1. Shell Programming
Winter Quarter 2005
Rolando V. Raqueño

2. Unix Power Tools Reference
Chapter 35
Shell Programming for the Uninitiated
Chapter 36
Shell Programming for the Initiated
Winter Quarter 2005
Rolando V. Raqueño

3. UNIX Tip of the Day
Directory maneuvering commands pushd, popd, and, dirs
% cd /usr/tmp
% pwd
/usr/tmp
% pushd ~rvrpci
/cis/staff/rvrpci
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4. UNIX Tip of the Day % dirs /cis/staff/rvrpci /usr/tmp % pushd % pwd
/usr/tmp /cis/staff/rvrpci
% pushd /usr/local/bin
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5. UNIX Tip of the Day % dirs /usr/local/bin /usr/tmp /cis/staff/rvrpci
% pwd
/usr/local/bin
% pushd
/usr/tmp /usr/local/bin
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6. UNIX Tip of the Day % dirs /usr/tmp /usr/local/bin /cis/staff/rvrpci
% pwd
/usr/tmp
% pushd +2
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7. UNIX Tip of the Day % dirs /cis/staff/rvrpci /usr/tmp /usr/local/bin
% popd
/usr/tmp /usr/local/bin
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8. IMPORTANT UNIX Concepts
Environment and Shell Variables
These allow you to customize your UNIX environment
They are different in terms of their SCOPE
SCOPE determines the visibility of a variable
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9. Other IMPORTANT UNIX Concepts
Environment Variable
Examples are TERM and DISPLAY
Set a particular variable to a value by using the setenv command
You can print the value of a particular variable or all the environment variable using the printenv command
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10. Shell Variables
Shell variables are similar to Environment variables except they have a limited scope, i.e., they exist only in the shell which they are defined.
Environment variables on the other hand, exist in all its children shells
Winter Quarter 2005
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11. Environment & Shell Variables
Why is this important?
UNIX uses Environment and Shell Variables control a number of processes
Customizes your working environment
Variables used for UNIX Scripts
They are typically defined and initialized in your .login and .cshrc files
Winter Quarter 2005
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12. Useful Shell Variables
filec #Allows file completion
path #List of command
directories
history #Number of commands to
remember
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13. What is shell programming?
automate a set of UNIX commands.
Just like any programming language
“wrappers”
black box a customized collection of UNIX commands.
Example of shell programs
.login
.cshrc
Shell programming is a way to automate a set of UNIX commands.
It behaves in much the same manner as other programming languages and allows you to process repetitive tasks (batch files)
Shell programming also allows you to create “wrappers” which essentially treat as a black box a customized collection of UNIX commands.
Winter Quarter 2005
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14. .login file set path=($HOME/bin /usr/local/bin \
/usr/ucb /usr/sbin /bin /usr/bin \
/usr/bin/X11 .)
stty dec new
tset -I -Q
set mail=/usr/spool/mail/$USER
set editmode = emacs
umask 077
biff n
date
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15. .cshrc file if ($?prompt) then set notify set history = 100
set savehist = 100
alias pd pushd
alias pop popd
alias vt100 "set term = vt100"
endif
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16. When these files are executed?
.cshrc
is automatically executed when you start a new shell
.login
only gets executed once when you first login
Can be re-executed by giving the source command
% source .cshrc
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17. Other useful .login and .cshrc entries
set filec
set cdpath=(~ ~rvrpci/pub ~/mythesis)
Other common entries
set path=( $path /usr/local/bin)
set path=(/usr/local/bin $path)
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18. User defined shell program
Determine name of command
Determine input, output, and option arguments
Determine UNIX commands to execute
Establish error trapping
Make shell program executable
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19. A simple shell program dd command to swap bytes
% dd if=input.dat of=output.dat bs=2 conv=swab
Very difficult to remember
Very little utility to non-UNIX geeks (normal people)
Unfortunately, this is very difficult to remember and you want other people to be able to use this command without being UNIX experts
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20. We would rather see... % swap_bytes input.dat output.dat
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21. Special Shell Variables Set
% swap_bytes input.dat output.dat $0 $1 $2
command
$argv[1]
$argv[2]
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22. Another Special Shell Variables
% swap_bytes input.dat output.dat
$#argv
Indicates how many arguments are present
In this case, 2
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23. shell program swap_bytes
#!/bin/csh -f
dd if=$1 of=$2 bs=2 conv=swab
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24. Making swap_bytes shell script executable
% ls -l swap_bytes
-rw swap_bytes
% chmod u+x swap_bytes
-rwx swap_bytes
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25. To run swap_bytes swap_bytes becomes just another unix command!
% swap_bytes input.dat output.dat
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26. Limitation of swap_bytes
No error trapping
Should give usage when typing command
% swap_bytes
usage: swap_bytes input_file output_file
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27. Improvement to swap_bytes
#!/bin/csh -f
if ( $#argv != 2 ) then
echo "usage: $0 input_file output_file"
exit 1
endif
dd if=$1 of=$2 bs=2 conv=swab
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28. Commad exit status By convention exit 0
Indicates successful command completion
exit 1 (or non-zero)
Indicates some error condition
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29. Informational message from swap_bytes
UNIX style informational message
% swap_bytes
usage: swap_bytes input_file output_file
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30. Interactive swap_bytes
If you want a “friendlier” shell program
Have it query the user for the inputs
Another special shell variable can be used
$<
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31. Interactive swap_bytes
#!/bin/csh -f
if ( $#argv != 2 ) then
echo -n "Please enter the input file> "
set input=$<
echo -n "Please enter the output file> "
set output=$<
endif
dd if=$input of=$output bs=2 conv=swab
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32. Interactive swap_bytes example
User simply types the command
% swap_bytes
Please enter the input file> input.dat
Please enter the output file> output.dat
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33. UNIX Quotes
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34. A note about quotes in UNIX
% set a=ls
% echo a
% echo $a
% set b=“$a”
% echo $b
% set b=‘$a’
% set b=`$a`
% echo $b
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35. A note about shell variables
Shell variables can also double up as arrays
Using the previous example,
% echo $b
% echo $b[1]
% echo $#b
% echo $b[$#b]
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36. A more complex shell program
In pbmplus utilities,
rawtopgm conversion exists
pgmtoraw conversion does not
A version of pgmtoraw in a programming language like C
Time consuming
Program will likely be used infrequently
Solution: shell program
If you study the pbmplus utilities, you will notice that there is a rawtopgm conversion, but there is no pgmtoraw.
Now you can write a version of rawtopgm in a programming language like C, but chances are the time it will take to get a working program will be long.
It is possible to write one using shell programming.
Winter Quarter 2005
Rolando V. Raqueño

37. pgmtoraw shell script design
Define input and output files
Figure out dimensions of input image
Determine number of bytes for input image
Determine number of bytes for header
Need to strip out the header bytes
Write out headerless image data
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38. Define input and output files pgmtoraw
#!/bin/csh -f
set command_name=$0
set number_args=$#argv
if( $number_args != 1 ) then
echo “usage:$command_name input_file_name”
exit 1
endif .
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39. Dimensions of input image ( pnmfile)
% more test_data.pgm P2
3 3
255
1 2 3
4 5 6
7 8 9
% pnmfile test_data.pgm
test_data.pgm: PGM plain, 3 by 3 maxval 255
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40. pgmtoraw (continued) set input_file=$1
set pnm_info = `pnmfile $input_file`
set image_type = $pnm_info[2]
set data_type = $pnm_info[3]
set width = $pnm_info[4]
set height = $pnm_info[6]
set maxval = $pnm_info[8]
set pixel_bytes = 1
@ image_bytes = $width * $height
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41. pgmtoraw (continued) set file_info=`wc -c $input_file`
set bytes_in_file = $file_info[1]
@ header = $bytes_in_file - $image_bytes
dd if=$input_file bs=$pixel_bytes skip=$header
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42. Resulting pgmtoraw utility
Uses existing routines to obtain information
pnmfile wc dd
Functional tool written in 20 command lines
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43. Current Limitations of Resulting pgmtoraw utility
No check between “ASCII” vs. “RAW” pgm
if( data_type == ‘plain,’) ...
No provisions for multibyte per pixel case
Use pnmfile results to check for above cases
endian case needs to be addressed for multibyte case
Above conditions can be addressed by suite of UNIX commands
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44. Shell Scripts Wrappers and IDL
Another utility formerly missing in pbmplus
jpegtopnm or pnmtojpeg
IDL has jpeg read/write capability
Create a “wrapper” that makes an idl program pbmplus-like
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45. pnmtojpeg.pro pro pnmtojpeg, input_file, output_file
read_ppm, input_file, image
write_jpeg, output_file, image
end
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46. Usage of pnmtojpeg.pro in IDL
IDL> pnmtojpeg,‘image.pnm’,’image.jpg’
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47. Usage of pnmtojpeg.pro in IDL
IDL> pnmtojpeg,‘image.pnm’,’image.jpg’
For IDL to automatically find pnmtojpeg.pro
It must be in the current working directory
Directory containing pnmtojpeg.pro must be defined in the ENVIRONMENT VARIABLE
IDL_PATH
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48. IDL_PATH environment variable
setenv IDL_DIR /cis/common/rsi/idl_5
setenv IDL_PATH \+$IDL_DIR/lib:\+$IDL_DIR/examples:
\+/dirs/common/rsi/idl_5:\+/dirs/common/lib/idl:\+~/lib/idl
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49. pnmtojpeg.csh #!/bin/csh -f
echo pnmtojpeg “,” “’”$1”’” “,” “’”$2”’” | idl
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50. Usage of pnmtojpeg.csh % pnmtojpeg.csh image.pnm image.jpg
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51. Limitation of pnmtojpeg.csh
Does not conform to pbmplus piping, i.e.,
% tifftopnm file.tif | pnmscale 2.0 > new_file.pnm
No error trapping
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52. Usage cases of pnmtojpeg (no command line arguments)
% tifftopnm file.tif | pnmscale 2.0 | pnmtojpeg > new_file.jpg
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53. Usage cases of pnmtojpeg (1 command line argument)
% pnmtojpeg image.pnm > image.jpg
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54. Usage cases of pnmtojpeg (2 command line arguments)
% pnmtojpeg image.pnm image.jpg
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55. Yet another wrapper pnmtojpeg
#!/bin/csh -f
# If user interrupts process, jump to stop
onintr stop
# $0 is the command name
# $#argv is the number of arguments
# $$ is the process id
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56. Code for no argument case
if($#argv == 0) then
set input_file = /usr/tmp/$0_input_$$
set output_file = /usr/tmp/$0_output_$$
cat > $input_file
pnmtojpeg.csh $input_file $output_file
cat $output_file .
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57. Code for 1 argument case else if($#argv ==1) then set input_file = $1
set output_file = /usr/tmp/$0_output_$$
pnmtojpeg.csh $input_file $output_file
cat $output_file .
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58. Code for 2 argument case else set input_file = $1 set output_file = $2
pnmtojpeg.csh $input_file $output_file
endif
#clean up when finished
stop:
rm -f /usr/tmp/$0_input_$$
rm -f /usr/tmp/$0_output_$$
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59. pnmtojpeg summary Produced a “new” pbmplus utility
Used UNIX shell scripting
Argument handling
Scratch space /usr/tmp
Process id handling
Clean up
Integrated IDL program and commands
21 lines of UNIX commands
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60. Summary The “dot” files Basics of Shell Scripting
Special Shell Variables
Seamless integration of UNIX to other utilities (IDL)
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61. Other Shell Constructs to keep in mind
foreach
while
case
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