Misc Linux usage

Network

  • Assign the current network interface to an IP address. Use ifconfig netmask . e.g.,

    $ ifconfig eth0 192.168.2.1 netmask 255.255.255.0

  • Use nmap to report whether your device uses a Linux TCP/IP stack:

    $ nmap -P0 -O

  • Show IP ports

    $ lsof -i -P -n

PATH environment

$ echo $PATH

In Unix systems, we sometimes want to execute a program in a terminal without specifying the its path. In this case, it’s easy to add the path of the program being a variable to the PATH environment. To do this, we could follow the command below:

$ export PATH=$PATH:[path to the program]

Notice when the terminal is terminated, the above command has no effect after a new terminal is opened. To ensure the PATH environment is modified permanently, we could add the above command in ~/.bashrc. Editor like vi/vim/nano could be used.

GNU screen

To connect to a serial on a console, we could use screen. The following is an example:

$ screen /dev/ttyUSB0 115200 8N1

On MacOS, you could kill the console (while you’re in the screen session) by command + A + , and typing [y]. Another way to kill a screen session is to first know what sessions are available:

$ screen -ls

Then, kill the specified session Id using the following command:

$ screen -X -S [Session ID] quit

Hexo

Refresh the build after modifications.

$ hexo clean
$ hexo generate

Publish to my github account.

$ hexo deploy

Create a page used for dropdown view:

$ hexo new page "name"

The created path will be: [hexo_directory]/source/[name]/index.md To add the page in the dropdown list, we need to modify the _config.yml file under [hexo_directory]/theme/next/ directory, e.g.,

IoT Ecosystems:
   SDK review: /sdk-review/
   Data model analysis: /data-model/

Git

  • How to save your ass from a local merge yet being pushed.

    The following command would go back to one commit.

    $ git reset --hard HEAD~1

  • How to add files that were copied from somewhere else (not in the repo). You need the following command to unstage the files:

    $ git reset HEAD path/to/file

    Then do git add and git commit.

  • How to revert:

    Git revert to a commit will eventually roll out a new commit. If git reset is used, there will be no commit.

    # Revert the latest commit.
$ git revert HEAD
# Revert a specific commit.
$ git revert commit [commit No.]

  • How to un-track a folder but keep the folder on your disk. Link: https://stackoverflow.com/questions/24290358/remove-a-folder-from-git-tracking

This also works for the situation where an unwanted submodule causing fatal error when ‘git add’. The basic idea is to remove the folder from git’s cache.

  1. Add the folder to the .gitignore

  2. Remove the folder from your local git tracking, but keep it on your disk. bash git rm -r --cached path_to_your_folder/

  3. Push your changes to your git repo.

The folder will be considered “deleted” from Git’s point of view (i.e. they are in past history, but not in the latest commit, and people pulling from this repo will get the files removed from their trees), but stay on your working directory because you’ve used –cached.

  • How to delete a local branch and a remote branch.

# Delete local branch
git branch -d {branch name}
# Delete remote branch
git push origin --delete {branch name}

  • How to add ssh keys to your stash or github account.

Instead of using Http connections, we could use ssh keys to push/pull a git repo. This prevents user from entering username and keys each time using Http connections. For a branch new machine, we need to firstly create ssh private and public keys (also known as key pairs).

$ ssh-keygen -t rsa -C "your_email@example.com"

Notice the email account is optional. You could pass key saving path and passphrase settings. After done, you should be able to see id_rsa.pub file under ~/.ssh/ directory. Copy the content of id_rsa.pub to your git or stash account (usually at security, adding keys tab).

Reference:

MacOS dylib

There could be an error when using dylib (dynamic library) on MacOS. The error could be:

dyld: Library not loaded: @rpath/libcmocka.0.dylib
  Referenced from: /Users/tozhao/Documents/Stash_Projects/customer-projects/xiaomi/6.3.0/tests-mac/./test
  Reason: image not found
[1]    8541 abort      ./test

The problem is this dylib does not specify the execution path. When checking the dylib with otool, we could find:

libcmocka.0.4.1.dylib:
	@rpath/libcmocka.0.dylib (compatibility version 0.0.0, current version 0.4.1)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)

To solve the problem, we could use install_name_tool like the following example:

$ install_name_tool -change @rpath/libcmocka.0.dylib [path/to/the/exefile] [exefile]

The exefile is the binary that is linked with the dylib.

References:

SSH

  • Remove ssh known hosts on a Linux machine.

$ ssh-keygen -R 192.168.42.1 -f "~/.ssh/known_hosts"

  • Remove ssh known hosts on a MacOS.

$ ssh-keygen -R 192.168.42.1

Markdown

  • Embedded an image in a markdown script. Ensure you know the url path of that image. The url path could be a github path. Usually, the url from the github would not be rendered, and you will see a broken link in the position. To solve this, add ?raw=true at the end of the url and place a line by following the syntax as bellow:

$ ![link text](url + `?raw=true`)

Run exe in back-end

Z3GatewayHost application forks a child process to handle communications with serial port. Use & at the end would cause assertion in the child process. A reliable way to run the application in the back-end could refer to the following init script which was developed for running on OpenWrt.

start() {
  touch /tmp/silabspipeopen
  mkfifo /tmp/silabs
  while [ -f /tmp/silabspipeopen ]; do sleep 1; done > /tmp/silabs &
  /opt/siliconlabs/zigbeegateway/siliconlabsgateway -n 1 -p ttyS1 < /tmp/silabs &
}

stop() {
  rm /tmp/silabspipeopen
  rm /tmp/silabs
  killall siliconlabsgateway
}

Bash syntax

  • Check if there is a specified directory.

    if [ -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY exists.
fi

    Or if there is NOT a specified directory.

    if [ ! -d "$DIRECTORY" ]; then
  # Control will enter here if $DIRECTORY exists.
fi

  • Check if the system is a MacOS or a Linux:

    SYSTEM=$(uname)

# Check if it's a MacOS
if [[ $SYSTEM == 'Darwin' ]]; then
  # Some actions.
fi

# Check if it's a Linux
if [[ $SYSTEM == 'Linux' ]]; then
  # Some actions.
fi

  • Conditionally exit a bash script.

    # exit n.

if [ $SOMECONDITION ]; then
  exit 0
fi

echo "This line will never be reached!"

  • Check if a long string contains a subString.

Reference: https://stackoverflow.com/questions/229551/how-to-check-if-a-string-contains-a-substring-in-bash

string='My long string'
if [[ $string == *"My long"* ]]; then
  echo "It's there!"
fi