Consider the following program.

Transcription

1 Consider the following program. #include <stdio.h> int do_sth (char *s); main(){ char arr [] = "We are the World"; printf ("%d\n", do_sth(arr)); } int do_sth(char *s) { char *p = s; while ( *s++!= \0 ) return s - p; } What does function do_sth intend to do? What is the output of the program?

2 In the linked list implementations in labtest 2 and lab, to make the code easier, we declare the head to be a global variable. An alternative way is to declare head as a local variable and pass it as a function parameter. Write a function return_type insertfirst(struct node* head, char d) which inserts in the beginning of the list a new node with data d. You need to define the return type of the function. You also needs to call this function to insert nodes with data A and E into the list. #include <stdio.h> struct node { }; char data; struct node *next; /************* DO NOT ADD MORE GLOBAL VARIABLES *************/ /*Define your function below:*/ struct node * insertfirst (struct node * list_head, char d){ /***** ADD YOUR CODE HERE *****/ struct node * newp = malloc (sizeof(struct node)); newp -> data = d; newp -> next = list_head; list_head = newp; } return list_head; main(){ struct node * head; /* call your function (twice) to insert nodes of data A E into list */ head = insertfirst (head, A ); head = insertfirst (head, E ); }

3 Question 7 short answers What are the four types of files maintained by the Unix file system? Regular file; directory; device file; link file Consider the following file system structure. Assume the current working directory is /home/will/play. Suppose there is a file called foo in /home/will/work. Using a relative path, and without changing the current working directoy, copy foo to /usr/bin cp../work/foo../../../usr/bin/ What does this utility do? find../ -mtime 3 exec ln s {} {}2 \; find all the files that has been modified within the 3 days, and then create a soft (symbolic) link of it named name2 The who utility displays who is logged on. An example is shown below: john pts/10 Jul 29 14:21 (deepthought.cs.yorku.ca) alice pts/13 Aug 1 21:50 (bas1-toronto dsl.bell.ca) xijunw pts/15 Jul 30 00:12 (grad resnet.yorku.ca) rouhan pts/16 Aug 1 15:10 (cpe0e.net.cable.rogers.com) midas pts/24 Jul 22 15:17 (monster.cs.yorku.ca) gaodon pts/26 Jul 31 16:01 ( ) tom pts/32 Jul 31 11:01 ( ) peter pts/37 Jul 27 10:09 ( )

4 Give one line of Unix commands/utilities, possibly connected by pipes but without using redirection (> symbol) and grouping ( ; symbol), to find and displays who is currently logged on, and sort and displays it based on the log on date and time (First by the month. For the same month records, sort by the date. For the same date records, sort by the time). The sorted results of the above list is shown below midas pts/24 Jul 22 15:17 (monster.cs.yorku.ca) peter pts/37 Jul 27 10:09 ( ) john pts/10 Jul 29 14:21 (deepthought.cs.yorku.ca) xijunw pts/15 Jul 30 00:12 (grad resnet.yorku.ca) tom pts/32 Jul 31 11:01 ( ) gaodon pts/26 Jul 31 16:01 ( ) rouhan pts/16 Aug 1 15:10 (cpe0e.net.cable.rogers.com) alice pts/13 Aug 1 21:50 (bas1-toronto dsl.bell.ca) Your solution should display on stdout both the original list and (followed by) the sorted list who tee /dev/tty sort +2M -3 +3n or who tee /dev/tty sort -k 3,3M -k 4,4n -k 5,5 Question 8 grep Assume that each non-empty line of file /etc/passwd file has the form username:password:uid:gid:gcos-field:home-dir:login-shell and that none of the fields uses : symbol, as shown in the following example huiwang:x:9226:3000:hui Wang:/cs/home/huiwang:/cs/local/bin/tcsh cse9100:x:11909:10000:cindy White:/cs/home/cse9100:/cs/local/bin/csh 8.1 Give Unix command(s)/utilitie(s), possibly connected by pipes but without using redirection (> symbol) and grouping ( ; symbol), to display the records in /etc/passwd whose user gid is and whose login-shell is csh. egrep ^[^:]*:[^:]*:[^:]*:3000: /etc/passwd egrep /bin/csh$ awk -F: '{print $4, $7}' passwd grep '^3000 ' grep /csh gives partial

5 8.2 Give Unix command(s)/utilitie(s), possibly connected by pipes but without using redirection (> symbol) and grouping ( ; symbol), to count the number of empty lines in /etc/passwd. egrep ^$ /etc/passwd wc -l Question 9 Write a Bourne shell script that takes as arguments first_name last_name job where job is a verb (such as read, write or meet), and then outputs Hello last_name, first_name. Your job today is jobing. If less than 3 arguments are provided, your script should output an error message as shown below. Sample input/ output (assume the script is named q9script): indigo 407 % q9script Cindy Usage: q9script first_name last_name job indigo 408 % q9script Cindy White read Hello White, Cindy. Your plan today is reading Don t use redirection (> symbol) #!/bin/sh if [ $# -lt 3 ] then echo Usage: $0 first_name last_name job exit 1 fi name=$1 name2=$2 job=$3 echo Hello $name2, $name. Your plan is ${job}ing # or $job"ing"

6 Question 10 Write an Bourne shell script that takes as arguments a file name, starting and ending line numbers, and displays all the lines between the given line numbers. Line numbers start from 1. For example, q10script foo 2 7 should display line 2 to line 7 of file foo. Assume that foo exists, and that both the line numbers are positive numbers greater or equal to 1. However, your program should still check if the line range is valid. Line range is valid if both the line numbers exist in the file, and that the starting line is not greater than the ending line. In other cases, the program should output an error message invalid range specification. Note: you will lose partial mark for using awk, cut or sed, since there are easier approaches. Don t use redirection (> symbol). #! /bin/sh lines=`cat $1 wc -l` if [ $2 -le $lines ] && [ $3 -le $lines ] && [ $2 -le $3 ] then tailmany=`expr $lines - $2 + 1` headmany=`expr $3 - $2 + 1` tail -$tailmany $1 head -$headmany else fi echo 'invalid range specification'

7 Aug 27, 2015 Name: Student ID: Page 17 of 15 C Operator Precedence (from high to low) and Associativity. Use the space below if you need space for your answers. Specify the question number.

8 26/08/2015 UNIX Reference Sheets Utilities II advanced utilities Regular Expression grep/egrep sort cut -v reverse -w whole word only -i case insensitive Start column 1, default delimiter: blank/tab grep -w v i ^[Tt]he file123 sort t: -k4 -r -n/m file cut -d -f2,3 -r reverse -n numerical -M month Start column 1 Default delimiter: tab awk awk F : {print $1,$2} Start column 1 Default delimiter blank/tab 1

9 26/08/2015 Some pre-defined Variables $# represents the number of command line arguments $* represents all the command line arguments represents all the command line arguments $$ represents the process ID of the shell $? represents the exit status code of the command lasxecuted /dev/null a special file (black hole) that discards / absorb all data written to it Unix file descriptors: 0,1,2 0: stdin 1: stdout 2: stderr 2

10 26/08/2015 if Statement and test Command if [ condition ] then command(s) elif condition_2 then command(s) else command(s) fi while Loops while [ condition ] do command(s) done Command test is often used in condition. Execute command(s)when condition is met. 3