CST8177 - Linux II
Review of Fundamentals (cont'd) 
 
Todd Kelley 
kelleyt@algonquincollege.com 

CST8207 - Todd Kelley 

1 





.change your password on CLS if you haven't already 

.the filesystem 

.access permissions 

.symbolic links 

.hard links 


 
     

2 

Topics





.Variables for general use (variables that are not environment variables) have lower case names 

.Environment variables are indicated by their UPPER CASE names: SHELL, VISUAL, etc 

.It's usually best to put variable expansions inside double quotes, to protect any special characters that might be inside the variable: 


echo "$somevar" 
.if somevar contained the * character, the double quotes stop the shell from globbing it 




Variables
CST8177 - Todd Kelley 

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.set the variable myvar to have value value 


myvar=value 
.Note, to make this variable setting visible in sub processes we use export 


export myvar=value 
or 
myvar=value 
export myvar 
 

Setting Variables
CST8177 - Todd Kelley 

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.set the myvar variable to have a null value, then run the value command with that variable setting in effect 



myvar= command 
 
.Notice that if you try mistakenly use this to try to set the value of myvar to value 


myvar= value 
 in this case you are actually trying to run a command called value 

Variable setting for command
CST8177 - Todd Kelley 

5 






The usual way to use this mechanism is something like 
VISUAL=nano vipd 
 
 
.This means to set the value of the environment VISUAL variable to nano, and use that while the vipd command runs 



Variable setting for command (cont'd)
CST8177 - Todd Kelley 

6 





.set the myvar variable to have a null value, then run the value command with that variable setting in effect 



myvar= value 
.run the myvar command with one argument, namely =value 



myvar =value 
 
.run the myvar command with two arguments, namely = and value 


myvar = value 
 

Setting Variables Mistakes
CST8177 - Todd Kelley 

7 







File Permissions
CST8177 - Todd Kelley 
8 






  
9 

Typical directory and file
inode 399303 
drwxr-xr-x 
access time 
modification time 
change time 
 etc  
 
. 
 inode 399303 
 
.. 
 inode 131074 
 
examples.desktop 
 inode 399306 
 
Assignments 
 inode 501292 
 
 etc  
  etc  
 

inode 399306 
-rw-r--r-- 
access time 
modification time 
change time 
 etc  
 
data blocks for the file 
there is no filename here 
the filename(s)  (at least one) are stored in directories 
 






 
File Permissions (cont'd)
CST8177 - Todd Kelley 
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inode 399303 
drwxr-xr-x 
access time 
modification time 
change time 
 etc  
 
. 
 inode 399303 
 
.. 
 inode 131074 
 
examples.desktop 
 inode 399306 
 
Assignments 
 inode 501292 
 
 etc  
  etc  
 

Need read (r) on directory to read this column 

Need search (x) on directory to access this column 





Need write (w) and search (x) on directory to change first column 






 
File Permissions (cont'd)
CST8177 - Todd Kelley 

11 
inode 399306 -rw-r--r-- access time modification time change time  etc   
data blocks for the file 
there is no filename here 
the filename(s)  (at least one) are stored in directories 
 

Need search (x) on directory this file is in to access this info on the file's inode 

Need read (r) / write (w) / execute (x) on file to read / write / execute this file (contents) 








File Attributes
CST8177 - Todd Kelley 

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.create a command with basic scripting 

.put "#!/bin/sh -u" at very beginning of file 

.put commands in file 

.make file executable   


.put the file in a directory that is in $PATH 

.http://teaching.idallen.ca/cst8207/13f/notes/400_search_path.html  

.Not a good  idea to put "." in PATH 

.Security implications of putting "current directory" , "." in PATH 

.PATH=.:$PATH 

.demonstration of how the bad guy can arrange for you to inadvertently run their malicious commands as you  


  

Extending Unix
CST8177 - Todd Kelley 
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