1. CIT 383: Administrative Scripting
Files and Directories
CIT 383: Administrative Scripting

2. CIT 383: Administrative Scripting
Topics
Creating File Objects
Reading Files
Writing Files
Directories
Inodes
File Management
Command Line Arguments
CIT 383: Administrative Scripting

3. CIT 383: Administrative Scripting
Files
File class constructor arguments
Pathname
Access type
a: append
a+: read-write (starts at end of file if file exists)
r: read
r+: read-write
w: write (truncate file to zero length)
w+: read-write (truncate file to zero length)
Examples
pwfile = File.new(‘/etc/passwd’, ‘r’)
usrfile = File.new(‘userlist’, ‘w’)
logfile = File.new(‘/var/log/mylog’, ‘a’)
CIT 383: Administrative Scripting

4. CIT 383: Administrative Scripting
Reading Files
getc: reads a single character at current pos
gets: reads a single line at current pos
seek: changes position in file
tell: returns position in file
read: reads entire file as a string
readlines: reads file as an array of lines
CIT 383: Administrative Scripting

5. Reading a file line by line
File class each_line iterator
fh.each_line do |line|
# do stuff with line here
end
While loop
while line = fh.gets
CIT 383: Administrative Scripting

6. CIT 383: Administrative Scripting
Writing Files
putc: writes a single character at current pos
print: writes a single line at current pos
puts: writes line at current pos with newline
seek: changes position in file
tell: returns position in file
write: writes to file, returns bytes written
CIT 383: Administrative Scripting

7. CIT 383: Administrative Scripting
Line Endings
Getting rid of line end characters
line_without_ending = line.chomp OR
line.chomp!
Outputting line ending characters
puts inserts a line ending
print does not add a line ending
CIT 383: Administrative Scripting

8. CIT 383: Administrative Scripting
Global File Objects
$stdin
Default object for getc and gets
$stdout
Default object for puts
$stderr
CIT 383: Administrative Scripting

9. CIT 383: Administrative Scripting
Closing Files
After reading/writing to a file, close it.
file.close
What if an error kills program before close?
Use open instead of new to auto close
File.open(pathname,’r’) do |fh|
fh.gets
end
CIT 383: Administrative Scripting

10. CIT 383: Administrative Scripting
Data Munging
Data munging: to convert data from one format to another format, possibly sorting, summarizing, or otherwise modifying it in the process.
CIT 383: Administrative Scripting

11. CIT 383: Administrative Scripting
Skipping lines
Skipping blank lines
file.each_line do |line|
next if line == "\n"
puts line
end
Skipping lines beginning with a character
next if line[0] == ?#
CIT 383: Administrative Scripting

12. CIT 383: Administrative Scripting
Delimited Data
pwfields = [:username, :password, :uid, :gid, :gcos, :homedir, :shell]
pwline = ‘root:x:0:0:root:/root:/bin/sh’
pwitems = pwline.split(‘:’)
pwhash = Hash.new
i = 0
while i < pwitems.size
pwhash[pwfields[i]] = pwitems[i]
i = i + 1
end
CIT 383: Administrative Scripting

13. CIT 383: Administrative Scripting
Directories
Directory: table of name to inode mappings
$ ls –i /
bin
65409 boot
49057 cdrom
2260 dev
etc
2 home
initrd
49075 initrd.img
49058 initrd.img.old
lib
CIT 383: Administrative Scripting

14. Accessing Directories
thisdir = Dir.new(pathname)
absolutedir = Dir.new(‘/home/smi/dir1’)
relativedir = Dir.new(‘smi/dir1’)
currentdir = Dir.new(‘.’)
uponedir = Dir.new(‘..’)
Methods
entries: returns array of filenames
each: iterates over each file in directory
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15. CIT 383: Administrative Scripting
Inodes
An inode is a disk structure that contains
Size of the file in bytes.
Device ID that identifies device where file is located.
UID of the file’s owner.
GID of the file’s group.
File mode (permissions.)
Timestamps
ctime: inode change time
mtime: time file content was last modified
atime: time file content was last accessed
Reference count that identifies how many directory entries point to inode.
Pointers to disk blocks containing file data.
CIT 383: Administrative Scripting

16. Hard and Symbolic Links
A hard link is
A directory entry that points to an inode.
Deleting a file with rm just removes the directory entry.
File data is not removed until all links removed.
A symbolic link is
A file that contains a pathname.
Deleting the link does not affect the file.
Deleting a file with rm invalidates the symbolic link but does not remove it from filesystem.
CIT 383: Administrative Scripting

17. Basic File Permissions
Three sets of permissions: owner, group, world
Each set represented by an octal digit.
Each permission (r,w,x) one bit in octal digit.
Special permissions: setuid, setgid, sticky.
ex: chmod 0644 file
u: rw, g: r, o: r
ex: chmod 0711 bin
u: rwx, g: x, o: x 4
read
setuid 2
write
setgid 1
execute
sticky
CIT 383: Administrative Scripting

18. CIT 383: Administrative Scripting
File::Stat
The File::Stat class allows access to inodes
stat = File::Stat.new(pathname) OR
file = File.new(pathname)
stat = file.stat
Methods
size: size of file in bytes
uid: UID of file owner
gid: GID of file owner
mode: file permissions in octal
mtime,ctime,atime: file timestamps
CIT 383: Administrative Scripting

19. Querying File Attributes
directory? – Is file a directory?
symlink? – Is file a symbolic link?
file? – Is file an ordinary file (not a directory, device, or symlink)?
readable? – Is file readable by me?
writable? – Is file writable by me?
executable? – Is file executable by me?
setuid? – Is file setuid?
setgid? – Is file setgid?
sticky? – Is the sticky bit set?
CIT 383: Administrative Scripting

20. CIT 383: Administrative Scripting
File Management
The FileUtils class provides methods that emulate the functionality of UNIX commands:
chmod
chown cp ln
mkdir mv rm
rmdir
CIT 383: Administrative Scripting

21. Command Line Arguments
Using command line arguments
./test.rb arg1 arg2 arg3
Accessing command line arguments
Ruby provides them via the ARGV array:
ARGV.each do |arg|
puts arg
end
CIT 383: Administrative Scripting

22. CIT 383: Administrative Scripting
References
Michael Fitzgerald, Learning Ruby, O’Reilly,
David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008.
Hal Fulton, The Ruby Way, 2nd edition, Addison- Wesley, 2007.
Robert C. Martin, Clean Code, Prentice Hall,
Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.
CIT 383: Administrative Scripting