. Explain the following unexpected result:
$ whereis date
date: /bin/date ...
$ echo $PATH
.:/usr/local/bin:/usr/bin:/bin
$ cat > date
echo "This is my own version of date."
$ ./date
Fri May 22 11:45:49 PDT 2009
I will annotate each statement for your information.
1.$ whereis date
date: /bin/date ...
2.$ echo $PATH
.:/usr/local/bin:/usr/bin:/bin
3.$ cat > date
echo "This is my own version of date."
4.$ ./date
Fri May 22 11:45:49 PDT 2009
Annotations for line:
1. "whereis date" tells us where the executable file "date" is located, according to the current $PATH value. The result tells us that the executable file "date" is found in /bin
2. echo $PATH
tells us what is the content of the $PATH environment parameter. Each path is separated by a colon ":".
3. "cat > date"
takes standard input (stdin) from the keyboard and put the keyed in content into a new file in the current directory called date. The input should be terminated by a control-d, which is not mentioned in the question.
The file "date" usually has a permission of 644 or 600 (depending on the computer implementation), which means that it is not executable.
4. ./date
attempts to execute the file ./date, but it is not executable (by default). Again, depending on the implementation of the system, it may return
"permission denied", or
possibly search for the next directory from the $PATH environment variable, which outputs the current date and time.
Ah, the perplexing case of the unexpected date! Well, it seems like you have stumbled upon a classic case of a "rogue" executable hiding in your current directory.
You see, when you type the command "date" in the terminal, the system looks for an executable file with that name in the directories listed in your PATH variable from left to right. In this case, it first finds the legitimate "date" executable in /bin, thanks to the "whereis" command.
However, when you create a file called "date" and run it with "./date," the system looks in the current directory (denoted by the dot (.) in your PATH) before checking the other directories. Since you have a file called "date" in your current directory, it takes precedence over the one in /bin.
Now, when you execute your "date" file, instead of the usual date format, it prints "This is my own version of date." Hilarious, right?
So, the unexpected result simply stems from your mischievous creation of a file named "date," causing the system to use your own version instead of the legit one. That's programming humor, making computers play dress-up and pretend to be something they are not!
In this scenario, the unexpected result is that the command "date" is not executing the system's default "date" command located in "/bin/date", but instead, it is executing a custom version of "date" created by the user.
To explain this unexpected result, let's break it down step-by-step:
1. The command "whereis date" is used to locate the binary executable file for the command "date". In this case, it shows that the system's default "date" command is located in "/bin/date".
2. The command "echo $PATH" is used to display the value of the environment variable "PATH". The "PATH" variable contains directories that the shell searches for executable files. In this case, the value of "PATH" is ".:/usr/local/bin:/usr/bin:/bin", where the dot (.) represents the current working directory.
3. The user creates a file named "date" using the "cat > date" command. This command allows the user to input text into a file. In this case, the user enters the text "echo "This is my own version of date.""". So, the newly created "date" file contains the custom command to echo the message.
4. The user executes the custom "date" command by running "./date". The dot slash notation "./" signifies execution of a file in the current directory. Since the custom "date" file is in the current directory and its execution permissions are granted, it is able to be executed.
5. The output of the custom "date" command is "Fri May 22 11:45:49 PDT 2009", which is the result of the "echo" command within the custom "date" file.
So, the unexpected result is due to the fact that the current directory (represented by the dot ".") is placed before the system directories in the "PATH" variable. This causes the custom "date" command in the current directory to be executed instead of the system's default "date" command.
The unexpected result in this scenario is that when the command "date" is executed, it prints "Fri May 22 11:45:49 PDT 2009" instead of the expected output of "This is my own version of date." This can be explained by the order in which the shell searches for executables when a command is issued.
When you run the command "whereis date", it shows that the "date" command is located at "/bin/date". The output of "echo $PATH" shows the directories that are searched by the shell when you enter a command. In this case, the directories searched are ".:/usr/local/bin:/usr/bin:/bin".
By typing "cat > date", you are creating a file named "date" in the current directory. This file contains the contents: "echo "This is my own version of date."".
When you execute "./date", you are telling the shell to run the "date" command, but the current directory is the first in the search order defined by the PATH variable. Therefore, instead of executing the "/bin/date" command, it executes the "date" file in the current directory.
Since the "date" file contains the command "echo "This is my own version of date"", that is what is printed as the output.