How do I detect CLI Special Characters in C?

Question

Note: This is not homework. I am still a beginner.

To clarify, and expand, upon my question:

In the command-line interface, we know that we can use special characters for special meanings such as the expansion character

ls -l *.c

expands the command to grab and list all files with the ".c" extension.

Other characters such as >, <, &, *, and combinations can be used as well.

Lets say I'm making a utility program in C and it's called findch , short for find a character, and I can call it in the CLI.

All findch does is take the first character of the first argument and attempts to find it in the following argument, whether it's a string or a file . If it finds the character, it reports the number of occurrences. If it fails, it reports that as well.

Usage 1: findch [character] [filename.ext]...

Usage 2: findch a "How many times does the letter a appear in this string?"

Usage 3: findch a filename.txt

the problem shows up when I do things like

findch * "hello, there!" findch ^ hello findch & hello

When I pass just a special character, the CLI interprets it that way and messes with my program. Is there a simple preventative measure I can implement? Or is this a bit more complicated and System Specific?

Here's the code so far.

findproto.h

/*
//C Primer Plus
//Chp 13 Programming Exercises
//Problem 13-08

*************************************************************
Written by: JargonJunkie
*************************************************************

************************************
    set the preprocessor directives
************************************/

/*
    SET THE SWITCHES
    ----------------
    To avoid conflicts while editing, and recompiling, I used a 
    "switch mechanism" to turn the function prototypes ON and OFF.
*/
#ifndef ON
#   define ON 1
#endif

#ifndef OFF
#   define OFF 0
#endif

/*
    maximum string length
    ---------------------
    11 //tiny
    21 //small
    41 //medium
    81 //large
    101 //extra
    ---------------------
    all sizes are offset by one
    to include the null character
*/
#ifndef STRLEN
#   define STRLEN 41
#endif

/*
    Set the ERROR macro
*/
#ifndef ERROR
#   define ERROR -1
#endif

/*
    Since I use MinGW, there is no standard boolean header.
    I have to define the bool type and create the true
    and false constants. If the bool type exists, then
    the bool type is not created. While my GCC is C99 compliant,
    I prefer to use bool over _Bool which, IMO, looks awkward.
*/
/*
    Set the boolean values for the
    variables true and false...
*/
#ifndef BOOL
#   define BOOL ON
#   if BOOL
#       define true 1
#       define false 0
        typedef _Bool bool;
#   endif
#endif

/*
    Define the file options.
    ------------------------
    These settings are used to decide whether the given 
    file extension type is a "valid" one.
    -----------------------------------
    More extensions can be added by simply changing the EXTLEN macro 
    value. The strings can be found in the fcfunct.c file.
    Keep in mind that the order of the values and string elements 
    must be the same.
*/
#ifndef FILE_OPTIONS

#   define EXTLEN 9

    enum file { 
        txt, asc, c, h, csv, html, log, xhtml, xml, 
        error = -1, missing = -2
    };

    const char * extension[EXTLEN];

#endif

/*
    Prototypes make a good reference and allows the "black-box" concept
    to stay in play. Once a function has been tested, and works, it 
    can be added to the prototype list.
*/
#ifndef PROTOTYPES
#   define PROTOTYPES ON
#   if PROTOTYPES == ON
        void string_to_lower(char * string);
        void display_help(const char * string);
        enum file known_file_extension(const char * filename);
        enum file string_or_file(const char * string);
        bool first_arg_is_char(char * string);
        unsigned int count_char_in_string(char * arg, char * string);
        unsigned int count_char_in_file(char * arg, char * filename);
#   endif
#endif

findfunct.c

//C Primer Plus
//Chp 13 Programming Exercises
//Problem 13-08

#include <stdio.h> 
#include <string.h>
#include <ctype.h>
#include "findproto.h"

/*
    Initialize EXTENSION Based Keywords
    ----------------------------------------------
    These keywords are used to compare against input supplied by the
    user. If the keyword is found to be valid, the function returns
    a value accordingly; else the function returns some type of error 
    based value.
*/
const char * extension[EXTLEN] = {
    ".txt", ".asc", ".c", ".h",
    ".csv", ".html", ".log",
    ".xhtml", ".xml"
};

/*
    The string_to_lower() function
    ---------------------------------------
    Transforms the string to lower case
*/
void string_to_lower(char * string)
{
    for (int index = 0; string[index]; index++)
    {
        if (isalpha(string[index]))
        {
            string[index] = tolower(string[index]);
        }
    }
}

/*
    The display_help() Function
    ---------------------------------------
    Prints the help menu to the standard display
*/
void display_help(const char * string)
{
    putchar('\n');
    printf("Usage: %s [character] [filename.ext]...\n\n", string);
    printf("This program takes a character and zero or more filenames\n"
        "as command-line arguments.\n\n"
        "If no arguements follow the character, the program reads from\n"
        "the standarad input instead. Otherwise, it opens each file in\n"
        "turn and reports how many times the character appeared in each\n"
        "file.\n\n" 
        "The filename, and the character itself, should be reported along\n" 
        "with the count.\n\n" 
        "If a file can't be opened, the program reports that fact and continues\n"
        "on to the next file.\n\n");
    printf("ALLOWED FILE EXTENSION TYPES\n"
           ".txt    .asc    .c    .h    .html\n"
           ".log    .xhtml  .dat  .xml  .csv\n");
}

/*
    The known_file_extension() Function
    ---------------------------------------
    IF a known file extension is found, return true on success. 
    ELSE return false on failure...
*/
enum file known_file_extension(const char * filename)
{
    enum file value = error;

    //find the last occurring period
    char * file_extension_type = strrchr(filename, '.');    

    if (NULL == file_extension_type)
    {
        value = missing;
        return value;
    }

    string_to_lower(file_extension_type);

    //find the file type
    for (enum file type = txt; type <= xml; type++)
    {
        if (0 == strcmp(file_extension_type, extension[type]))
        {
            value = type;
            break;
        }
    }

    return value;
}

/*
    Find the first argument and make sure the string is
    only one character long. 

    If the the string is not one character long, grab 
    only the first character and replace the remaining 
    characters with the null character.

    If the function succeeds, it returns true.
    If the function fails, it returns false.
*/
bool first_arg_is_char(char * string)
{
    int length;
    bool status = false;

    length = strlen(string);

    putchar('\n');

    if (isalpha(string[0]))
    {
        if (length == 1)
        {
            puts("Found character literal...\n");
            status = true;
        }

        if (length >= 2)
        {
            puts("Stripping string to first character...\n");

            for (int i = 1; i <= length; i++)
            {
                string[i] = '\0';
            }

            status = true;

            puts("Successfully modified the string...\n");
        }

        if (status)
        {
            printf("Character literal: '%s'\n\n", string);
        }
    }

    return status;
}

/*
    Determine whether the given string is just a
    string or a filename.
*/
enum file string_or_file(const char * string)
{
    enum file type = known_file_extension(string);

    switch (type)
    {
        case missing: 
            puts("The file extension seems to be missing.");
            puts("Assuming string is string literal.\n");
            return missing;
        case error:
            puts("Oops! An invalid file extension was used.");
            return error;
        default:
            puts("Found valid file extension...");
            return type;
    }
}

/*
    Count the number of occurrences that the given character
    arugment is found in the given string.
*/
unsigned int count_char_in_string(char * arg, char * string)
{
    unsigned int count = 0, index;

    for (index = 0, count = 0; string[index] != '\0'; index++)
    {
        if (arg[0] == string[index])
        {
            ++count;
        }
    }

    return count;
}

/*
    Count the number of occurrences that the given character
    arugment is found in the given file.
*/
unsigned int count_char_in_file(char * arg, char * filename)
{
    FILE * source;
    char ch;
    unsigned int count, index;

    if (NULL == (source = fopen(filename, "r")))
    {
        fprintf(stderr, "Oops! The file %s failed to open...", filename);
        return 0;
    }

    for (index = 0, count = 0; EOF != (ch = fgetc(source)); index++)
    {
        if (arg[0] == ch)
        {
            ++count;
        }
    }

    return count;
}

findchar.c

//C Primer Plus
//Chp 13 Programming Exercises
//Problem 13-08
/*

*************************************************************
Written by: JargonJunkie
*************************************************************
CHP 13 Problem #8
*************************************************************
Write a program that takes as command-line arguments a character 
and zero or more filenames. 

If no arguements follow the character, have the program read the 
standarad input. Otherwise, have it open each file in turn and 
report how many times the character appears in each file. 

The filename and the character itself should be reported along 
with the count. 

Include error-checking to see whether the number of arguments is 
correct and whether the files can be opened. If a file can't be 
opened, have the program report that fact and go on to the next
file.
*************************************************************
*/

#include <stdio.h> 
#include <string.h>
#include <ctype.h>
#include "findproto.h"

int main(int argc, char * argv[])
{
    enum file status;
    unsigned number_of_chars = 0; 

    if (argc <= 1)
    {
        display_help(argv[0]);
        return 1;
    }

    if (!first_arg_is_char(argv[1]))
    {
        puts("The first argument must be a character.");
        return 1;
    }

    /* 
        Determine whether or not the string is a referenced
        file name.
    */
    for (int count = 2; count < argc; count++)
    {
        status = string_or_file(argv[count]);

        switch (status)
        {
            case missing:
                //assume string literal
                number_of_chars = count_char_in_string(argv[1], argv[count]);

                if (!number_of_chars)
                {
                    printf("[Null Character]! The character '%s' was not found.\n", argv[1]);
                }

                if (number_of_chars)
                {
                    printf("String: %s\n\n", argv[count]);
                    printf("The character '%s' was found %u time(s).\n", argv[1], number_of_chars); 
                }

                break;
            case error:
                //something went wrong...
                puts("Error! Possible invalid argument...");
                break;
            case txt:
            case asc:
            case c:
            case h:
            case csv:
            case html:
            case log:
            case xhtml:
            case xml: 
                //assume string is of FILE type
                //calculate the number of occurances
                //for the given letter X...
                break;
            default:
                //something went horribly wrong...
                puts("Oops! Something went HORRIBLY wrong...");
                break;
        }
    }


    return 0;
}

Answer 1

Yes, that is system specific and depends on which CLI/shell you use. For most unix like shells, you can just quote those arguments with single quotes, so the shell doesn't interpret those special constructs, eg run:

findch '*' "hello, there!" 
findch '&' "hello, there!" 

That will pass the & and * character to your program as argv[1] , without the single quotes, and without the shell interpreting them. See eg here for more info if you're using the bash shell.