Lab46 Wiki

A shell script is an executable text file that contains a list of commands and operations to be performed by a command line interpreter.

#!/bin/bash
echo "Please enter your birth year"
read birth
let year=`date +%Y`
let age=$year-$birth
echo $age

lab46:~$ ./age.sh
Please enter your birth year
1986
26

Filtering can be applied to a set of data in order to exclude unnecessary information.

This example shows how a filter can be applied to display only the first 3 lines of a text file.

lab46:~$ head -3 sample.db
name:sid:major:year:favorite candy*
Jim Smith:105743:Economics:Sophomore:Lollipops*
Adelle Wilson:594893:Sociology:Junior:Ju-Ju Fish*

Regular files is a term used to distinguish them from other types of files called special files. Regular files are specified by a “-: for the file flag in a long directory listing. Types of regular files are text files, binary data files, and executable files.

A long listing displaying a regular file.

lab46:~$ ls -l file
-rw-r----- 1 rhensen lab46 90 May  8 21:46 file

A directory is a file type that is able to contain other files. A directory is marked with a “d” for the file flag in a long directory listing. Directories are the most common type of special files found in Unix.

A long listing of a directory file.

lab46:~$ ls -ld bin
drwxr-xr-x 2 rhensen lab46 18 Mar  9 22:49 bin

Permissions specify how users are able to access a file. Permissions include being able to read, write to, and search or execute a file depending on the type of file it is. Permissions are defined separately for the owner of the file, the security group associated with the file, and everyone else. These are displayed symbolically as a list of three sets of rwx bits.

A long directory listing displays the symbolic permissions for each type of user. The first set applies to the owner of the file, the second to the group, and the third to the world. The owner of the file and the group associated with it are displayed afterwards.

drwxr-xr-x 2 rhensen lab46    4096 Apr  7 02:10 courselist
-rw-r--r-- 1 rhensen lab46     666 Feb  9 21:50 courses.html
-rwxrwxrwx 1 rhensen lab46     794 May  3 13:03 cs0xd.sh
-rw-r----- 1 rhensen lab46    8186 Apr 19 16:59 data.file

The umask is a value that can be specified by the user to set the permissions for new files that are created. A umask specifies which permissions should not be given when a new file is created.

This shows that when a umask is set to 000 (will not change default permissions) a regular file is created with the permission 666. When a umas of 022 is set, a new regular file that is created has a permission of 644.

lab46:~$ umask 000
lab46:~$ touch test1
lab46:~$ ls -l test1
-rw-rw-rw- 1 rhensen lab46 0 May  9 23:17 test1
lab46:~$ umask 022
lab46:~$ touch test2
lab46:~$ ls -l test2
-rw-r--r-- 1 rhensen lab46 0 May  9 23:18 test2

The data dump program (dd) can be used to transfer data from one file into another file. The user has a great deal of control when specifying which bytes of data are to be copied over and where to place them in the output file.

An example of dd being used to extract all the information of a file called “pattern” into a file called “test1”.

lab46:~$ dd if=pattern of=test1
0+1 records in
0+1 records out
30 bytes (30 B) copied, 0.0437978 s, 0.7 kB/s 

This concept is similar to a text editor, only dealing with binary data instead of text. A binary editor, such as bvi, allows a user to view and edit each byte of a file.

This is what a screen in bvi looks like. The leftmost column is the line numbers. The far right is the ASCII equivalent of the binary data (although this is typically meaningless when not dealing with text files). The middle shows the values of the bytes written in hexadecimal.

00000000  6E 61 6D 65 3A 73 69 64 3A 6D 61 6A 6F 72 3A 79 name:sid:major:y
00000010  65 61 72 3A 66 61 76 6F 72 69 74 65 20 63 61 6E ear:favorite can
00000020  64 79 2A 0A 4A 69 6D 20 53 6D 69 74 68 3A 31 30 dy*.Jim Smith:10
00000030  35 37 34 33 3A 45 63 6F 6E 6F 6D 69 63 73 3A 53 5743:Economics:S
00000040  6F 70 68 6F 6D 6F 72 65 3A 4C 6F 6C 6C 69 70 6F ophomore:Lollipo
00000050  70 73 2A 0A 41 64 65 6C 6C 65 20 57 69 6C 73 6F ps*.Adelle Wilso
00000060  6E 3A 35 39 34 38 39 33 3A 53 6F 63 69 6F 6C 6F n:594893:Sociolo
00000070  67 79 3A 4A 75 6E 69 6F 72 3A 4A 75 2D 4A 75 20 gy:Junior:Ju-Ju
00000080  46 69 73 68 2A 0A 53 61 72 61 68 20 42 69 6C 6C Fish*.Sarah Bill
00000090  69 6E 67 73 3A 39 33 38 33 38 39 3A 41 63 63 6F ings:938389:Acco
000000A0  75 6E 74 69 6E 67 3A 46 72 65 73 68 6D 61 6E 3A unting:Freshman:
000000B0  54 69 63 2D 54 61 63 73 2A 0A 45 72 69 63 20 56 Tic-Tacs*.Eric V
000000C0  69 6E 63 65 6E 74 3A 31 30 30 31 31 31 39 3A 42 incent:1001119:B
000000D0  69 6F 6C 6F 67 79 3A 46 72 65 73 68 6D 61 6E 3A iology:Freshman:
000000E0  4C 6F 6C 6C 69 70 6F 70 73 2A 0A 4C 69 6E 75 73 Lollipops*.Linus
000000F0  20 54 6F 72 76 61 6C 64 73 3A 34 34 33 32 30 30  Torvalds:443200
00000100  31 3A 43 6F 6D 70 75 74 65 72 20 53 63 69 65 6E 1:Computer Scien
00000110  63 65 3A 53 65 6E 69 6F 72 3A 53 6E 69 63 6B 65 ce:Senior:Snicke
00000120  72 73 2A 0A 41 6C 61 6E 20 43 6F 78 3A 34 30 30 rs*.Alan Cox:400
00000130  34 39 33 30 30 3A 43 6F 6D 70 75 74 65 72 20 53 49300:Computer S
00000140  63 69 65 6E 63 65 3A 53 65 6E 69 6F 72 3A 57 68 cience:Senior:Wh
00000150  6F 70 70 65 72 73 2A 0A 41 6C 61 6E 20 54 75 72 oppers*.Alan Tur
00000160  69 6E 67 3A 34 30 30 33 30 33 33 33 3A 43 6F 6D ing:40030333:Com
"sample.db" 898 bytes                          00000000  \156 0x6E 110 'n'

To become familiar with more complex Unix concepts.

My objective for this portion of the semester was to become more familiar with some of the more complex concepts in Unix, such as regular expressions and scripting.

To determine my progress in this area I will look at my comprehension of how to use the different symbols of regular expressions. It is also useful to be able to think in terms of regular expressions and use them to obtain desired results.

I will be able to measure my progress by looking at how I am doing at solving exercises that involve the use of regular expressions and writing scripts.

I believe that in this portion of the semester I have become more proficient in using regular expressions effectively. When the concept was first introduced to me it reminded me of wildcards, which are also used to find matches to patterns. I felt that wildcards were fairly easy to use and straightforward. Understanding how to use them has made regular expressions easier, however the complexity of RegEx is much greater and there is much more that can be done with them. I feel that after working through the various labs in this portion of the semester I have a better understanding of how regular expressions are used. I believe I still sometimes forget the way that different tools handle RegEx differently, such as grep being unable to handle extended characters. I still find regular expressions challenging especially when implementing them into a script to perform complex functions, but I feel that I have a good understanding of the concept and can use them effectively.