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519 lines
9.2 KiB
Markdown
519 lines
9.2 KiB
Markdown
---
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title: Linux
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created: 2021-11-09
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updated: 2021-11-09
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type: summary
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tags: [tech, reference]
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external: https://github.com/wahyd4/knowledge/blob/master/categories/devops/linux.md
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---
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# Linux
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## Useful commands and tips
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### Split strings
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```bash
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# split ip list string by comma and get first one
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export IP=127.0.0.2,127.0.0.3
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echo $IP | cut -d ',' -f1
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```
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### Create a systemd service
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Create a file called `/etc/systemd/system/hello.service`
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```
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[Unit]
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Description=Hello Service
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After=network.target
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StartLimitIntervalSec=0
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[Service]
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Type=simple
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Restart=always
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RestartSec=1
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User=centos
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ExecStart=/app/hello.sh
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[Install]
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WantedBy=multi-user.target
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```
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Automatically start the service when boot the vm
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```bash
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sudo systemctl enable hello
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```
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Start service
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```bash
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sudo systemctl start hello
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```
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Learned from [here](https://medium.com/@benmorel/creating-a-linux-service-with-systemd-611b5c8b91d6)
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### ssh
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Do not use default `22` port
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```bash
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ssh user@ip -p 22222
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```
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Login without password
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```bash
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# adds private key identities to the authentication agent
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ssh-add ~/.ssh/id_rsa
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# use locally available keys to authorise logins on a remote machine
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ssh-copy-id user@ip -p 22222
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```
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### Add user to the sudo group
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```bash
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usermod -aG sudo youruser
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```
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### IP table
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### Cron job
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List cron jobs for current user
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```bash
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crontab -l
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```
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List cron jobs for some user
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```bash
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crontab -u user -l
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```
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Edit cron jobs
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```bash
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crontab -e
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```
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#### An example cron job
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For example, you can run a backup of all your user accounts
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```bash
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# at 5 a.m every week with:
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# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
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```
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#### Check cron jobs logs
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```bash
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grep CRON /var/log/syslog
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```
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#### Cron job expression
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`MIN HOUR DOM MON DOW`
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Field Description Allowed Value
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MIN Minute field 0 to 59
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HOUR Hour field 0 to 23
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DOM Day of Month 1-31
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MON Month field 1-12
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DOW Day Of Week 0-6
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### PS
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```bash
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ps aux | grep 'ruby'
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```
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List process, using `grep` to filter processes
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### tail
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Tailing texts from file in realtime
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```bash
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tail -f a.log
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```
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`f` means following, auto append texts to console
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### nohup
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Simply running application in background
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### df
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```bash
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df -h
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```
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Show disk usage and left, `h` means show in human readable way. Normally by MB/GB.
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### free
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```bash
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total used free shared buff/cache available
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Mem: 3960624 416720 270024 22252 3273880 3264060
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Swap: 4104188 109688 3994500
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```
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Show memory usage and
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### Watch
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```
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watch kubectl get pods
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```
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Continuous running the commands, watching the changes. By default it refresh the result every `2s`.
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### Change timezone
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```bash
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TZ=Asia/Shanghai;
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ln -snf /usr/share/zoneinfo/$TZ /etc/localtime
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```
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### Check release
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```bash
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cat /etc/*-release
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```
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### Debian check system version
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```bash
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cat /etc/issue
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cat /etc/debian_version
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```
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### htop
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like top, but more
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#### mosh
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auto reconnect ssh
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### show last something time
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```bash
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last shutdown
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last reboot
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```
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### Count files in current folder
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```bash
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find . -type f | wc -l
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```
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### Check if a port is open
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Used Netcat
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```bash
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if ! nc -z localhost 5672; then
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sleep 3;
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fi
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```
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### scp copy files between host
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#### Copy files from remote server
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```bash
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scp your_username@remotehost.edu:foobar.txt /local/dir
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```
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### Copy local files to remote server
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```bash
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scp /path/to/local/file user@server:/path/to/remote/file
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```
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### Merge video files into one
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```bash
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ffmpeg -f concat -safe 0 -i ./files.txt -c copy output.mp4
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```
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In the `files.txt` you should list all the video clips like the following
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```txt
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file 1.flv
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file 2.flv
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file 3.flv
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```
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### Curl
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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.
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The following are some useful commands:
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#### Post request with file
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```bash
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curl --data "@$(pwd)/fixure/sample.json" --user "foo:bar" localhost:8080/api/abc
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```
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### Mount
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#### Mount NFS(Network file system) to local
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```bash
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mkdir /to-local-folder
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# mount
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mount -t nfs -O user=abc,pass=pass 192.168.1.2:/nfs-folder /to-local-folder
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# remount
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mount -o remount /to-local-folder
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```
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#### Mount folder permanently
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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
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```
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192.168.1.2:/nfs-folder /to-local-folder nfs defaults 0 0
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```
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You can specify more options
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```
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192.168.0.216:/nfs-folder /to-local-folder nfs defaults,proto=tcp,port=2049 0 0
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```
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### Rsync
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Sync folders and files between folders and hosts
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### sync between directories
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```bash
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rsync -avzh /root/rpmpkgs /tmp/backups/
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```
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#### sync folders between hosts
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```bash
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rsync -avz --exclude downloads --exclude 'some-folder' /mnt rsync://user@192.168.1.2/rsync/ -v -c
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```
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You can export `RSYNC_PASSWORD` to pass password to the command.
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#### Check rsync logs
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```bash
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grep -ir rsync /var/log
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```
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### Check debian version
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```bash
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cat /etc/issue
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cat /etc/debian_version
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```
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### Debian error bash: netstat: command not found
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```bash
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apt-get install net-tools
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```
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### Create CPU loads
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```bash
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dd if=/dev/urandom | gzip -9 >> /dev/null &
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```
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### Kill the load process
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```bash
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kill %1
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```
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### IP Address
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```bash
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sudo /sbin/ifconfig
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ip address # new command
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```
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### IP range, CIDR
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```bash
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# ip range ,CIDR network
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/16 /20 /24
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```
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### trace route tool
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```bash
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sudo apt-get install traceroute
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sudo traceroute google.com -I
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```
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### linux list all the service with status
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```bash
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systemctl --full --type service --all
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```
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### To see details about the RAM installed on your VM, run the following command:
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```bash
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sudo dmidecode -t 17
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```
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### verify the number of processors, run the following command:
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```bash
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nproc
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```
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### CPU information
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```bash
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lscpu
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# Check cpu platform
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cat /proc/cpuinfo
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```
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### Running application in background with screen
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```bash
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sudo apt-get install -y screen
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sudo screen -S mcs java -Xms1G -Xmx7G -d64 -jar /home/minecraft/minecraft_server.1.11.2.jar nogui
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detach
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# 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:
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sudo screen -r mcs
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# send commands to screen
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sudo screen -r -X stuff '/stop\n'
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```
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### ApacheBench To place a load on the load balancer, run the following command
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```bash
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ab -n 50000 -c 1000 http://ip
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```
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### Mount drives
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```bash
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1. Find what the drive is called
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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):
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lsblk
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sudo blkid
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sudo fdisk -l
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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.
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2. Create a mount point (optional)
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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:
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sudo mkdir /media/usb
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3. Mount!
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sudo mount /dev/sdb1 /media/usb
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When you're done, just fire off:
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sudo umount /media/usb
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```
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### Change dns server
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```bash
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vim /etc/resolv.conf
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```
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### Last boot
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```bash
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last reboot | less
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last -x shutdown
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```
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### Ports scanner
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```bash
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#nmap scan ports
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nmap -v 203.192.94.53
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```
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### Check and turn off swap
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```bash
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# swap
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1. Identify configured swap devices and files with cat /proc/swaps.
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2. Turn off all swap devices and files with swapoff -a.
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3. Remove any matching reference found in /etc/fstab.
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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.
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```
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### Reset timezone
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```bash
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sudo dpkg-reconfigure tzdata
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```
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### List all timezones
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```bash
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ls /usr/share/zoneinfo/
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# check ssh failed retries
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grep "Failed password" /var/log/auth.log
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```
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### Linux disk analyse
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```bash
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### Ncdu analyse linux disk
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ncdu
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```
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### VIM setup
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```bash
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# https://github.com/amix/vimrc
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git clone --depth=1 https://github.com/amix/vimrc.git ~/.vim_runtime
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sh ~/.vim_runtime/install_awesome_vimrc.sh
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```
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### check ssh failed retries
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```bash
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grep "Failed password" /var/log/auth.log
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# In order to display extra information about the failed SSH logins, issue the command as shown in the below example.
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egrep "Failed|Failure" /var/log/auth.log
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```
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## Popular Distributions
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### Debian
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### Ubuntu
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### Manjaro
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### Fedora
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Fedora is the main project, and it’s a community-based, free distro focused on quick releases of new features and functionality.
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### CentOS
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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.
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### RHEL (RedHat Enterprise Linux)
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RHEL is the corporate version based on the progress of that project, and it has slower releases, comes with support, and isn’t free.
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### Arch Linux
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## Package manager
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* [snap](https://snapcraft.io/)
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* [Brew](https://brew.sh/)
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