Tips on Linux

Things on this page are fragmentary and immature notes/thoughts of the author. Please read with your own judgement!

Tricks and Traps

Use full paths of commands when you use a batch account or schedule a cron job. The reason is that batch account and/or cron might have different environment from your personal account. So if a shell script works in your personal account, it might not work in a batch account or in cron.
explainshell.com is a great place for learning shell.
Broken symbolic link .bashrc (and probably other configuration files/scripts) stops Xfce4 (and probably other DEs) from login to the GUI. Fix the broken symbolic links resolves the issue.

Installation

After a new kernel is installed, the old image is usually kept and you can still boot into the old kernel if you like. This make is convenient if the new kernel has some issues.

Remote

The remote desktop software Remmina leaks the super key, which is problematic ... I think the problem is Ubuntu, not the remote apps ...
When you connect to a server, you'd better not use the super key. Because it is active on both the server and the local machines.

File System

You cannot change permission of files on NTFS filesystem ..., they are determined when mounted
Linux hard disk saves properties, user, group of files, a different user must have enough permission to mount it
do not use gui to remove large file/directories, use command line instead

Good Practice

The home directory is where you should work in, you should put all your personal data in your home directory. When you need to reinstall the system or backup your data, you can make a copy of your home directory. However, I go one step further to make things even more convenient. I create a directory named archives in my home directory, put all my persoanl data including configuration files there (except cloud, such as Dropbox, TeamDrive, etc.)

Terminal

nautilus-open-terminal allow you to open a terminal in any directory via short menu.
A good backgroup color setting for terminal Hue: 85, Saturation: 36, Value: 71, Red: 155, Green: 182 and Blue: 117. color name: #9BB675. Cursor color: #B93A3A. The theme goes well with this color setting is "Xterm".
```
^P : move up through history list, one command at a time. (Up arrow key) 
^N : move down (or Down arrow key).
```
Use Shift+PageUp and Shift+PageDown to scroll up and down in virtual console.

Shell

White spaces matters a lot in bash. To assign a string to a variable, you can use v=some_content but not v = some_content. The reason is that space is used to separate bash commands and arguments. Bash will treat v as a command instead of a variable.
To source in a bash file "a-bash-file" so that commands in the file will be executed, you can use source a-bash-file, or you can use a period "." in place of "source", i.e., you can also use . a-bash-file.
return returns a bash function and thus can only be used in a bash function. The argument of return must be a numeric number. exit exits the bash enviroment, i.e., terminate the terminal you are using.
To add a path to the $PATH environment variable, use the following command
```
PATH=$PATH:path_to_add
```
If you want to add the path permanently, you can put the command into the .bashrc file under your home directory.

Path in Bash Script

Full path of current directory
```
$PWD
```
base name of current directory
```
${PWD##*/}
```
or
```
basename $PWD
```

cd

use - to go back to last directory
use $CDPATH
use pushd and popd

File Association

The user-specific file association information is save in the file /.local/share/applications/mimeapps.list, and the global file association information is saved in file /etc/gnome/defaults.list.

Change Hostname

To change hostname permanently, change the hostname to what you want in files [] /etc/hostname and [] /etc/hosts, and then use the following code to make the change in effect without restarting your computer.
```
/etc/init.d/hostname.sh start
```

sudo

To change the timeout of the sudo password, add the following line to the end of the /etc/sudoers file.
```
Defaults:your_username timestamp_timeout=SECONDS
Defaults syslog=auth,passwd_timeout=10
```
j

users and groups
id displays information of a user.

Skills

If you select some text, right click on it but nothings happens (i.e., no short menu poped up), it means that the selected text has already been copied to the mouse buffer. This is one place that Linux is different from Windows. Notice that no all selct and copy process is like this. You still have to copy the selected text into the buffer manually sometimes.
Hard drive related operations: http://www.skullbox.net/newsda.php
check cpu information cat /proc/cpuinfo and memory cat /proc/meminfo
Linux system support true random number generator, which is implemented by collecting the noise generate by hardwares. To use generate true random numbers, you can use the command.

head -1 /dev/random | od -i -N 10 | awk '{ print $2 }'

Note that /dev/random will block until additional environmental noise is gathered if the entropy pool in the machine is empty. If you do not want to be block, you can use /dev/urandom to generate random numbers, which reuses the internal pool to produce more pseudo-random bits when the entropy pool is empty. Though /dev/urandom might contain less entropy than /dev/random does, it is still valide for usual use.

Copy using UI

Many Linux/Unix realted applications use copy on selection. This features makes things confusing sometimes. For example, if you copy things from one place and want to paste it into a terminal which has copy on selection enable. Then you click on the terminal to paste. The thing is that when you click on the terminal, you often select a blank line which means that you have copied a blank line before pasting. This often make your copy/paste fail to work. If copy/paste fail to work in some application, it is possibly because of the copy on selection feature. A workaround is to right click ...

References

http://blog.scoutapp.com/articles/2015/02/24/understanding-linuxs-cpu-stats

https://h-node.org/home/index/en

http://www-03.ibm.com/systems/power/software/aix/linux/toolbox/alpha.html

AIX - The getconf Command for System Information

Ben Chuanlong Du's Blog

It is never too late to learn.