# Shell

### SSH

* `ssh` to see if ssh is installed (openssh-client is installed by default)
* `ssh-keygen` or `ssh-keygen -t rsa` generate ssh key by RSA algorithm
* `cat ~/.ssh/id_rsa.pub` print public key (for settings on Github)
* `ssh -T git@github.com` testing
* `sudo apt-get install openssh-server` install server for incoming ssh connections
* `ssh-copy-id username@hostname` copy your ssh key to remote computer to avoid verifying password on every connection
* `ssh username@ip` ssh login to a remote computer with this ip and username
* `ssh username@hostname` if mapped this hostname to its ip in /etc/hosts file
* `ssh -X username@hostname` login with GUI support
* `ssh -p 1234 username@hostname` login via port 1234
* `sudo gedit /etc/hosts` modify hosts file
* `sudo gedit /etc/ssh/sshd_config` modify ssh server configuration

### Devices and Filesystem

* `sudo gparted`
* `ls -l /dev/ttyS0`
* `groups <username>`
* `sudo gpasswd -a <username> <groupname>`
* `sudo gpasswd -d <username> <groupname>`
* `sudo groupadd <newgroupname>`
* `sudo usermod -a -G <groupname> <username>`
* `lscpu` , `lsmem` , `lshw` list CPU, memory, hardware info
* `lsusb` list USB devices
* `lsblk` list block devices, often used to check external ssd
* `lspci` list PCI devices
* `lspci | grep VGA` check GPUs
* `sudo lshw -C video` check GPU display details
* `du -h -d 1` check file size/usage in current directory
* `df -h` check hard disk usage

### Environment Variables

* `which pip` to see which `pip` command you are running - useful for checking environment!
* uncomment `#force_color_prompt=yes` in `.bashrc` for a colored prompt
* `sudo !!` where `!!` is for the last command
* `python3 -m site` show searching path for python3
* `sudo python -m pip install --force-reinstall pip==8.1.1` if you [inadvertently upgraded your system pip](https://stackoverflow.com/questions/49836676/error-after-upgrading-pip-cannot-import-name-main). More info can be found at [this link](https://stackoverflow.com/questions/50776299/how-to-downgrade-pip-version-10-0-0-to-pip-version-9-0-1), and [this link](https://github.com/pypa/pip/issues/5221#issuecomment-382069604).
* `sudo ldconfig` after `sudo make install` to refresh the cache

### Date and Time

* `date`
* `sudo systemctl stop systemd-timesyncd`
* `sudo systemctl start systemd-timesyncd`
* `timedatectl`
* `sudo timedatectl set-ntp false`
* `sudo timedatectl set-time 'YYYY-MM-DD'`
* `sudo timedatectl set-time 'YYYY-MM-DD HH:MM:SS'`

### Network

* `sudo systemctl restart network-manager`
* `nmap -sP 192.168.0.1/24` scan ports
* `nmap -A 192.168.10.112` detect OS and services

### Compression

* `tar -cf filename.tar file1 folder2` packed without compression
* `tar -xf filename.tar`  create an archive by `-c`, extract an archive by `-x`
* `tar -cvzf filename.tar.gz file1 folder2` compress by `-z`; verbose by `-v`
* `tar -xvzf filename.tar.gz`  decompress
* `tar -xvzf filename.tar.gz -C {path}` decompress to a designated path
* `sudo apt-get install zip unzip`
* `zip -r archive_name.zip myfolder`
* `unzip archive_name.zip`

### running in the background

* `Ctrl + Z` to stop the process, followed by `bg` to run in the background or `fg` to run in the foreground.
* Use command `./test.sh 1>stdout.log 2>stderr.log &` and it will return a process ID.&#x20;
* Processes running in the background can be monitored by `top` command.&#x20;

### apt commands to find dependencies ([link](https://askubuntu.com/questions/13296/how-do-i-find-the-reverse-dependency-of-a-package))

* `apt-cache showpkg <pkgname>` show package info, both depends and reverse depends
* `apt-cache depends ros-melodic-desktop` show one-level dependencies
* `apt-cache rdepends --installed ros-melodic-desktop`&#x20;
* `sudo apt install apt-rdepends`
* `apt-rdepends ros-melodic-desktop | pager` recursively show all dependencies&#x20;
* `apt-rdepends --reverse protobuf-compiler | grep ros-melodic-desktop` &#x20;

### apt commands to locate packages

* `dpkg -L <package-name>`
* `apt list <package-name>`
* `apt-cache madison` &#x20;
* `locate <filename>` find a file in database (globally)
* `find <filename>` find a file in local directory

### vim commands

* `vi -b <filename>` open file in binary mode
* `:%!xxd` switch to hex mode
* `:%!xxd -r` switch back to binary mode
* `:wq` save and exit

### terminal

* `sudo update-alternatives --config x-terminal-emulator`

### References

* [The Unix Shell](http://swcarpentry.github.io/shell-novice/)
* [How do I find the reverse dependency of a package?](https://askubuntu.com/questions/13296/how-do-i-find-the-reverse-dependency-of-a-package)