1. Shell Programming (ch 10)
IT244 - Introduction to Linux / Unix
Instructor: Bo Sheng

2. Vim Commands vim filename Add contents: i/a Back to command mode: ESC
Save the file: :w
Delete: x
Quit: :q

3. Control Flow
The order of executing commands
if..then

4. Control Flow The order of executing commands if..then Example
~it244/it244/scripts/if1

5. Control Flow The order of executing commands if..then
Built-in utility test
man test
Example (-eq)
~it244/it244/scripts/chkargs
$#: the number of arguments
exit 1: terminate with a status code ‘1’

6. Control Flow The order of executing commands if..then Read arguments
Example
~it244/it244/scripts/read3args

7. Control Flow The order of executing commands if..then
test (-f) and arguments
Example
~it244/it244/scripts/is_ordfile
Other options (Table 10-1)

8. Control Flow The order of executing commands if..then
test ([ ]) and arguments
Example
~it244/it244/scripts/chkargs2
White spaces [ $# -eq 0 ]
Output error message: echo … … 1>&2

9. Control Flow
The order of executing commands
if..then..else

10. Control Flow The order of executing commands if..then..else
Read arguments
Example ( shift )
~it244/it244/scripts/shift2args
Example ( )
~it244/it244/scripts/allargs

11. Control Flow The order of executing commands if..then..else
Optional arguments
Example
~it244/it244/scripts/out

12. Control Structure
if..then..elif

13. Control Structure if..then..elif 1 & 2 & 3, 1 & 2, 1 & 3, 2 & 3,
Example: Compare 3 input words and find the match (word1, word2, word3)
1 & 2 & 3,
1 & 2,
1 & 3,
2 & 3,
no match

14. Control Structure if..then..elif
Example: Compare 3 input words and find the match (word1, word2, word3)
if word1=word2 =word3
then …
elif word1=word2
elif word1=word3
elif word2=word3
else fi
1 & 2 & 3
1 & 2
1 & 3
2 & 2
No match

15. Control Structure if..then..elif
Example: Compare 3 input words and find the match (word1, word2, word3)
Combine two conditions
test condition1 -a condition2
test condition1 -o condition2
~it244/it244/scripts/if3

16. Control Structure for loop for loop-index in argument-list do commands
done
Example
~it244/it244/scripts/fruit
~it244/it244/scripts/dirfiles

17. Control Structure for loop A range for argument-list Example
{1..10}, {a..z}
for loop-index in range do
commands
done
Example
~it244/it244/scripts/for_range
~it244/it244/scripts/for_range2

18. Control Structure for loop Repeat operations
Example
~it244/it244/scripts/for_print
Nested loops
~it244/it244/scripts/for_print2

19. Control Structure for loop for loop-index (in $@) do commands done
Example
~it244/it244/scripts/for_test

20. Control Structure
while loop
while test-command do
commands
done

21. Control Structure while loop while test-command do commands done
Example: ~it244/it244/scripts/count
Arithmetic evaluation: ((a=b+2))
~it244/it244/scripts/for_while_print

22. Control Structure
until loop
until test-command do
commands
done

23. Control Structure until loop until test-command do commands done
Example: ~it244/it244/scripts/guessnumber
~it244/it244/scripts/guessnumber2

24. Control Structure break and continue break: jump out of the loop
continue: proceed to “done”
Example: ~it244/it244/scripts/brk

25. Control Structure select select varname [in arg …] do commands done
Example
~it244/it244/scripts/select1

26. Control Structure select select varname [in arg …] do commands done
PS3: select prompt
$REPLY: index of the selected item
Terminate: exit, break
Example: ~it244/it244/scripts/select2

27. Parameters and Variables
Special parameters
$#: number of arguments
$1-$n: arguments
$0: name of the script
Example: ~it244/it244/scripts/abc
./abc
~it244/it244/scripts/abc
basename: ~it244/it244/scripts/abc2
basename ~it244/it244/scripts/abc

28. Parameters and Variables
Special parameters
$$: PID number
echo $$ ps
Example: ~it244/it244/scripts/id2
./id2

29. Parameters and Variables
Special parameters
$?: exit status
ls; echo $?
ls xyz; echo $?
cat yyy; echo $?
Example: ~it244/it244/scripts/es
./es
echo $?

30. Parameters and Variables
Initialize command-line arguments
set arg1 arg2 …
~it244/it244/scripts/set_it
date
~it244/it244/scripts/dateset

31. Some More Examples Get file sizes ~/it244/it244/scripts/filesize
$ls -l
-rwxr-xr-x 1 it244 libuuid :09 abc
-rwxr-xr-x 1 it244 libuuid :10 abc2
-rwxr-xr-x 1 it244 libuuid :49 bb1
-rwxr-xr-x 1 it244 libuuid :49 bb2
… … …
for i in * do
set -- $(ls -l "$i")
echo $i: $5 bytes
done

32. File Descriptors An index number of a open file
3 system file descriptors
0(std input), 1(std output), 2(std err)
Create a file descriptor
exec n > outfile
exec m < infile
exec n<>file
exec n>>outfile

33. File Descriptors Duplicate a file descriptor Close a file descriptor
exec n<&m
exec n>&m
Close a file descriptor
exec n<&-
Example: ~it244/it244/scripts/exec_read
~it244/it244/scripts/exec_write

34. File Descriptors Example: ~it244/it244/scripts/exec_read
while read <&3 myline do
echo "*** $myline ***"
done
File descriptor is “3” ($1 is the first argument)
&3 represents the input file in the program (a pointer)
“read <&3 myline” read a line from the file and assign it to variable “myline” (pointer &3 moves to the next line)

35. File Descriptors Example: ~it244/it244/scripts/exec_read
while read <&3 myline do
echo "*** $myline ***"
done
Command as a condition
True: if the exit status is 0 (successfully executed)
False: if the exit status > 0 (sth went wrong)
“read” gets an error when EOF is reached

36. Built-in Commands Do not fork a new process “read” user input
type cat echo who if
“read” user input
read firstline
echo “$firstline”
~it244/it244/scripts/read1
~it244/it244/scripts/read1_no_quote
type in this is *

37. Built-in Commands “read” user input
“REPLY”: default variable for “read”
~it244/it244/scripts/read1a
“-p” option: with prompt
read -p “Enter a command: ” cmd
~it244/it244/scripts/read2
“read” multiple variables
~it244/it244/scripts/read3
type in this is sth else

38. Built-in Commands “read” user input “read” from files
read line1 < names
echo $line1
read line2 < names
echo $line2
(read line1; echo $line1;
read line2; echo $line2) < names

39. Built-in Commands “read” user input “read” from files
~it244/it244/scripts/readfile
./readfile < names
-u option
exec 3<names
read –u3 line1 (read <&3 line1)
echo $line1
read –u3 line2
echo $line2
exec 3<&-

40. Built-in Commands “trap” : catch a signal
Signal: a report about a condition (Table 10-5)
kill -l, trap -l, man 7 signal
trap [‘commands’] [signal]
‘’ is not required but highly suggested
Example: ~it244/it244/scripts/inter

41. Built-in Commands “trap” : catch a signal
trap ‘’ signal: ignore the signal
Example
2: CTRL+C, 20: CTRL+Z
~it244/it244/scripts/locktty
stty (input secret/passwd)
~it244/it244/scripts/locktty2
Multiple traps
~it244/it244/scripts/cleanfile

42. Built-in Commands “kill” : abort a process kill [-signal] PID
The KILL signal (9)
kill -9 %1

43. Expressions Arithmetic evaluation a=10; b=5; let a=a*10+b; echo $a
No path expansion (*)
((a=a*10+b))
let “a=a+1” b=a*b
((a=a+1, b=a*b))

44. Expressions Logic expression (( )), [[ ]]
In [[]], must precede a variable name with $
>, <, -lt, -gt, …
&& (and), || (or)
~it244/it244/scripts/age2
String comparison
[[ “aa” < “bbb” ]]
[[ artist = a* ]]; echo $?
[[ a* = artist ]]; echo $?

45. Expressions Arithmetic operators +,-,*,/,% var++, var--, ++var, --var
a=5;b=3;echo $((a%b))
var++, var--, ++var, --var
echo $((a++)); echo $a
echo $((--a)); echo $a
echo $((a+++3))
echo $((a++---b))
~it244/it244/scripts/printstar

46. Expressions Logical operators true, false !, ==, != &&, ||
true; echo $?; false; echo $?
!, ==, !=
((1==1)); echo $?; echo $((1==1))
((1!=1)); echo $?; echo $((1!=1))
&&, ||
true && false; echo $?
true || false; echo $?
ls –l testop && cat testop; echo $?
ls –l testop || cat testop; echo $?

47. Expressions Logical operators &&, || Short-circuiting: Example
Cond1 && Cond2: does not evaluate Cond2 if Cond1 is false
Cond1 || Cond2: does not evaluate Cond2 if Cond1 is true
Example
mkdir bkup && cp testop bkup
rm bkup/*
mkdir bkup && cp testop bkup; ls bkup
mkdir bkup || echo “mkdir failed”
a=1;b=2;echo $((a||b++));echo $b