Giteveryday – A useful minimum set of commands for Everyday Git(kernel.org)

  ssize_t write(int fd, const void *buf, size_t count);

  DESCRIPTION
  write()  writes  up  to  count  bytes  from  the buffer pointed buf to the file referred to by the file descriptor fd.

  The number of bytes written may be less than count if, for example, there is insufficient space on  the underlying  physical  medium,  or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or call was interrupted by a signal handler after having written less than  count  bytes.   (See  also (7).)

  For  a  seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the file offset, and the file offset is incremented by  the  number  of  bytes  actually written. If the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing.  The adjustment of the file offset and the write operation are performed as  an  atomic step.