1. Shell Control Statements and More
CS465 - UNIX
Shell Control Statements and More

2. while statement while [ condition ] do command(s) done
Same condition syntax as if statement
Begin and end of command block defined by keywords do...done
Loops while condition is TRUE
while [ condition ] do
command(s)
done

3. while Loop Example #1 $ cat wake #!/bin/sh resp="n"
while [ "$resp" != "y" ] do
echo "Wakeup [y/n]? "
read resp
done $
$ wake
Wakeup [y/n]? n Y
Wakeup [yes/no]? y $

4. while Loop Example #2 #!/bin/sh echo "Enter number: " read num fac=1
$ cat fac
#!/bin/sh
echo "Enter number: "
read num
fac=1
loop=1
while [ $loop -le $num ] do
fac=`expr $fac \* $loop`
loop=`expr $loop + 1`
done
echo "The factorial of $num is $fac" $
$ fac
Enter number: 5
The factorial of 5 is 120 $

5. while example #3 $ cat lines while [ "$input" != done ] do
echo 'Enter a filename, or "done":'
read input
if [ "$input" != done ]
then
lines=`wc –l < $input`
echo "$input has $lines lines" fi
done $
$ lines
Enter a filename, or "done":
shoes
shoes has lines
done $

6. while example #4 $ cat sayhi #! /bin/sh # $* = list of names count=$#
while [ count -gt 0 ] do
echo Hello $1
count=`expr $count - 1`
shift
done
exit 0 $
sayhi Sue Joe Bob
Hello Sue
Hello Joe
Hello Bob $

7. Student Exercise
Write a shell script called up, that will move you up in the directory structure
If no arguments, move up ONE directory
If one argument, it should be a number, telling how many directories to move up
Usage Example:
$ pwd
/usr/home/faculty/small000
$ . up 2
/usr/home
$ . up
/usr

8. Exercise Sample Solution
#! /bin/sh
# $1 = number of levels to go up
# (if no parameters, go up one level) #
if [ $# -eq 0 ]
then
count=1
else
count=$1 fi
while [ $count -gt 0 ] do
cd ..
count=`expr $count - 1`
done
exit 0

9. for Statement
The for statement is used to repeat commands for known or “fixed” values
Unlike C programming, the for loop usually repeats tasks for “arguments” that are either issued from the script or a stated directory after for statement.

10. for statement for variable in list do command(s) done
variable is a variable name; don't use $
list is a space-separated list of strings. Each element of the list will be assigned to variable one at a time, and the command block will be executed.
Within the command block, use $variable to use the value that has been assigned.
for variable in list do
command(s)
done

11. for example #1 $ cat colorscript #!/bin/sh
for color in red yellow blue do
echo $color
done
echo "the end"
$ colorscript
red
yellow
blue
the end $

12. for example #2
$ cat userdirs
#!/bin/sh
for user in $* do
echo Directory for: $user
ls -F /home/${user}
done $
Note: If the “in ___” part is omitted, for defaults to “in $*
$ userdirs jmsmith krjones
Directory listing for: jmsmith
cprogs/ dotask* xfile
diskfile mbox
Directory listing for: krjones
mbox prog1.c prog2.c $

13. for example #3 $ cat printall #!/bin/sh for file in * do
if [ -f $file ]
then
echo "Print $file [y/n]? "
read resp
if [ $resp = "y" ]
lpr $file fi
done $
$ printall letter1 names
Print letter1 [y/n]? y
Print names [y/n]? n

14. case statement
Very similar to C switch statement: executes the commands after a choice that matches string.
Double-semicolon ends a block of commands, like the C break statement. If you skip the ;; you will keep going into the next block .
* as a choice will match any string, and can be used to set a default
esac ends the statement.
case string in
choice) command(s) ;;
choice) command(s)
esac

15. case Example #1 echo Enter command and filename read cmd file
case "$cmd" in
list)
ls -l "$file" ;;
count)
wc -l "$file" *)
echo "command $cmd is not implemented"
esac

16. case Example #2 $ cat yesno #! /bin/sh echo –n 'Yes or No (y/n)? '
read choice
case $choice in
"Y" | "y") echo You chose Yes;;
"N" | "n") echo You chose No;;
*) echo Invalid choice;;
esac
exit $
yesno
Yes or No (Y/N)? N
You chose No $

17. case Example #3 The following will be a script to:
Give user a choice of what to do with the files listed as arguments:
Copy to a subdirectory
Concatonate
or Delete
Carry out the action chosen, prompting for the subdirectory or file to concatonate into, as needed.
Display a message confirming action was done.

18. case Example #3 (Continued on next slide) $ cat files #! /bin.ksh
# script parameters = files to operate on
cat << STOP
M) Move Files
C) Concatonate Files
D) Delete Files
Enter choice:
STOP
read choice
(Continued on next slide)

19. case Example #3 (continued)
case $choice in
"m"|"M") echo Move files to which subdir?
read subname
if [ ! -d $subname ]
then
mkdir $subname fi
mv $* $subname
echo Files moved to subdir $subname ;;
"c"|"C") echo File to place concatonation in?
read fname
if [ -f $fname ]
echo Error - $fname already exists
else
cat $* > $fname
echo Files concated into $fname

20. case Example #3 (continued)
"d"|"D") rm $*
echo Files $* have been deleted ;;
*) echo Invalid Choice -- No Can Do
esac
exit 0 $
files file1 file2
M) Move Files
C) Concatonate Files
D) Delete Files
Enter choice: C
File to place concatonation in?
combo
Files concated into combo $

21. How the Shell Finds a Command
The shell searches a list of directories for an executable file with the same name as the command given.
The list of directories is stored in the PATH variable $ PATH=/bin:$PATH (sh/ksh)
If there is a match in more than one directory, the shell uses the first one it finds.
To run a command not in one of these directories, give a pathname (relative or absolute) instead.
$ ~/progs/dosomething

22. Built-In Commands Some commands are built into the shell kernel.
The echo command, for example, is often builtin, for efficiency.
You can find out where the shell is getting a particular command using the “which” command in any shell:
$ which echo
echo: shell built-in command.
$ which cat
/usr/bin/cat $

23. More Environment Variables
Predefined Environmental Variables:
PPID shell’s parent’s process id
IFS list of command line word delimiters (default is list is space, tab & newline)
PS1 your shell prompt (default is $)
PS2 your input prompt (default is >)
SHENV directory where your .profile is located (default is $HOME)

24. Files Run at Login System-wide login file /etc/profile
Personal login file
$HOME/.profile
Personal environment file
Set by $ENV
Usually $HOME/.kshrc

25. Sample Bourne .profile # Set PATH PATH=$PATH:. export PATH
# Set environmental variable file
ENV=$HOME/.envsetup
# Set shell variables
PS1='Command? '
# Display status information
date
echo "Currently logged in users:“
users

26. Directory in prompt
To include directory in prompt, put these lines in your .profile file:
PS1='$PWD $ ' or
PS1="`pwd`$ "
export PS1