system() in C/C++

The system() function is used to invoke an operating system command from a C/C++ program. For example, we can call system("dir") on Windows and system("ls") in a Unix-like environment to list the contents of a directory.

It is a standard library function defined in <stdlib.h> header in C and <cstdlib> in C++.

Syntax

The syntax of system() function is:

int system(const char *command);

Parameters

• command: A pointer to a null-terminated string that contains the command we want to execute.

Return Value

• It returns 0 if the command is successfully executed.
• It returns a non-zero value if command execution is not completed.

Example: Program to Illustrate the system() Function

In this program, we will use the echo command to print the "Hello World" string. To learn about its usage and best practices, the C++ Course offers detailed explanations and practical examples.

Hello, World!
Command executed successfully.

Writing a C/C++ program that compiles and runs other programs?

We can invoke gcc from our program using system(). See below the code written for Linux. We can easily change code to run on Windows.

To convert the above code for Windows we need to make some changes. The executable file extension is .exe on Windows. So, when we run the compiled program, we use a.exe instead of ./a.out.

system() Function vs Using Library Functions

Some common uses of system() in Windows OS are:

• system(“pause”): This command is used to execute the pause command and make the screen/terminal wait for a key press.
• system("cls"): This command is used to make the screen/terminal clear.

However, making a call to system command should be avoided due to the following reasons:

• It's a very expensive and resource-heavy function call.
• Using system() makes the program non-portable to some extent which means this works only on systems that have the pause command at the system level, like DOS or Windows. But not Linux, MAC OSX, and most others.

Let us take a simple C++ code to output Hello World using the system(“pause”): 

The output of the above program in Windows OS:

Hello World!
Press any key to continue…

This program is OS-dependent and uses the following heavy steps:

• It prints "Hello World!" on the screen.
• It displays a message "Press any key to continue…".
• The system() function opens the shell of the Operating System that will first scan the string passed inside the system() function and then execute the command.
• The "pause" command waits for the user input and the shell window remains open until the user presses any key.
• When the user presses any key, the "pause command" receives an input, and the shell window is closed.

Instead of using the system(“pause”), we can also use the functions that are defined natively in C. Let us take a simple example to output Hello World with cin.get():

Hello World!

Thus, we see that both system(“pause”) and cin.get() are actually performing a wait for a key to be pressed, but, cin.get() is not OS dependent and neither does it follow the above-mentioned steps to pause the program.

What is the common way to check if we can run commands using system() in an OS?

The common way to check if we can run commands using system() in an OS is to check if a command processor (shell) exists in the operating system.

Using the following way, we can check if a command processor exists in an OS:

If we pass a null pointer in place of a string for the command parameter,

• The system returns a nonzero value if a command processor exists (or the system can run).
• Otherwise returns 0.

Command processor exists

Note: The above programs may not work on online compiler as System command is disabled in most of the online compilers including GeeksforGeeks IDE.