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 you’ve 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 = <STDIN>); 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: @all = <*>; @b = <[a-m]*.cpp>; @c = glob("[a-m]*.cpp"); # same as @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 @files = <[a-m]*>; foreach $file (@files){ print "$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 @files = </etc/host*>; foreach $file (@files){ print "$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 @files = </etc/host*>; foreach $file (@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: @bill_files = <gates* clinton*>; • The argument to glob is variable interpolated: $ cat glob4 #!/usr/local/bin/perl5 -w if(-d "/homes/horner/111"){ $where = "/homes/horner/111"; }else{ $where = "/homes/horner"; } @files = <$where/*>; print "@files\n"; $ glob4 /homes/horner/111/letter1 /homes/horner/111/names

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($what = <STDIN>); unlink($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 <*.cpp>; # delete all .cpp files • The return value on unlink is the number of files successfully deleted. foreach $file (<*.cpp>){ if(unlink($file) == 0){ print "trouble deleting $file\n"; } }

13. Removing a File • With no arguments to unlink, $_ is used as the default. foreach (<*.cpp>){ 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); #u:rwx g:rx o:rx chmod(0750, "gatesdir"); #u:rwx g:rx o: chmod(0531, "gates"); #u:rx g:wx o:x chmod(0642 , "gates", "cheap"); #u:rw g:r o:w