CIT 383: Administrative Scripting

CIT 383: Administrative Scripting Slide #1

CIT 383: Administrative Scripting

Files and Directories


Topics

1. Creating File Objects

2. Reading Files

3. Writing Files

4. Directories

5. Inodes

6. File Management

7. Command Line Arguments


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)

Examplespwfile = File.new(‘/etc/passwd’, ‘r’)usrfile = File.new(‘userlist’, ‘w’)logfile = File.new(‘/var/log/mylog’, ‘a’)


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


Reading a file line by line

File class each_line iteratorfh.each_line do |line|

# do stuff with line here

end

While loopwhile line = fh.gets

# do stuff with line here

end


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


Line Endings

Getting rid of line end charactersline_without_ending = line.chomp

OR

line.chomp!

Outputting line ending charactersputs inserts a line ending

print does not add a line ending


Global File Objects

$stdinDefault object for getc and gets

$stdoutDefault object for puts

$stderr


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 closeFile.open(pathname,’r’) do |fh|

fh.gets

end


Data Munging

Data munging: to convert data from one format to another format, possibly sorting, summarizing, or otherwise modifying it in the process.


Skipping lines

Skipping blank linesfile.each_line do |line|

next if line == "\n"

puts line

end

Skipping lines beginning with a characterfile.each_line do |line|

next if line[0] == ?#

puts line

end


Delimited Datapwfields = [:username, :password, :uid, :gid, :gcos, :homedir, :shell]

pwline = ‘root:x:0:0:root:/root:/bin/sh’

pwitems = pwline.split(‘:’)pwhash = Hash.new

i = 0while i < pwitems.size

pwhash[pwfields[i]] = pwitems[i] i = i + 1

end


Directories

Directory: table of name to inode mappings

$ ls –i /2272929 bin 65409 boot 49057 cdrom 2260 dev 850305 etc 2 home2371041 initrd 49075 initrd.img 49058 initrd.img.old 948417 lib


Accessing Directories

thisdir = Dir.new(pathname)absolutedir = Dir.new(‘/home/smi/dir1’)

relativedir = Dir.new(‘smi/dir1’)

currentdir = Dir.new(‘.’)

uponedir = Dir.new(‘..’)

Methodsentries: returns array of filenames

each: iterates over each file in directory


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.


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.


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 fileu: rw, g: r, o: r

ex: chmod 0711 binu: rwx, g: x, o: x

4 read setuid

2 write setgid

1 execute sticky


File::Stat

The File::Stat class allows access to inodesstat = File::Stat.new(pathname)ORfile = File.new(pathname)stat = file.stat

Methodssize: size of file in bytesuid: UID of file ownergid: GID of file ownermode: file permissions in octalmtime,ctime,atime: file timestamps


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?


File Management

The FileUtils class provides methods that emulate the functionality of UNIX commands:

chmod

chown

cp

ln

mkdir

mv

rm

rmdir


Command Line Arguments

Using command line arguments./test.rb arg1 arg2 arg3

Accessing command line argumentsRuby provides them via the ARGV array:

ARGV.each do |arg|puts arg

end

CIT 383: Administrative Scripting Slide #22

References1. Michael Fitzgerald, Learning Ruby, O’Reilly,

2008.2. David Flanagan and Yukihiro Matsumoto, The

Ruby Programming Language, O’Reilly, 2008.3. Hal Fulton, The Ruby Way, 2nd edition, Addison-

Wesley, 2007.4. Robert C. Martin, Clean Code, Prentice Hall,

2008.5. Dave Thomas with Chad Fowler and Andy Hunt,

Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.

CIT 383: Administrative Scripting

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