CTEC 1430 Enterprise Computing I

1
CTEC 1430 Enterprise Computing I

• 4. UNIX I/O Redirection

2
I/O Redirection Filters

• Recall
• each UNIX command is written as a general purpose
  program
• Inputs come from STDIN outputs go to STDOUT

3
File Descriptors

• When a UNIX command is invoked, shell opens 3 I/O
  files
• Unless you specify differently, the default files
  will be opened
• These default files can easily be changed when
  the command is invoked

4
Redirecting Output

• E.g. redirect output of ls to go into a disk file
  instead of going to the screen
• ls gt MYFILE
• To see the contents of MYFILE
• cat MYFILE
• .profile
• adir
• bdir
• report
• MYFILE

5
Redirecting Output (2)

• When ls output is redirected to MYFILE, shell
  opens the following file descriptors

6
Redirecting Input

• Not as common as output redirection
• Most commands take input from files, rather than
  STDIN
• If command isnt provided a file, input will come
  from the keyboard until a Ctrl-D is read.
• Example of a command using STDIN as the input
  file
• cat
• Some text is typed here.
• This text will be buffered and sent to STDOUT
  when a Ctrl-D is read
• D
• Some text is typed here.
• Or you could put the above text into MYFILE and
  direct cat to use MYFILE as STDIN
• cat lt MYFILE

7
Redirecting Diagnostic Output

• E.g. send error messages from program to
  ERRORFILE, rather than screen
• cc thisfile.c 2gtERRORFILE
• For STDERR you must precede the gt by the file
  descriptor number 2
• The file descriptor for STDOUT is number 1, and
  is optional for redirecting
• Assumed if not provided

8
I/O Redirection

• The following commands are equivalent
• cat 0ltMYFILE
• cat ltMYFILE
• cat lt MYFILE
• catltMYFILE
• The following commands are equivalent
• ls 1gtMYFILE
• ls gtMYFILE

9
Appending

• Special case of redirection STDOUT
• Use gtgt instead of gt
• Instead of wiping out output file and then
  directing output to it, output appended to end of
  the output file
• cat MYFILE
• THIS TEXT EXISTS.
• pwd gtgt MYFILE
• cat MYFILE
• THIS TEXT EXISTS.
• /usr/user1

10
Pipes

• Pipes take output of one command as input to
  another command
• Example sort MYFILE, and send it to the lpr
  program to be spooled to the printer
• cat MYFILE sort lpr

11
I/O Redirection vs. Pipes
12
Filters

• Most UNIX commands are filters
• Filters take input from STDIN and put their
  output to STDOUT
• Filter commands can occur between two 's in a
  pipeline, e.g. print

13
Filters (2)

• tee
• program which sends its inputs to both STDOUT and
  a file, e.g. try
• sort file.txt tee sorted.txt
• tee can also be used as a filter