Adding Any Current Directory './' to the Search Path in Linux

What does / , ./ , ../ represent while giving path?

Root directory, current working directory, and parent directory, respectively.

Difference between ./ and ~/

./ means "starting from the current directory". . refers to the current working directory, so something like ./foo.bar would be looking for a file called foo.bar in the current directory. (As a side note, .. means refers to the parent directory of the current directory. So ../foo.bar would be looking for that file one directory above.)

~/ means "starting from the home directory". This could have different meanings in different scenarios. For example, in a Unix environment ~/foo.bar would be looking for a file called foo.bar in your home directory, something like /home/totzam/foo.bar. In many web applications, ~/foo.bar would be looking for a file called foo.bar in the web application root, something like /var/http/mywebapp/foo.bar.

Add a bash script to path

Try this:

Save the script as apt-proxy (without the .sh extension) in some directory, like ~/bin.
Add ~/bin to your PATH, typing export PATH=$PATH:~/bin
If you need it permanently, add that last line in your ~/.bashrc. If you're using zsh, then add it to ~/.zshrc instead.
Then you can just run apt-proxy with your arguments and it will run anywhere.

Note that if you export the PATH variable in a specific window it won't update in other bash instances.

Path difference between ../ and ./

./ means the current directory

../ means the parent of the current directory, not the root directory

/ is the root directory

myfile.text is in the current directory, as is ./myfile.text

../myfile.text is one level above you and /myfile.text lives in your root directory.

Why do you need ./ (dot-slash) before executable or script name to run it in bash?

Because on Unix, usually, the current directory is not in $PATH.

When you type a command the shell looks up a list of directories, as specified by the PATH variable. The current directory is not in that list.

The reason for not having the current directory on that list is security.

Let's say you're root and go into another user's directory and type sl instead of ls. If the current directory is in PATH, the shell will try to execute the sl program in that directory (since there is no other sl program). That sl program might be malicious.

It works with ./ because POSIX specifies that a command name that contain a / will be used as a filename directly, suppressing a search in $PATH. You could have used full path for the exact same effect, but ./ is shorter and easier to write.

EDIT

That sl part was just an example. The directories in PATH are searched sequentially and when a match is made that program is executed. So, depending on how PATH looks, typing a normal command may or may not be enough to run the program in the current directory.

extending default lib search path in ubuntu

create (as root) a new file in /etc/ld.so.conf.d/ containing, the new path. For example:

sudo echo "/path-to-your-libs/" >> /etc/ld.so.conf.d/your.conf

after that run

sudo ldconfig

No need to change libc.conf.

find: paths must precede expression: How do I specify a recursive search that also finds files in the current directory?

Try putting it in quotes -- you're running into the shell's wildcard expansion, so what you're acually passing to find will look like:

find . -name bobtest.c cattest.c snowtest.c

...causing the syntax error. So try this instead:

find . -name '*test.c'

Note the single quotes around your file expression -- these will stop the shell (bash) expanding your wildcards.

windows equivalent of ./ (current directory)

A period denotes the current directory in Windows.

For your example you would use the following:

c:\> cd c:\windows
c:\Windows> .\System32\ipconfig.exe

Alternately, you could forego the .\ and do it like this:

c:\Windows> System32\ipconfig.exe

What does ./ (dot slash) refer to in terms of an HTML file path location?

./ is the the folder that the working file is in:

So in /index.htm ./ is the root directory

but in /css/style.css ./ is the css folder.

This is important to remember because if you move CSS from /index.htm to /css/style.css the path will change.