Input/output redirection

Standard input and standard output files
UNIX treats the keyword as the standard input file and vdu (visual display unit) as the standard output file as well as the standard error file. But input can be taken from sources other than the standard input and output can be passed to any source other than the standard output. Such a process is called redirection. This may be input redirection or output redirection. These three streams i.e. the “standard input”, the” standard output” and the “standard error” are denoted by the numbers 0, 1, 2 respectively.