How to create a file with a given size in Linux?

Question

For testing purposes I have to generate a file of a certain size (to test an upload limit).

What is a command to create a file of a certain size on Linux?

Answer 1

Use this command:

dd if=$INPUT-FILE of=$OUTPUT-FILE bs=$BLOCK-SIZE count=$NUM-BLOCKS

To create a big (empty) file, set $INPUT-FILE=/dev/zero.
Total size of the file will be $BLOCK-SIZE * $NUM-BLOCKS.
New file created will be $OUTPUT-FILE.

Answer 2

dd if=/dev/zero of=my_file.txt count=12345

Answer 3

you could do:

[dsm@localhost:~]$ perl -e 'print "\0" x 100' > filename.ext

Where you replace 100 with the number of bytes you want written.

Answer 4

dd if=/dev/zero of=upload_test bs=file_size count=1

Where file_size is the size of your test file in bytes

Answer 5

The trouble with the approaches using dd and perl is that they actually have to write every byte of the file. Using an approach like this you wouldn't have to.

#include <stdio.h>

int main ()
{
  FILE * pFile;
  int filesize = 12345;
  pFile = fopen ( "myfile.txt" , "w" );
  fseek ( pFile , filesize , SEEK_SET );
  fputs ( "" , pFile );
  fclose ( pFile );
  return 0;
}

This will just allocate a large file and doesn't actually need to write many bytes to do so.

Disclaimer: I have not compiled or tested this code.

Answer 6

Just to follow up Tom's post, you can use dd to create sparse files as well:

dd if=/dev/zero of=the_file bs=1 count=0 seek=12345

This will create a file with a "hole" in it on most unixes - the data won't actually be written to disk, or take up any space until something other than zero is written into it.

Answer 7

You can do it programmatically:

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>

int main() {
    int fd = creat("/tmp/foo.txt", 0644);
    ftruncate(fd, SIZE_IN_BYTES);
    close(fd);
    return 0;
}

This proceeding is especially useful to subsequently mmap the file into memory.

use the following command to check that the file has the correct size:
# du -B1 --apparent-size /tmp/foo.txt

But:
# du /tmp/foo.txt
will return 0 because it is allocated as Sparse file.

see also: man 2 open and man 2 truncate

Answer 8

I suppose the most efficient approach would involve creating the file using open (with the O_CREAT and O_WRONLY flags, unless you want to read from the file in the same program) and setting the size with ftruncate.