[C language] [Linux] Try to create a simple Linux command * Just add! !!

Introduction

I didn't use C language at all, so I wanted to use it for the first time in a while. So, as a review of Linux, I will try to make my own command that just adds.

• Not practical at all. ..

The command specification is to output the total value of the numerical values after the second argument on the command line.

Purpose

・ By making a simple one, you can imagine what the command looks like. -Understand the concept of PATH that Linux refers to when executing commands. For the above reasons, detailed explanation of implementation is omitted.

environment

・ Amazon Linux AMI release 2018.03

• Cloud9.

procedure

1. Create a test directory

After that, work will be done under this directory.

Directory creation command

$ mkdir test; cd test

2. Create a source code file

I'm using vi here, but any text editor can be used.

File creation command

$ vi test_sum.c

3. Implementation

Copy the following source code.

test_sum.c

#include <stdio.h>

int main(int argc, char *argv[])
{
  int sum = 0;
  int count = 0;

  if (argc < 3) {
    printf("ERROR: invalid of arguments number\n");
    return 1;
  }

  /* argv[0]Is the command itself, so it is excluded from the calculation, so count=Start from 1*/
  for (count=1; count<argc; count++) {
    sum += atoi(argv[count]);
  }
  printf("%d\n", sum);

  return 0;
}

Atoi converts from the most significant digit and stops the conversion when it detects a character other than a number. Example) "a": Returns 0 because there is nothing that can be converted. "1a3": The most significant 1 is converted, but conversion is stopped because a is detected, and 1 is returned.

4. Compile

Compile the source code (test_sum.c) using gcc

Compile command

$ gcc -o test_sum test_sum.c

The operation is simple, but this completes the command test_sum!

5. Simple operation check

Try using your own test_sum. Below, try some patterns.

test_Calculation using sum

$ ./test_sum 123
ERROR: invalid of arguments number
$ ./test_sum 123 23456
23579
$ ./test_sum 123 23456 812
24391

Great success! !!

• The first error is the error that is output when there are few arguments (less than 3).

6. PATH setting

If this is left as it is, it will be necessary to specify the path ("./") every time, so set it so that it operates without specifying the path like other commands.

• You need to specify even if you are in the same directory.

6.1. What is PATH?

An environment variable that summarizes the paths that Linux goes to look for files when it executes an executable file (ELF). Linux searches the paths set in the PATH in order and executes the first executable file found.

• Ls and find can be used without specification because the location is saved in PATH.

6.2. PATH setting

Use export to set environment variables such as PATH. Register the path of the current directory (current directory) in the PATH with the following command.

Set the path of the current directory to PATH

$ export PATH=$PATH:$(pwd)

Now you don't have to specify a path in any directory! Note that this setting disappears when you drop the shell! !! </ strong>

$ test_sum 1 2 3
6
$ cd ~
$ test_sum 1 2 3
6
$ cd /tmp
$ test_sum 1 2 3
6

$ which ls
alias ls='ls --color=auto'
        /bin/ls
$ which find
/bin/find
$ which test_sum
~/environment/test/test_sum