Shell Programming and Scripting

Hello All, I have a question in Unix

Let's say when the command is executed in Unix, where the shell search for that command in Unix. I mean where
that command is defined in Unix.

And how Can we make our own command.

Code:

echo $PATH

Please consult your shell's manual page for further information.

when the command is executed from your console, it looks whether the command is there in your $PATH location.

Code:

 
echo $PATH

to know, where the ls command is there.. execute which ls

Based on the distribution you are using the executable for the command may be stored in some of the common directories like /usr/sbin. You can execute command

Code:

 which <command>

or

Code:

 whereis <command>

to know the source directory of the command executable.

When you execute a command, it is searched across the directories exported in PATH variable. Perform

Code:

echo $PATH

to know the list of directories exported.

If the executable is not found in any of the directories listed, then you will get an error stating command not found.

How Can we make our own command and use?

Create a shell script, and place in one of the directories referred to in your $PATH variable. Or, place in a custom location, and update $PATH appropriately:

Code:

# vi /path/to/shellscript.sh
# chmod +x /path/to/shellscript.sh
# export $PATH=/path/to:$PATH
# which shellscript.sh
/path/to/shellscript.sh
# shellscript.sh
...

Keeping your custom scripts/commands/executables in an appropriate location (normally /usr/local/bin, or ~/bin) will prevent them from becoming mixed with system provided files, and should prevent them being touched during upgrades/etc.

I'd always recommend placing your custom directories at the front of $PATH, so you can override system-provided scripts/binaries if required (with your own wrappers, for example).

Cheers,
ZB

Hello All,
How to run the scripts automatically when the user log into the shell