RH033 Unit7 Standard I/O and Pipes

Objectives

1) Upon completion of this unit, you should be able to:

- Redirect I/O channels to files

- Connect commands using pipes

- Use the for loops iterate over sets of values

Standard Input and Output

1) Linux provides three I/O channels to Programs

- Standard input (STDIN) �C keyboard by default

- Standard output (STDOUT) �C terminal window by default

- Standard error (STDERR) �C terminal window by default

Redirecting Output to a File

1) STDOUT and STDERR can be redirected to files:

- command operator filename

2) Supported operators include:

  > Redirect STDOUT to file

  2> Redirect STDERR to file

  &> Redirect all output to file

3) File contents are overwritten by default.
   .>> appends

Redirecting Output to a File Examples

1) This command generates output and errors when run as non-root:

   $ find /etc �Cname passwd

2) Operators can be used to store output and errors:

   $ find /etc �Cname passwd > find.out

   $ find /etc �Cname passwd 2> /dev/null

   $ find /etc �Cname passwd > find.out 2> find.err

Redirecting STDOUT to a program (piping)

1) Pipes (the | character) can connect commands:

- command1 | command2

- Sends STDOUT of comand1 to STDIN of command2 instead of the screen.

- STDERR is not forwarded across pipes

2) Used to combine the functionality of multiple tools

- command1 | command2 | command3 …

Redirecting STDOUT to a program Examples:

1) less: view input e page at a time:

   $ ls �Cl /etc | less

- Input can be searched with /

2) mail: Send input while email:

   $ echo “test email” | mail �Cs “test” [email protected]

3) lpr: Send input to a printer

   $ echo “test print” | lpr

   $ echo “test print” | lpr �Cp printer_name

Combining Output and Errors

1) Some operators affect both STDOUT and STDERR
   &> Redirect all output:

    $ find /etc �Cname passwd &> find.all

   2>&1: Redirects STDERR to STDOUT

    Useful for sending all output through a pipe

    $ find /etc �Cname passwd 2>&1 | less

-  (): Combines STDOUTs of multiple programs

    $ ( cal 2007; cal 2008) | less

Redirecting to Multiple Targets (tee)

1) $ command1 | tee filename | command2

2) Store STDOUT of comand in filename, then pipes to command2

3) Uses:

- Troubleshooting complex pipelines

- Simultaneous viewing and logging of output

Redirecting STDIN from a file

1) Redirect standard input with <

2) Some commands can accept data redirected to STDIN from a file

   $ tr ‘A-Z’ ‘a-z’ < .bash_profile

- this command will translate the uppercase characters in .bash_profile to lowercase

3) Equivalent to:

   $ cat .bash_profile | tr ‘A-Z’ ‘a-z’

Sending Multiple Lines to STDIN

1) Redirect multiple lines from keyboard to STDIN with <<WORD

- All text until WORD is sent to STDIN

- Sometimes called a heretext

   $ mail �Cs “Please Call” [email protected] << END

   Hi Jane,

   Please give me a call when you get in…

   Details when you’re site

   Thanks,

   Jackie

   END

Scripting: for loops

1) Performs actions each member of a set of values

2) Example:

   for NAME in joe jane julie

   do

ADDRESS=”$ [email protected]”

MESSAGE=’Projects are due today!’

echo $MESSAGE | mail �Cs Reminder $ADDRESS

   done

Scripting: for loops continued

1) Can also use command-output and file lists:

- for num in $(seq 1 10)

   Assigns 1-10 to $num

   seq X Y pirnts the numbers X through Y

- for file in *.txt

   Assign names of text files to $file

End of Unit7

1) Questions and Answers

2) Summary

- Standard I/O channels

- File redirection

   Standard input (<)

   Standard output (>)

   Standard Error (2>)

- Pipes redirect standard output to standard input

- for loops can perform commands items from a program’s standard output or an explicit list