Dave’s Ramblings

File Manipulation

• Perl provides a large number of functions to perform various operations on files
• These are very similar to the corresponding UNIX system call or command
• See the UNIX man pages for details

File Test Operators

• Operates on a filename or filehandle argument (except for -t which only operates on a filehandle argument)
• Tests associated file to determine if something is true or not about the file
• If the argument is omitted, $_ is tested (except for -t which tests STDIN)
• Most of these operators return 1 for True and the empty string for False, or undef if the file does not exist (except for -s which returns the file size and -M, -A and -C which return the
  file age)
• Precedence is higher than logical and relational operators, but lower than arithmetic operators
• For superuser, -r, -R, -w and -W always return True and -x and -X return True if ANY execute bit is set

Example;

File Test Operator List

• -r File is readable by effective uid
• -w File is writable by effective uid
• -x File is executable by effective uid
• -o File is owned by effective uid
• -R File is readable by real uid
• -W File is writable by real uid
• -X File is executable by real uid
• -O File is owned by real uid
• -e File exists
• -z File exists and has zero size
• -s File exists and has nonzero size (returns size in bytes)
• -f File is a plain file
• -d File is a directory
• -l File is a symbolic link
• -p File is a named pipe (FIFO)
• -S File is a socket
• -b File is a block special file
• -c File is a character special file
• -u File has its setuid bit set
• -g File has its setgid bit set
• -k File has its sticky bit set
• -t Filehandle is a tty
• -T File is a text file
• -B File is a binary file
• -M Modification age in days
• -A Access age in days
• -C Inode-modification age in days

Stat Function

• stat FILEHANDLE
• stat (FILENAME)

Useful for file info which the file test operators do not provide (such as number of links) or for finding the true mode when superuser

- Typical use:

($dev, $ino, $mode, $nlink, $uid, $gid, $rdev, $size, $atime, $mtime, $ctime, $blksize, $blocks) = stat ($filename);

$file = "toy1.c";
($uid, $gid) = stat ($file) [4,5];

Lstat Function

• Same as the stat() function, but gives info on a symbolic link itself
• lstat (FILEHANDLE)

• lstat FILEHANDLE
• lstat (FILENAME)

ul>

The _ Filehandle

• Whenever a file test operator, stat function or lstat function is used, Perl invokes the proper system call (stat(2) on UNIX) to get the required info
• Doing a file test, stat or lstat on the special _ filehandle, causes Perl to use the existing memory cache (stat buffer) of file info from the previous file test, stat or lstat

-Example

PLAIN TEXT
CODE:
1. if (-r $file && -w _)
2.          {
3.            print ("$file is both readable and writable\n");
4.          }

The above does only one invocation of stat(2) which is more efficient than the following which causes two invocations of stat(2):

PLAIN TEXT
CODE:
1. if (-r $file && -w $file)
2.          {
3.            print ("$file is both readable and writable\n");
4.          }

File Name Expansion (Globbing)

• If the string inside angle brackets is NOT a filehandle, it is interpreted as a C-Shell Filename Expansion (Globbing) pattern (versus the input operator)
• All C-Shell globbing metacharacters are valid: *, ?, [], -, {}, ~
• In an array context, the glob returns a list of all filenames that match (or an empty list if none match). In a scalar context, the next filename that matches is returned (or undef if there are no more matches). This is all similar to how the input operator with a filehandle works.
• One level of scalar variable interpolation is done
• But since < $x> indicates an indirect filehandle, use < ${x}> for globbing

-Example

PLAIN TEXT
CODE:
1. <*.c>                     # All files that end in c
2.          <ch [1-3]>                 # Files ch1, ch2 and ch3
3.          
4.          $x = "*.c";
5.          <${x}>                    # All files that end in c

Unlink Function

• Removes one or more files (actually deletes links)
• unlink (LIST)
• unlink LIST
• Returns the number of files successfully deleted
• On failure $! is set to the value of errno
• Uses unlink(2)

Typical use:

PLAIN TEXT
CODE:
1. $count = unlink ("toy1.c", "toy2.c", "toy3.c");
2.  
3.          $count = unlink (<*.c>);

-Example

PLAIN TEXT
CODE:
1. #!/usr/bin/perl
2.          # Simple rm program
3.  
4.          foreach $file (@ARGV)
5.          {
6.            unlink ($file) || print ("Could not unlink $file: $!\n");
7.          }

Rename Function

• Renames a file
• rename (OLDNAME, NEWNAME)
• Returns 1 for success, 0 for failure
• On failure $! is set to the value of errno
• Similar to mv(1), but does NOT rename across filesystems and does not work if OLDNAME is a regular file and NEWNAME is an existing directory

- Typical use:

PLAIN TEXT
CODE:
1. $status = rename ("toy1.c", "toy2.c");
2.          $status = rename ("toy1.c", "toys/toy1.c");

Link Function

• Creates a new hard link for a file
• link (OLDNAME, NEWNAME)
• Returns 1 for success, 0 for failure
• On failure $! is set to the value of errno
• Uses link(2)

- Typical use:

PLAIN TEXT
CODE:
1. $status = link ("toy1.c", "toy2.c");

Symlink Function

• Creates a new symbolic (soft) link for a file
• symlink (OLDNAME, NEWNAME)
• Returns 1 for success, 0 for failure
• On failure $! is set to the value of errno
• Uses symlink(2)

- Typical use:

PLAIN TEXT
CODE:
1. $status = symlink ("toy1.c", "toy2.c");

Readlink Function

• Reads the contents of a symbolic link file
• readlink (FILENAME)
• readlink FILENAME
• Returns link contents on success, undef on failure
• On failure $! is set to the value of errno
• Uses readlink(2)
• Uses $_ if FILENAME is omitted

- Typical use:

PLAIN TEXT
CODE:
1. $link = readlink ("toy2.c");

Chmod Function

• Changes the mode (permissions) of a list of files
• chmod (LIST)
• chmod LIST
• Returns the number of files successfully changed
• On failure $! is set to the value of errno
• Uses chmod(2)
• The first element of the list must be the numerical mode

- Typical use:

PLAIN TEXT
CODE:
1. $count = chmod (0755, "toy1.c");

Chown Function

• Changes the owner and group of a list of files
• chown (LIST)
• chown LIST
• Returns the number of files successfully changed
• On failure $! is set to the value of errno
• Uses chown(2)
• The first two elements of the list must be the numerical uid and gid

- Typical use:

PLAIN TEXT
CODE:
1. $count = chown ($uid, $gid, <toy *.c>);

Utime Function

• Changes the access (atime) and modification (mtime) times of a list of files
• utime (LIST)
• utime LIST
• Returns the number of files successfully changed
• On failure $! is set to the value of errno
• Similar to touch(1)
• The first two elements of the list must be the numerical access and modification times
• The inode modification time (ctime) is set to the current time

- Typical use:

PLAIN TEXT
CODE:
1. $count = utime ($atime, $mtime, "toy1.c");

Borrowed and reformatted from http://umbc7.umbc.edu/~tarr/perl/perl4/ch12-filemanip.html so I wouldn't loose it.