perl file and directory access learning objectives: 1. to learn how to change directories being...

Perl File and Directory Access

Learning Objectives:1. To learn how to change directories being

accessed in a Perl program

2. To learn the Perl’s command involving file system management / accessing

COMP111COMP111Lecture 20 / Slide Lecture 20 / Slide 22

Perl File and Directory AccessTable of Content Changing Directories Globbing Removing a File Renaming a File Hard Links Soft Link Making and Removing Directories Permissions Modifying Permissions Recursive Directory Processing


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";

Changing Directories (1)


Changing Directories (2) Like other functions, parentheses are optional:

$ cat cd1#!/usr/local/bin/perl5 -wprint "where to go? ";chomp($where = <STDIN>);if(chdir $where){

print "chdir succeeded, now in $where\n";}else{

print "chdir did not succeeded\n";}$ cd1where to go? /binchdir succeeded, now in /bin$


Changing Directories (3)

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.


Globbing (1)

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.


Globbing (2)

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).


Globbing (3)

Example:$ lsaddress letter1 names sort$ cat glob1#!/usr/local/bin/perl5 -w@files = <[a-m]*>;foreach $file (@files){

print "$file\n";}$ glob1addressletter1$


Globbing (4)

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$


Globbing (5)

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 slashprint "$file ";}print "\n";$ glob3hostname.le0 hosts hosts.allow hosts.deny$


Globbing (6)

Multiple patterns are permitted inside the glob:@bill_files = <gates* clinton*>;

The argument to glob is variable interpolated:$ cat glob4#!/usr/local/bin/perl5 -wif(-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


Removing a File (1)

The Perl unlink function deletes a file, exactly like the UNIX rm command.

unlink("bill"); # bye bye bill

Example:$ lsletter1 names unlink1$ cat unlink1#!/usr/local/bin/perl5 -wprint "what file to delete? ";chomp($what = <STDIN>);unlink($what);$ unlink1what file to delete? letter1$ lsnames unlink1$


Removing a File (2)

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";

}

}


Removing a File (3)

With no arguments to unlink, $_ is used as the default.

foreach (<*.cpp>){

if(unlink == 0){

print "trouble deleting $_\n";

}

}


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";

}


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";

}


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


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");


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

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

Modifying Permissions


Here is the skeleton of a program for recursive directory

processing:

#!/usr/local/bin/perl5 -wsub dirtree { my @files = <$_[0]/*>; # local variable required print "Directory listing for: $_[0]\n"; foreach (@files){ print "$_\n"; dirtree($_) if (-d $_); # recursion }}dirtree($ARGV[0]);

Recursive Directory Processing

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