# Linux

## Useful commands and tips

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

### 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
```

## 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/)