Command-line interface

Command-line interface#

command-line interface

a means of interacting with software via commands. Also called command-line shell.

main arguments#

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
  for (size_t i = 0; i < argc; ++i)
    puts(argv[i]);
}

$ code-wi/main-arguments.exe geko --1 🦎
code-wi/main-arguments.exe
geko
--1
🦎
🤔 Question to ponder

Three arguments are provided to main-arguments.exe. But why do we see 4 lines that are printed?

Activity 33 (Basic CLI)

Create a program that accepts any number of words as arguments and prints them back as a single, space-separated string in reverse order (last argument printed first), but only if the verb is reverse. Otherwise it prints usage:

$ ./main reverse a b c
c b a
$ ./main a b c
Usage: ./main reverse STRING1 STRING2 ...

Exit status#

If main returns, then the program exits. The program can communicate a status to the command line interface by returning a corresponding integer called exit status.

exit status

an integer returned by a terminated program that is made available to its caller, e.g., the command line interface.

Every function with a return type other than void must return a value. However, main is special. If no return value is not specified, it returns automatically return 0:

int main() {}

exit code

meaning

EXIT_SUCCESS or 0

successful termination

EXIT_FAILURE or 1

unsuccessful termination

any int other than {0, 1}

unsuccessful termination with a special meaning

CLI design#

Here are good examples for CLI programs.

VS Code considerations#

You cannot use F5 directly, because our program’s runtime is now dependent on arguments. There are two solutions:

  1. Only build using CtrlShiftb. Then run manually on the command line:

    ./main e caesar LEVE

  2. You change launch.json to cater for command-line arguments, e.g.,

    {
"version": "0.2.0",
"configurations": [
  {
    // ...
    "program": "${workspaceFolder}/main",
    "args": [
      "${input:verb}",
      "${input:cipher}",
      "${input:plaintext}"
    ],
    // ...
  }
],
"inputs": [
  {
    "id": "verb",
    "type": "promptString",
    "description": "Enter verb"
  },
  // ...
 ]
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