CIT 383: Administrative Scripting

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

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’)

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

5. CIT 383: Administrative Scripting 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 # do stuff with line here end

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

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

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

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

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.

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 file.each_line do |line| next if line[0] == ?# puts line end

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

13. CIT 383: Administrative Scripting Directories Directory: table of name to inode mappings $ ls –i / 2272929 bin 65409 boot 49057 cdrom 2260 dev 850305 etc 2 home 2371041 initrd 49075 initrd.img 49058 initrd.img.old 948417 lib

14. CIT 383: Administrative Scripting 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

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.

16. CIT 383: Administrative Scripting 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.

17. CIT 383: Administrative Scripting 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

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

19. CIT 383: Administrative Scripting 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?

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

21. CIT 383: Administrative Scripting 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

22. CIT 383: Administrative Scripting References • Michael Fitzgerald, Learning Ruby, O’Reilly, 2008. • 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, 2008. • Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.