Perl File and Directory Access

1
Software Tools

• Perl File and Directory Access

2
Changing Directories

• You can change the current working directory
  within Perl, just like the cd shell command.
• In Perl, the chdir function, takes a single
  argument -- the directory name to change to.
• chdir returns true if youve successfully changed
  to the requested directory, and false if you
  could not.
• chdir("/etc") die "cannot cd to /etc"

3
Changing Directories

• Like other functions, parentheses are optional
• cat cd1
• !/usr/local/bin/perl5 -w
• print "where to go? "
• chomp(where ltSTDINgt)
• if(chdir where)
• print "chdir succeeded, now in where\n"
• else
• print "chdir did not succeeded\n"
• cd1
• where to go? /bin
• chdir succeeded, now in /bin

4
Changing Directories

• When your Perl program starts, the current
  working directory will be inherited from the
  shell that invoked it.
• However, using chdir in Perl will not change the
  working directory of the shell when your program
  ends.
• The chdir function without a parameter defaults
  to taking you to your home directory.

5
Globbing

• The shell takes a and turns it into a list of
  all the filenames in the current directory.
• Similarly, a-m.cpp turns into a list of all
  filenames in the current directory that begin
  with a letter in the first half of the alphabet,
  and end in .cpp.
• The expansion of or /etc/host into the list of
  matching filenames is called globbing.

6
Globbing

• Perl supports globbing.
• Just put the globbing pattern between angle
  brackets or use the glob function
• _at_all ltgt
• _at_b lta-m.cppgt
• _at_c glob("a-m.cpp") same as _at_b
• In a list context, the glob returns a list of all
  names that match the pattern (or an empty list if
  none match).

7
Globbing

• Example
• ls
• address letter1 names sort
• cat glob1
• !/usr/local/bin/perl5 -w
• _at_files lta-mgt
• foreach file (_at_files)
• print "file\n"
• glob1
• address
• letter1

8
Globbing

• If you use a full pathname in your glob, you will
  get full pathnames as a result
• cat glob2
• !/usr/local/bin/perl5 -w
• _at_files lt/etc/hostgt
• foreach file (_at_files)
• print "file "
• print "\n"
• glob2
• /etc/hostname.le0 /etc/hosts /etc/hosts.allow
  /etc/hosts.deny

9
Globbing

• If you want just the simple filename, you can use
  substitute to chop off the directory part of the
  string
• cat glob3
• !/usr/local/bin/perl5 -w
• _at_files lt/etc/hostgt
• foreach file (_at_files)
• file s./ delete to last slash
• print "file "
• print "\n"
• glob3
• hostname.le0 hosts hosts.allow hosts.deny

10
Globbing

• Multiple patterns are permitted inside the glob
• _at_bill_files ltgates clintongt
• The argument to glob is variable interpolated
• cat glob4
• !/usr/local/bin/perl5 -w
• if(-d "/homes/horner/111")
• where "/homes/horner/111"
• else
• where "/homes/horner"
• _at_files ltwhere/gt
• print "_at_files\n"
• glob4
• /homes/horner/111/letter1 /homes/horner/111/name
  s

11
Removing a File

• The Perl unlink function deletes a file, exactly
  like the UNIX rm command.
• unlink("bill") bye bye bill
• Example
• ls
• letter1 names unlink1
• cat unlink1
• !/usr/local/bin/perl5 -w
• print "what file to delete? "
• chomp(what ltSTDINgt)
• unlink(what)
• unlink1
• what file to delete? letter1
• ls
• names unlink1

12
Removing a File

• unlink can take a list of names as well
• unlink("bill", "gates")
• unlink lt.cppgt delete all .cpp files
• The return value on unlink is the number of files
  successfully deleted.
• foreach file (lt.cppgt)
• if(unlink(file) 0)
• print "trouble deleting file\n"

13
Removing a File

• With no arguments to unlink, _ is used as the
  default.
• foreach (lt.cppgt)
• if(unlink 0)
• print "trouble deleting _\n"

14
Renaming a File

• The Perl function rename allows you to rename
  files.
• Here is how to rename the file gates into cheap
• rename("gates", "cheap")
• rename returns a true value if successful.
• if(rename("gates", "cheap"))
• print "Bill is now cheap\n"
• else
• print "Bill is still gates\n"

15
Hard Links

• The Perl function link allows you to create a
  hard link.
• Here is how to link from the file gates to cheap
• link("gates", "cheap")
• link returns true if successful.
• if(link("gates", "cheap"))
• print "Bill is now also cheap\n"
• else
• print "Bill is still only gates\n"

16
Soft Links

• The Perl function symlink allows you to create a
  soft (symbolic) link.
• Here is how to symbolic link from gates to cheap
• symlink("gates", "cheap")
• readlink returns the name pointed at by the
  specified symbolic link
• symlink("gates", "cheap")
• x readlink("cheap")
• print "cheap points at x\n" cheap points at
  gates

17
Making and Removing Directories

• The Perl functions mkdir and rmdir allow you to
  make and remove directories.
• mkdir takes the name of the new directory and a
  mode that determines the permissions
• Use 0755 for the mode to allow user full (rwx)
  permission, and no write permission for group and
  other (rx only).
• mkdir("gatesdir", 0755)
• rmdir("gatesdir")

18
Permissions

• The leading mode number is always 0, and the
  other 3 numbers are permissions for user, group,
  and other respectively.
• The mode values are octal, and have the following
  meanings
• 0 ---
• 1 --x
• 2 -w-
• 3 -wx
• 4 r--
• 5 r-x
• 6 rw-
• 7 rwx

19
Modifying Permissions

• The Perl function chmod allows you to change file
  and directory permissions
• mkdir("gatesdir", 0755) urwx grx orx
• chmod(0750, "gatesdir") urwx grx o
  chmod(0531, "gates") urx gwx ox
• chmod(0642 , "gates", "cheap") urw gr ow