Introduction

Most UNIX commands:

• Read input from the standard input, stdin.
• Write output to the standard output, stdout.
• Write error messages to the standard error, stderr.

Notes

• By default, stdin, stdout, and stderr are your terminal.
• Your shell will let you run commands so that one or all those are files instead of the terminal.

stdin

• To make a command read from a file instead of a terminal, use < filename on the command line.
• To mail the content saved in a file called message:

  
  % mail kensmith < message
  
  

stdout

• If you want to save the output of a command to a file, type the command as normal, including any arguments it needs, and end the command with >  filename where filenameis the name of the file you want the output saved in.
• Some commands will check to see if stdout is a file instead of a terminal and will change the output format:

  
  % ls /usr/local > listing
  % cat listing
  bin
  etc
  games
  include
  lib
  lib64
  libexec
  sbin
  share
  src
  % ls /usr/local
  bin  etc  games  include  lib  lib64  libexec  sbin  share  src
  % 
  
  

• Example: you can combine all files beginning with mail. into a single file called MAIL with:

  
  % cat mail.* > MAIL
  
  

• If MAIL didn't exist before, it is created.
• If MAIL did exist before, it is overwritten, and the previous contents of the file are gone.
• You can set the shell variable noclobber to have shell give an error if MAIL exists instead of overwriting it
• You can append to a file instead of overwriting it by using >> instead of > for output redirection.
• Be careful when using wildcarding and I/O redirection!
• Your shell creates the output file first, then does the wildcarding:

  
  % ls
  file.1  file.2
  % cat file.* > file.3
  cat: input/output files 'file.3' identical
  % 
  
  

stderr

• If error messages should be saved in file too use >& for output redirection.