CS 497C - Introduction to UNIX
Lecture 21: - The Shell
Chin-Chih Chang chang@cs.twsu.edu

Redirection (Standard Output)
* Using the symbols > and >>, you can redirect the output to a disk file.
* For example, when we issue who > new-file, the shell opens the file 
  new-file, writes the stream into it and the closes the file.
* Using the > symbol will overwrite the existing file. To append to a 
  file, use the >> symbols:
who >> new-file

Redirection (Standard Output)
* Redirection also becomes a useful feature when concatenating the 
  standard output of a number of files.
cat chap?? > textbook
* Two or more commands can be combined and their aggregate output can be 
  redirected to a file. A pair of parentheses groups the commands, and a 
  single > symbol can be used to redirect both of them:
(ls -l ; who) > lsfile

Redirection (Standard Output)
* You can redirect a message to the terminal /dev/pts/6 provided the 
  terminal is enabled accordingly.
echo hello > /dev/pts/6
* There are three destinations of standard output: terminal (default), 
  file, and pipe.
* Some commands are designed to take their input as a stream. This 
  stream represents the standard input to a command.

Redirection (Standard Input)
* The wc command expects input from the keyboard when the filename is 
  omitted:
$ wc
This is a test of the wc comand.
It reads the input from the standard input.
[Ctrl-d]
      2      16      77

Redirection (Standard Input)
* The standard input stream has three sources: the keyboard (default), a 
  file using redirection with <, and another program using a pipeline.
* You can make calculations in a batch as shown in the following example:
$ cat calc.lst
2^32
25*50

Redirection (Standard Input)
$ bc < calc.lst > result.lst
$ cat result.lst
4294967296
1250
* When a command takes input from multiple resources – say a file and 
  standard input, the – symbol must be used to indicate the sequence of 
  taking the input.

Redirection (Standard Error)
* For example. cat - foo indicates first reading from standard input and 
  then from foo.
* Each of three standard files has a number, called a file descriptor:
 - 0 - standard input: < or 0<
 - 1 - standard output: > or 1>
 - 2 - standard error: 2>
* You need to use the descriptor 2> for the standard error:
$ cat bar 2>errorfile

Redirection (Combining Streams)
* In C Shell, use >& for the standard error.
* The following Stream combinations are equivalent:
wc < infile > newfile
wc > newfile < infile
> newfile < infile wc
* The standard output and error symbols can also used in the same 
  command line:
cat newfile nofile 2> errorfile > outfile