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

You can do it programmatically:

#include 
#include 
#include 
#include 
#include 

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

This approach 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

Be careful:

# du /tmp/foo.txt

will probably print 0 because it is allocated as Sparse file if supported by your filesystem.

see also: man 2 open and man 2 truncate