Passing arguments through bash script to C program

Question

I'm trying to write a program that accept file names as arguments in a bash script, then passes them to a C program that replaces spaces with underscores, then the bash script uses that to rename the file.

For example, the input would be

Bash bash_script "test test test.txt"

and the file would be renamed test_test_test.txt.

My problem is that when I run this, it tells me that I'm using mv incorrectly. Why is this? I'm new to bash, so I'm sorry for using program/script incorrectly.

My C program:

#include <stdio.h>

int main(int argc, char *argv[])
{
    int i = 0;
    char * file = argv[1];

    while(i<=256){  //max file size is 256 characters on mac
        if (argc != 2)
            printf("Please provide one file name.");
        else if(file[i] == ' ')
            file[i] = '_';
        i++;
    }
    printf("%s \n", file);
    return 0;
}

My Bash program:

#! /bin/bash

VAR = "C_program '$@'"
mv $1 $VAR

One answer is to actually use tr in your shell script (ref: linux.die.net/man/1/tr ) — mctylr
– mctylr, Commented Aug 11, 2014 at 20:26
What about tabs and newlines in the file name? What about other control characters? What happens if the name passed to the script doesn't contain spaces? What happens if the argument is "abc def" but there is already a file "abc_def"? — Jonathan Leffler
– Jonathan Leffler, Commented Aug 11, 2014 at 23:18

mctylr · Accepted Answer · 2014-08-11 21:09:53Z

1

This line:

VAR = "C_program '$@'"

doesn't do what you want. And your mv line is broken too.

VAR=$(C_program "$@")
mv "$1" "$VAR"

Also, your C program doesn't exit with an error when an error is detected.

Also, sed and tr are existing programs that are suitable alternatives to writing your C program to transliterate (translate) characters in strings.

Also, rename/prename are existing (Perl) programs that handle rename files with regular expression pattern functionality to rename files, which may be already available on your system(s).

edited Aug 11, 2014 at 21:09

mctylr

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answered Aug 11, 2014 at 20:29

Ignacio Vazquez-Abrams

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Comments

nneonneo · Accepted Answer · 2014-08-11 20:38:23Z

0

In your specific example, I would not shell out to a custom C program to do this.

Here's an equivalent shell script (not requiring tr, sed or any programs besides bash and mv):

mv "$1" "${1// /_}"

In your specific problem, you are not setting your VAR properly. Shell scripts cannot accept spaces around the = when setting variables, and you need to use backticks or $() to execute an external program. So, properly written, you want

VAR="$(C_program "$@")"

answered Aug 11, 2014 at 20:38

nneonneo

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Comments

mctylr · Accepted Answer · 2014-08-11 21:11:47Z

If you just want the results than I would suggest simply replace the bash script and custom C program with a single short Bourne or POSIX shell script.

#!/bin/sh
NEW_FILE_NAME= `echo $1 | tr ' ' _  `
mv $1 $NEW_FILE_NAME

Otherwise

You want the shell script to run your C program (I'll refer to it as todash for simplicity) before setting the shell variable VAR. This is done using the backtick ` (located near upper right corner of US keyboards with tilde, '~') operation.

#!/bin/sh
VAR= `todash $1`
mv $1 $VAR

For todash.c I'll suggest a couple of mostly small improvements.

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

int main(int argc, char *argv[])
{
   char * filename;  
   /* Program operates on filename, not file or its contents, so use a variable
      name that reflect that. Good variable names make debugging easier.
    */

   if (argc != 2) {
     printf("Please provide one file name.");
     return EXIT_FAILURE;  /* or exit(EXIT_FAILURE); */
   } else {
     /* Only set filename, once known that argv[1] is not NULL (empty) */
     filename = argv[1];
   }

   /* Use a simple for loop
    * PATH_MAX is a standard system constant included with limits.h
    */
   for (i = 0; (i < PATH_MAX) && (filename[i] != '\0'); i++) {
      if (filename[i] == ' ') {
         filename[i] = '_';
      }
   }
   printf("%s \n", filename);
   return EXIT_SUCCESS;
}

The added length is only my additional inline comments.

The largest change was untangling the argc if comparison from the while loop, which once simplified, become a classic example of where to use a for loop.

And for your sanity, and those around you, use braces (curly brackets) around conditional blocks of code. Just because you are allowed to not include them, does not mean you should (not include them). Programs tend to live beyond their original intention and expand in the future. Avoid making mistakes later by including them now.

technosaurus · Accepted Answer · 2014-08-11 23:43:55Z

0

2 problems with the mv $1 "C_program '$@'"

$1 needs double quotes -> "$1"
"C_program '$@'" should be `C_program '$1'` or $(C_program '$@')

however this can be done more efficiently with

IFS="
"
for x in $@; do
    mv "$x" "${x// /_}"
done

edited Aug 11, 2014 at 23:43

answered Aug 11, 2014 at 20:41

technosaurus

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3 Comments

Jonathan Leffler Over a year ago

Use for x in "$@" because the input names generally have spaces in them, which will cause troubles if you don't double quote "$@".

technosaurus Over a year ago

@JonathanLeffler Thanks, I initially had while ([ "$1" ]) do mv "$1" "${1// /_}";shift;done but (over-)simplified it to this. Fixed with IFS, hopefully none of the files have return characters (though it is possible since its not a '/')

Jonathan Leffler Over a year ago

Better not to mess with IFS and simply use the notation that works regardless.

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