03. Shell Programming (part 2)

Make 03_ShellProgramming subdirectory and store all the code there.

As always, part of this tasks is about reading [[man1:man|manual pages

1. while.sh

      1  #!/bin/sh
      2 
      3  A=.
      4  while echo "Run"; ls $A 2> /dev/null; do
      5          read A
      6  done
      7  echo "Exit status: $?"
   

   • Like if, while executes a command chain (echo "Run"; ls $A in example) and check exit status of last command executed (ls)

     • if it's 0 (successful execution) this means true

     • if it is non-zero, this means false (when ls got an error

   • exit status on any last command executed is stored to «?» variable

   • (optionally) you can use && and || instead of ; to force all exit statuses into account, e. g. cmd1 || cmd2 is true if either of cmd1 or cmd2 was successful, and cmd && cmd2 && cmd3 is true only if all commands was successful.

2. (was in homework, but can get it from help read). Write a program while2.sh that simulates cat: reads from standard input (one read per line):

      1 $ fortune > file
      2 $ cat < file
      3 Isn't it ironic that many men spend a great part of their lives
      4 avoiding marriage while single-mindedly pursuing those things that
      5 would make them better prospects?
      6 $ sh while2.sh < file
      7 Isn't it ironic that many men spend a great part of their lives
      8 avoiding marriage while single-mindedly pursuing those things that
      9 would make them better prospects?
     10 
   

   • Also: what we ought to do to make ./whil2.sh run?

3. test. Can compare strings, numbers, check is string is a real file name etc.

   • Research: read the manual page. What these commands do:

   • try test $A = $B ; echo $? for various A and B

     • why we got an error if either A or B is empty?

   • $A -lt $B

   • -f $A

   • -z "$A"

4. Write a while3.sh script that print all the lines from standard input, but stops if QUIT is entered

   • (optional) it must also stop when standard input is closed

5. for.sh

      1 #!/bin/sh
      2 
      3 for n in 1 2 3 10 20 30 qwe asd xcv; do
      4         echo "Next value $n"
      5 done
      6 
      7 echo "$#" / "$*"
      8 for n ; do
      9         echo "What is this: $n?"
     10 done
   

   • The for cycle is like python's (strictly speaking, python's for is like shell's one ). It assigns to a variable (n in example) all words from word sequence

   • Research:

     • What are # and * variables?

     • What the for variable; construction do?

       1. You can read all the way though bash

       2. or you can just play with this script a little

6. Functions:
   • Function in shell is like a (temporally defined) command
   • Arguments are passing to the function just like command line arguments does

   • inside a function all the command-line aware constructions (like $0, $1, $#, $* etc) deals with function arguments instead

   • See funct.sh file:

      1 #!/bin/sh
      2 
      3 fun() {
      4   echo "$0: $# $*"
      5   for arg; do
      6         echo "> $arg"
      7   done
      8 }
      9 
     10 fun 1   QWE  "3 4"
     11 fun "$*"
     12 fun  $*
     13 fun "$@"
   

   • Research:

     • Describe the difference between "$*", $*, and especially "$@" (yes, it is special construction)

7. Reminder. Command output substitution, `command` and $(command) forms (equivalent)

   •    1 $ date
        2 Чт апр 23 20:13:35 MSK 2020
        3 $ A=`date`
        4 $ echo $A
        5 Чт апр 23 20:13:44 MSK 2020
        6 $ B=$(cal)
        7 $ echo $B
        8 Апрель 2020 Пн Вт Ср Чт Пт Сб Вс 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
        9 $ echo "$B"
       10      Апрель 2020
       11 Пн Вт Ср Чт Пт Сб Вс
       12        1  2  3  4  5
       13  6  7  8  9 10 11 12
       14 13 14 15 16 17 18 19
       15 20 21 22 23 24 25 26
       16 27 28 29 30
       17 
     

     • Why echo $B looks like this?

   • Create funct2.sh program

      1 #!/bin/sh
      2 
      3 sum() {
      4   # insert your code here
      5 }
      6 
      7 while read a b; do
      8         sum $a $b
      9 done
   

   • Note how read var1 var2 … varN works

   • read expr and improve the program so it will print the sum of $a and $b. Assume input is always correct

8. Copy funct2.sh to funct3.sh and modify it so it shall also stop if $A+$B>100.

   • Hint: use substitution here.

   • You can use break to quit from cycle

   • Hint: debug shell script with sh -x shellscript.sh

H/W

• Complete all tasks

• expr again

• Write a shell script sumsum.sh that

  • Has function sum() that

    • sums all of it's arguments (any number of arguments are permitted)
    • prints this sum if there's no errors

    • prints 0 if there was error (e. g. try to sum non-numbers)

    • redirects all error messages to /dev/null

  • Reads two lines of numbers
  • Prints if their sums was equal or not

       1 $ ./sumsum.sh
       2 1 3 5
       3 2 4 6
       4 Not equal
       5 $ ./sumsum.sh
       6 1 5 6
       7 4 4 4
       8 Equal
       9 $ ./sumsum.sh
      10 1 2 w
      11 3 4 e
      12 Equal
      13 $ ./sumsum.sh
      14 1 2 1
      15 2 2 Q
      16 Not equal
      17 $ ./sumsum.sh
      18 qwe 3 4
      19 10 20 -30
      20 Equal
      21