How to create file with size in Linux.

 To create file with size you can use following commands on different platform.

On Solaris, you can use this command:

$ mkfile file-size file-name

$ mkfile 10m output.dat

On RHEL Linux, you can use the dd command:

dd if=/dev/zero of=output.dat  bs=24M  count=1

or

dd if=/dev/zero of=output.dat  bs=1M  count=24

[root@kernal ~]# dd if=/dev/zero of=file.example bs=10M count=1 1+0 records in 1+0 records out 10485760 bytes (10 MB) copied, 0.0559974 s, 187 MB/s

For Check size of the file:

[root@kernal ~]# ls -hl file.example -rw-r--r-- 1 root root 10M Jul 26 19:47 file.example

Under non-embedded Linux or Cygwin (or any system with GNU coreutils) and FreeBSD:

# truncate -s 10G file-name

[root@kernal ~]# truncate -s 20m file [root@kernal ~]# [root@kernal ~]# ls -hl file -rw-r--r-- 1 root root 20M Jul 26 19:57 file

This creates a file full of null bytes. If the file already exists and is smaller, it is extended to the requested size with null bytes. If the file already exists and is larger, is is truncated to the requested size.

The null bytes do not consume any disk space, the file is a sparse file.

# fallocate -l 5G file-name

[root@kernal ~]#  fallocate  -l  25M  file1 [root@kernal ~]# ls  –hl  file1 -rw-r--r-- 1 root root 25M Jul 26 19:57 file1

You can do it using Program:

#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  is useful to subsequently mmap the file into memory.

use the following command to check  file size:

# du -B1 --apparent-size /tmp/foo.txt

OR

# du  /tmp/foo.txt

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


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