The dd command copies a input file to an output file. It takes the number of loops and the block size in input.
dd if=/dev/zero of=output.txt count=1024 bs=1024
This example creates a file from /dev/zero and copies 1024 times 1024 blocks. This way the output file is 1MB. The ls -alh shows the output file size.
$ dd if=/dev/zero of=output.txt count=1024 bs=10241024+0 records in 1024+0 records out 1048576 bytes (1.0 MB) copied, 0.00607721 s, 173 MB/s $ ls -alh total 1.1M drwxrwxrwt 2 root root 4.0K May 3 14:50 . drwxr-xr-x 22 root root 4.0K Nov 24 23:48 .. -rw-r--r-- 1 root root 1.0M May 3 14:50 output.txt /tmp$