

# systemctl enable tmp.mount

Adding a Disk

# parted /dev/sdx

mklabel gpt

unit s

mkpart primary 2048s 100%

set 1 lvm on

quit

# pvcreate /dev/sdx1

# vgcreate rl_ssd /dev/sdx1

# lvcreate –L 50GB -n mysql rl_ssd

# mkfs.xfs /dev/rl-ssd/mysql

# blkid /dev/sdc

# chown mysql:mysql /var/lib/mysql

Growing a lvm partition

# parted /dev/sdc

(parted) unit b

(parted) print free

Number Start End Size Type File system Flags

1 31744B 5368709119B 5368677376B primary

5368709120B 21474836479B 16106127360B Free Space

(parted) resizepart 1 21474836479B

(parted) quit

# pvresize /dev/sdc1

Updating Bootloader configuration

/etc/default/grub

grub2-mkconfig -o /boot/grub2/grub.cfg

NMAP Scan for all Open Ports

TCP

sudo nmap -sT -p- onling.com

UDP

sudo nmap -sU -p- onling.com