CSE II-Sem)

Transcription

1 a) Write a shell script that displays a list of all the files in the current directory to which the user has read, write and execute permissions. b) Develop an interactive script that asks for a word and a file name and then tells how many times that word occurred in the file. c) Write a shell script to perform the following string operations: i. To extract a sub-string from a given string. ii. To find the length of a given string. a) Write a shell script that displays a list of all the files in the current directory to which the user has read, write and execute permissions. Program: [CSESTAFF@localhost ex9]$ vi perm.sh echo "The list of File Names in the curent directory." echo "Which have Read,Write and Execute permisions. " for file in * do if [ -f $file ] then if [ -r $file -a -w $file -a -x $file ] then ls -l $file fi fi done Output: [CSESTAFF@localhost ex9]$ ls -l -rw-rw-r--. 1 CSESTAFF CSESTAFF 16 Jan 31 20:03 t1.txt [CSESTAFF@localhost ex9]$ chmod 777 t1.txt [CSESTAFF@localhost ex9]$ ls -l -rwxrwxrwx. 1 CSESTAFF CSESTAFF 16 Jan 31 20:03 t1.txt [CSESTAFF@localhost ex9]$ sh perm.sh The list of File Names in the curent directory. Which have Read,Write and Execute permisions. -rwxrwxrwx. 1 CSESTAFF CSESTAFF 16 Jan 31 20:03 t1.txt Page 1 of 9

2 b) Develop an interactive script that asks for a word and a file name and then tells how many times that word occurred in the file. Program: [CSESTAFF@localhost ex9]$ cat f1.txt day by day week by end week by week month by end month by month year by end but friendship is never end [CSESTAFF@localhost ex9]$ vi wcount.sh echo " Enter the word to be searched" read word echo "Enter the filename to be used" read flname echo "the no. of times the word ['$word'] occured in the file." grep -o $word $flname wc -l echo " the no of line that contains ['$word'] is" grep -c $word $flname Output: [CSESTAFF@localhost ex9]$ sh wcount.sh Enter the word to be searched by Enter the filename to be used f1.txt the no. of times the word ['by'] occured in the file. 6 the no of line that contains ['by'] is 3 Page 2 of 9

3 c) Write a shell script to perform the following string operations: a. To extract a sub-string from a given string. b. To find the length of a given string. Program: [CSESTAFF@localhost ex9]$ vi substr.sh echo "enter the string" read str strlen=${#str} echo "The length of the given string '$str' is:$strlen " echo "enter the string possibion in main str" read s1 echo "ending possition" read f1 echo $str cut -c$s1-$f1 [CSESTAFF@localhost ex9]$ Output: [CSESTAFF@localhost ex9]$ sh substr.sh enter the string krishnasai The length of the given string 'krishnasai' is:10 enter the string possibion in main str 8 ending possition 10 sai [CSESTAFF@localhost ex9]$ Page 3 of 9

4 File ownership is an important component of UNIX that provides a secure method for storing files. Every file in UNIX has the following attributes: Users A UNIX system serves many users. Users are an abstraction that denotes a logical entity for assignment of ownership and operation privileges over the system. A user may correspond to a real-world person, but also a type of system operation. So, in my system, I have user 'nick' that corresponds to me, but I also have user 'www' which corresponds to the privileges necessary to operate the local webserver. UNIX doesn't care about what the user means for me. It just knows what belongs to any given user and what each user is allowed to do with any given thing (file, program, device, etc) on the system. UNIX identifies each user by a User ID (UID) and the username (or login) such as 'nick' and 'www' is just an alias to the UID that makes humans more comfortable. Groups Users can be organized in groups. A user may belong to one or more groups of users. The concept of groups serves the purpose of assigning sets of privileges for a given resource and sharing them among many users that need to have them. (perhaps because they a re all members of a project working team and they all need access to some common project files) So, on my system user 'nick' and user 'www' both belong to the group 'perlfect'. This way, they can have some shared privileges over the files for this site. User 'nick' needs them to edit the site, and user 'www' needs them to manage the webserver that will be publishing the site. Ownership Every file in UNIX has an owner user and an owner group. So, for any file in the system, user 'nick' may have one of the following ownership relations: nick owns the file, i.e. the file's owner is 'nick'. nick is a member of the group that owns the file, i.e. the file's owner group is 'perlfect'. nick is neither the owner, nor belonging to the group that owns the file Permissions Every file on the system has associated with it a set of permissions. Permissions tell UNIX what can be done with that file and by whom. There are three things you can (or can't) do with a given file: read it, write (modify) it and Page 4 of 9

5 execute it. Unix permissions specify which of the above operations can be performed for any ownership relation with respect to the file. In simpler terms, what can the owner do, what can the owner group do, and what can everybody else do with the file. For any given ownership relation, we need three bits to specify access permissions: the first to denote read (r) access, the second to denote (w) access and the third to denote execute (x) access. We have three ownership relations: 'owner', 'group' and 'all' so we need a triplet for each, resulting in nine bits. Each bit can be set orclear. (not set) We mark a set bit by it's corresponding operation letter (r, w or x) and a clear bit by a dash (-) and put them all on a row. An example might be rwxr-xr-x.what this means is that the owner can do anything with the file, but group owners and the rest of the world can only read or execute it. Usually in UNIX there is also another bit that precedes this 9-bit pattern. You do not need to know about it, at least for the time being. So if you try ls -l on the command prompt you will get something like the following: [nick@thekla src]$ ls -l -rwxr-xr-x 1 nick users 382 Jan 19 11:49 bscoped.pl drwxr-xr-x 3 nick users 1024 Jan 19 11:19 lib/ -rwxr-xr-x 1 nick users 1874 Jan 19 10:23 socktest.pl The first column here shows the permission bit pattern for each file. The third column shows the owner, and the fourth column shows the owner group. By the time, the information provided by ls -l should be enough for you to figure out what each user of the system can do with any of the files in the directory. Directories Another interesting thing to note is that lib/ which is a directory has permissions, too. Permissions take a different meaning for directories. Here's what they mean: read determines if a user can view the directory's contents, i.e. do ls in it. write determines if a user can create new files or delete file in the directory. (Note here that this essentially means that a user with write access toa directory can delete files in the directory even if he/she doesn't have write permissions for the file! So be careful with this.) execute determines if the user can cd into the directory. Page 5 of 9

6 chmod To set/modify a file's permissions you need to use the chmod program. Of course, only the owner of a file may use chmod to alter a file's permissions. chmod has the following syntax: chmod [options] mode file(s) The 'mode' part specifies the new permissions for the file(s) that follow as arguments. A mode specifies which user's permissions should be changed, and afterwards which access types should be changed. Let's say for example: chmod a-x socktest.pl This means that the execute bit should be cleared (-) for all users. (owner, group and the rest of the world) The permissions start with a letter specifying what users should be affected by the change, this might be any of the following: u the owner user g the owner group o others (neither u, nor g) a all users This is followed by a change instruction which consists of a +(set bit) or -(clear bit) and the letter corresponding to the bit that should be changed. Let's see some examples: $ ls -l socktest.pl -rwxr-xr-x 1 nick users 1874 Jan 19 10:23 socktest.pl* $ chmod a-x socktest.pl $ ls -l socktest.pl -rw-r--r-- 1 nick users 1874 Jan 19 10:23 socktest.pl $ chmod g+w socktest.pl $ ls -l socktest.pl -rw-rw-r-- 1 nick users 1874 Jan 19 10:23 socktest.pl $ chmod ug+x socktest.pl $ ls -l socktest.pl -rwxrwxr-- 1 nick users 1874 Jan 19 10:23 socktest.pl* Page 6 of 9

7 $ chmod ug-wx socktest.pl $ ls -l socktest.pl -r--r--r-- 1 nick users 1874 Jan 19 10:23 socktest.pl Strange numbers... You might have encountered things like chmod 755 somefile and of course you will be wondering what this is. The thing is, that you can change the entire permission pattern of a file in one go using one number like the one in this example. Every mode has a corresponding code number, and as we shall see there is a very simple way to figure out what number corresponds to any mode. Every one of the three digits on the mode number corresponds to one of the three permission triplets. (u, g and o) Every permission bit in a triplet corresponds to a value: 4 for r, 2 for w, 1 for x. If the permission bit you add this value to the number of the permission triplet. If it is cleared, then you add nothing. (Some of you might notice that in fact, the number for a triplet is the octal value corresponding to the three-bit pattern - if you don't know what an octal value is, it doesn't really matter, just follow the intstructions) So if a file has rwxr-xrx permissions we do the following calculation: Triplet for u: rwx => = 7 Triplet for g: r-x => = 5 Tripler for o: r-x => = 5 Which makes : 755 So, 755 is a terse way to say 'I don't mind if other people read or run this file, but only I should be able to modify it' and 777 means 'everyone has full access to this file' chmod is used to change the permissions of files or directories. In general, chmod commands take the form: chmod options permissions filename If no options are specified, chmod modifies the permissions of the file specified byfilename to the permissions specified by permissions. permissions defines the permissions for the owner of the file (the "user"), members of the group who owns the file (the "group"), and anyone else ("others"). There are two ways to represent these permissions: with symbols ( alphanumeric characters), or withoctal numbers (the digits 0 through 7). Let's say you are the owner of a file named myfile, and you want to set its permissions so that: Page 7 of 9

8 1. the user can read, write, and execute it; 2. members of your group can read and execute it; and 3. others may only read it. This command will do the trick: chmod u=rwx,g=rx,o=r myfile This is an example of using symbolic permissions notation. The letters u, g, and o stand for "user", "group", and "other". The equals sign ("=") means "set the permissions exactly like this," and the letters "r", "w", and "x" stand for "read", "write", and "execute", respectively. The commas separate the different classes of permissions, and there are no spaces in between them. Here is the equivalent command using octal permissions notation: chmod 754 myfile Here the digits 7, 5, and 4 each individually represent the permissions for the user, group, and others, in that order. Each digit is a combination of the numbers 4, 2, 1, and0: 4 stands for "read", 2 stands for "write", 1 stands for "execute", and 0 stands for "no permission." So 7 is the combination of permissions (read, write, and execute), 5 is 4+0+1(read, no write, and execute), and 4 is (read, no write, and no execute). Following are the symbolic representation of three different roles: u is for user, g is for group, and o is for others. Following are the symbolic representation of three different permissions: r is for read permission, w is for write permission, x is for execute permission. Following are few examples on how to use the symbolic representation on chmod. 1. Add single permission to a file/directory Changing permission to a single set. + symbol means adding permission. For example, do the following to give execute permission for the user irrespective of anything else: $ chmod u+x filename Page 8 of 9

9 2. Add multiple permission to a file/directory Use comma to separate the multiple permission sets as shown below. $ chmod u+r,g+x filename 3. Remove permission from a file/directory Following example removes read and write permission for the user. $ chmod u-rx filename 4. Change permission for all roles on a file/directory Following example assigns execute privilege to user, group and others (basically anybody can execute this file). $ chmod a+x filename 5. Make permission for a file same as another file (using reference) If you want to change a file permission same as another file, use the reference option as shown below. In this example, file2 s permission will be set exactly same as file1 s permission. $ chmod --reference=file1 file2 6. Apply the permission to all the files under a directory recursively Use option -R to change the permission recursively as shown below. $ chmod -R 755 directory-name/ 7. Change execute permission only on the directories (files are not affected) On a particular directory if you have multiple sub-directories and files, the following command will assign execute permission only to all the sub-directories in the current directory (not the files in the current directory). $ chmod u+x * Page 9 of 9