Whats Wrong With This Script

1
Whats Wrong With This Script?

• !/bin/sh
• for filename
• do
• if test -d filename
• then
• echo filename is a directory
• else
• cat filename
• fi
• done

2
What if I use it like this?

• clyde myscript.sh file1-9.txt wc -w
• Is the output from wc correct?
• What if one of the files specified is a
  directory?

3
Using STDERR

• Whenever one of your scripts outputs prompts or
  error messages, they should use standard error,
  not standard out
• This keeps your script from "creating data"
• This keeps your script from injecting spurious
  "noise" into a pipeline

4
Standard Files and Device Files

• A commands standard output is a default place
  for it to send output
• A commands standard input is a default place to
  receive input
• Standard error is the default place to send
  errors from a command
• The Unix treatment of files as a stream of bytes
  extends to I/O devices also. They are a special
  type of file known as a device file.
• Device files are commonly found in the /dev
  directory

5
Redirection

• Redirection encompasses the various ways you can
  cause the shell to change where a command gets
  its standard input from or sends its standard
  output to
• By default, these two files are associated with
  the terminal but the user can change this
  behavior
• Standard error is also normally associated with
  the terminal but it also can be redirected

6
Redirecting Standard Output

• The gt operator is used to redirect standard
  output
• command options arguments gt output_filename
• command is any executable program that doesnt
  hardcode its output destination
• options and arguments are the same as you are
  familiar with
• output_filename is the place (file) that you want
  the output of command saved in
• If output_filename doesnt exist, it is created.
  If output_filename does exist, it is overwritten
  and the original contents are destroyed! (Unless
  the shell variable "noclobber" is set.
• gt! over-rides "noclobber"

7
Appending Standard Output

• The gtgt operator gets around the last problem
• command options arguments gtgt output_filename
• gtgt appends standard output to output_filename,
  preserving whatever was originally there and
  appending the new information
• CAUTION You must ensure that you use both gtgt
  symbols. If you accidentally use only one gt,
  output_filename will be overwritten with the new
  data and all previous contents will be lost!

8
Redirecting Standard Input

• Redirection of standard input works in a similar
  manner
• lt is the standard input redirection operator
• command options arguments lt input_filename
• command is any executable program
• options and arguments are the same as you are
  familiar with
• input_filename is the place (file) that contains
  the data you want to use for input to command
• Obviously, input_filename must exist

9
Using Input and Output Redirection

• Standard input and standard output redirection
  can be used together on the same command
• command options arguments lt input_filename gt
  output_filename
• or
• command options arguments lt input_filename gtgt
  output_filename

10
Redirecting Standard Error

• Under the C shell, you can redirect both standard
  output (STDOUT) and standard error (STDERR) by
  using the gt or gtgt symbols followed by an
• If you dont do this, commands like find may
  clutter up your screen with error messages while
  you are trying to accomplish other work
• find / -name stdlib.c -print gt whereitis
• or
• find / -name stdlib.c -print gtgt whereitis

11
Redirecting STDERR STDOUT

• You can also redirect STDIN and STDOUT to
  separate files using the Bourne shell (sh)
• A file descriptor is associated with each of the
  standard files
• STDIN is file descriptor 0
• STDOUT is file descriptor 1
• STDERR is file descriptor 2
• Using the file descriptor, we can redirect STDOUT
  and STDERR independently or together

12
Bourne Shell Examples

• To redirect STDERR
• command 2gt file
• To redirect STDERR and STDOUT to the same place
• command gt file 2gt1
• Note that order is paramount!
• command 2gt1 gt file is NOT the same thing!
• The first redirect sends STDOUT (file descriptor
  1) to file and then redirects file descriptor 2
  (STDERR) to the same place as file descriptor 1
• The second sends file descriptor 2 (STDERR) to
  the same place as file descriptor 1 (STDOUT, the
  tty) and then redirects file descriptor 1 to file

13
How Do I Do This In My Scripts?

• For a Bourne shell script, you can use
• echo "This is a message" gt2
• For Korn shell, you can use
• print "This is a message" gt2
• print -u2 "This is a message"
• For awk, you can use something like
• 'print "What a nice day!" gt "/dev/tty" '

14
Correct Way to Script

• !/bin/sh
• for filename
• do
• if test -d filename
• then
• echo filename is a directory gt2
• else
• cat filename
• fi
• done