Lecture Four

1
Lecture Four

• Unix Shell Programming II

2
More If Example

• send an email to a user on local system
• if test 1 why 1?
• then
• echo Usage 0 username 1gt2 exit 1
• fi
• groups 1 gt /dev/null 2gt1
• if ((? ! 0))
• then
• echo User 1 is invalid exit 1
• fi
• mail 1

3
For and While Example

• c1
• for i
• do
• echo "Arg c i"
• cexpr c 1
• done
• c1
• while (( ! 0))
• do
• echo "Arg c 1"
• cexpr c 1
• shift
• done

4
Until Example

• PATH/bin/use/bin
• case in
• 0) echo Usage 0 1gt2 exit 1
• esac
• until who egrep 1
• do
• sleep 60
• done

5
Case Statement

• case word in
• pattern) commands
• pattern) commands
• esac

6
Case Example

• PATH/bin/usr/bin
• for i in _at_
• do
• echo -n i? gt /dev/tty
• read response
• case response in
• y) echo i
• q) break
• esac
• done

7
Why Use _at_

• showeacharg a b c
• and _at_ are the same when not enclosed by
  double quotes
• showeacharg has for i in echo arg i
  done
• output
• arg a
• arg b
• arg c

8
Why Use _at_

• one word
• showeacharg has for i in echo arg i
  done
• output
• arg a b c
• _at_ same as the args passed to the script
• showeacharg has for i in _at_ echo arg i
  done
• output
• arg a b
• arg c

9
Trap

• Whats wrong with the script
• PATH/bin/usr/bin
• new/tmp/wwho1.
• old/tmp/wwho2.
• gtold
• while is a built-in command that
  returns true, like command true
• do
• who gt new
• diff old new
• mv new old
• sleep 60
• done awk /gt/ 1 in print
• /lt/ 1 out print

10
Use Trap to Clean up

• PATH/bin/usr/bin
• new/tmp/wwho1.
• old/tmp/wwho2.
• trap rm -f new old exit 1 1 2 15
• gtold
• while
• do
• who gt new
• diff old new
• mv new old
• sleep 60
• done awk /gt/ 1 in print
• /lt/ 1 out print

11
Script with a Flaw

• We want to do
• sed s/UNIX/UNIX(TM)/g doc.txt overwrite
  doc.txt
• Overwrite script
• PATH/bin/usr/bin
• case in
• 1)
• ) echo Usage 0 file 1gt2 exit 1
• esac
• new/tmp/0.
• trap rm -f new exit 1 1 2 15
• cat gt new
• cp new 1
• rm -f new

12
Almost Right Answer

• PATH/bin/usr/bin
• case in
• 1)
• ) echo Usage 0 file 1gt2 exit 1
• esac
• new/tmp/0.1.
• old/tmp/0.2.
• trap rm -f new old exit 1 1 2 15
• cat gt new
• cp 1 old
• trap 1 2 15
• cp new 1
• rm -f new old

13
The Reason

• What would happen if we have syntax error
• sed s/abc/ABCg doc.txt overwrite doc.txt

14
The Right Answer

• OPATHPATH
• PATH/bin/usr/bin
• case in
• 01) echo Usage 0 file cmd args 1gt2 exit
  1
• esac
• file1shift
• new/tmp/0.1.old/tmp/0.2.
• trap rm -f new old exit 1 1 2 15
• if PATHOPATH _at_ gt new
• then
• cp file old
• trap 1 2 15
• cp new file
• else
• echo 0 1 failed, file unchanged gt 1gt2
• exit 1
• fi
• rm -f new old