# Linux

## Useful commands and tips

### Split strings

```bash
# split ip list string by comma and get first one
export IP=127.0.0.2,127.0.0.3
echo $IP | cut -d ',' -f1
```

### Create a systemd service

Create a file called `/etc/systemd/system/hello.service`

```
[Unit]
Description=Hello Service
After=network.target
StartLimitIntervalSec=0
[Service]
Type=simple
Restart=always
RestartSec=1
User=centos
ExecStart=/app/hello.sh

[Install]
WantedBy=multi-user.target
```
Automatically start the service when boot the vm

```bash
sudo systemctl enable hello
```

Start service

```bash
sudo systemctl start hello
```
Learned from [here](https://medium.com/@benmorel/creating-a-linux-service-with-systemd-611b5c8b91d6)

### ssh
Do not use default `22` port
```bash
ssh user@ip -p 22222
```

Login without password

```bash
# adds private key identities to the authentication agent
ssh-add ~/.ssh/id_rsa

# use locally available keys to authorise logins on a remote machine

ssh-copy-id user@ip -p 22222
```

### Add user to the sudo group

```bash
usermod -aG sudo youruser
```

### IP table

### Cron job

List cron jobs for current user

```bash
crontab -l
```

List cron jobs for some user

```bash
crontab -u user -l
```

Edit cron jobs

```bash
crontab -e
```

#### An example cron job

For example, you can run a backup of all your user accounts

```bash
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
```
#### Check cron jobs logs

```bash
grep CRON /var/log/syslog
```

#### Cron job expression

`MIN HOUR DOM MON DOW`

Field    Description    Allowed Value
MIN      Minute field    0 to 59
HOUR     Hour field      0 to 23
DOM      Day of Month    1-31
MON      Month field     1-12
DOW      Day Of Week     0-6

### PS
```bash
ps aux | grep 'ruby'
```
List process, using `grep` to filter processes
### tail
Tailing texts from file in realtime

```bash
tail -f a.log
```
`f` means following, auto append texts to console
### nohup
Simply running application in background


### df
```bash
df -h
```
Show disk usage and left, `h` means show in human readable way. Normally by MB/GB.
### free
```bash
              total        used        free      shared  buff/cache   available
Mem:        3960624      416720      270024       22252     3273880     3264060
Swap:       4104188      109688     3994500
```
Show memory usage and

### Watch
```
watch kubectl get pods
```
Continuous running the commands, watching the changes. By default it refresh the result every `2s`.

### Change timezone
```bash
TZ=Asia/Shanghai;
ln -snf /usr/share/zoneinfo/$TZ /etc/localtime
```

### Check release
```bash
cat /etc/*-release
```

### Debian check system version
```bash
cat /etc/issue

cat /etc/debian_version
```

### htop

like top, but more

#### mosh

auto reconnect ssh

### show last something time

```bash
last shutdown
last reboot
```

### Count files in current folder

```bash
find . -type f | wc -l
```
### Check if a port is open

Used Netcat

```bash
if ! nc -z localhost 5672; then
sleep 3;
fi
```

### scp copy files between host

#### Copy files from remote server

```bash
scp your_username@remotehost.edu:foobar.txt /local/dir
```

### Copy local files to remote server

```bash
scp /path/to/local/file user@server:/path/to/remote/file
```

### Merge video files into one

```bash
ffmpeg -f concat -safe 0 -i ./files.txt -c copy output.mp4
```

In the `files.txt` you should list all the video clips like the following

```txt
file 1.flv
file 2.flv
file 3.flv
```

### Curl

Curl is a very popular and powerful HTTP client, there is also another very good tool called `httpie` written in Python which is even eaiser to use.

The following are some useful commands:

#### Post request with file
```bash
curl --data "@$(pwd)/fixure/sample.json" --user "foo:bar" localhost:8080/api/abc
```

### Mount

#### Mount NFS(Network file system) to local

```bash

mkdir /to-local-folder
# mount
mount -t nfs -O user=abc,pass=pass 192.168.1.2:/nfs-folder  /to-local-folder

# remount
mount -o remount /to-local-folder
```

#### Mount folder permanently

In the example above, the mounted folder will lost the connection when you restart your system, in order to permanently mount the folder, you will need to edit `/etc/fstab` by adding one more line

```
192.168.1.2:/nfs-folder   /to-local-folder    nfs    defaults    0 0
```
You can specify more options

```
192.168.0.216:/nfs-folder   /to-local-folder    nfs    defaults,proto=tcp,port=2049    0 0
```

### Rsync

Sync folders and files between folders and hosts

### sync between directories

```bash
rsync -avzh /root/rpmpkgs /tmp/backups/
```
#### sync folders between hosts

```bash
rsync -avz --exclude downloads --exclude 'some-folder' /mnt rsync://user@192.168.1.2/rsync/ -v -c
```

You can export `RSYNC_PASSWORD` to pass password to the command.

#### Check rsync logs

```bash
grep -ir rsync /var/log
```

### Check debian version

```bash
cat /etc/issue

cat /etc/debian_version
```

### Debian error bash: netstat: command not found

```bash
apt-get install net-tools
```

### Create CPU loads

```bash
dd if=/dev/urandom | gzip -9 >> /dev/null &
```

### Kill the load process

```bash
kill %1
```

### IP Address

```bash
sudo /sbin/ifconfig

ip address # new command
```

### IP range, CIDR

```bash
# ip range  ,CIDR network

/16 /20 /24
```

### trace route tool

```bash
sudo apt-get install traceroute

sudo traceroute google.com -I
```

### linux list all the service with status

```bash
systemctl --full --type service --all
```

### To see details about the RAM installed on your VM, run the following command:

```bash
sudo dmidecode -t 17
```

### verify the number of processors, run the following command:

```bash
nproc
```

### CPU information

```bash
lscpu

# Check cpu platform

cat /proc/cpuinfo
```

### Running application in background with screen

```bash
sudo apt-get install -y screen

sudo screen -S mcs java -Xms1G -Xmx7G -d64 -jar /home/minecraft/minecraft_server.1.11.2.jar nogui

detach

# To detach the screen terminal, press Ctrl+A, D. The terminal continues to run in the background. To reattach the terminal, run the following command:

sudo screen -r mcs

# send commands to screen

sudo screen -r -X stuff '/stop\n'
```

### ApacheBench To place a load on the load balancer, run the following command

```bash
ab -n 50000 -c 1000 http://ip
```

### Mount drives

```bash
1. Find what the drive is called
You'll need to know what the drive is called to mount it. To do that fire off one of the following (ranked in order of my preference):
lsblk
sudo blkid
sudo fdisk -l
You're looking for a partition that should look something like: /dev/sdb1. The more disks you have the higher the letter this is likely to be. Anyway, find it and remember what it's called.
2. Create a mount point (optional)
This needs to be mounted into the filesystem somewhere. You can usually use /mnt/ if you're being lazy and nothing else is mounted there but otherwise you'll want to create a new directory:
sudo  mkdir /media/usb
3. Mount!
sudo mount /dev/sdb1 /media/usb
When you're done, just fire off:
sudo umount /media/usb
```

### Change dns server

```bash
vim /etc/resolv.conf
```

### Last boot

```bash
last reboot | less
last -x shutdown
```

### Ports scanner

```bash
#nmap scan ports
nmap -v 203.192.94.53
```

### Check and turn off swap

```bash
# swap

1. Identify configured swap devices and files with cat /proc/swaps.
2. Turn off all swap devices and files with swapoff -a.
3. Remove any matching reference found in /etc/fstab.
4. Optional: Destroy any swap devices or files found in step 1 to prevent their reuse. Due to your concerns about leaking sensitive information, you may wish to consider performing some sort of secure wipe.
```

### Reset timezone

```bash
sudo dpkg-reconfigure tzdata
```

### List all timezones

```bash
ls /usr/share/zoneinfo/

# check ssh failed retries

grep "Failed password" /var/log/auth.log
```

### Linux disk analyse

```bash
###  Ncdu analyse linux disk

ncdu
```

### VIM setup

```bash
# https://github.com/amix/vimrc

git clone --depth=1 https://github.com/amix/vimrc.git ~/.vim_runtime
sh ~/.vim_runtime/install_awesome_vimrc.sh
```

### check ssh failed retries

```bash
grep "Failed password" /var/log/auth.log

# In order to display extra information about the failed SSH logins, issue the command as shown in the below example.

 egrep "Failed|Failure" /var/log/auth.log
```

## Popular Distributions

### Debian

### Ubuntu

### Manjaro

### Fedora

Fedora is the main project, and it’s a community-based, free distro focused on quick releases of new features and functionality.

### CentOS

CentOS is basically the community version of Redhat. So it’s pretty much identical, but it is free and support comes from the community as opposed to Redhat itself.

### RHEL (RedHat Enterprise Linux)

![redhat](https://raw.githubusercontent.com/wahyd4/knowledge-mind-mapping/master/assets/images/redhat.webp)


RHEL is the corporate version based on the progress of that project, and it has slower releases, comes with support, and isn’t free.

### Arch Linux

## Package manager

* [snap](https://snapcraft.io/)
* [Brew](https://brew.sh/)