Softpanorama

May the source be with you, but remember the KISS principle ;-)
Home Switchboard Unix Administration Red Hat TCP/IP Networks Neoliberalism Toxic Managers
(slightly skeptical) Educational society promoting "Back to basics" movement against IT overcomplexity and  bastardization of classic Unix

Files Predicates


Introduction

There are several dozens operators in Perl that can help to test the status of the file. There are several dozens operators in Perl that can help to test the status of the file. They are derived from and are similar to used in Unix shells. They can be considered as shortcuts for stat function. . Actually in Perl documentation they are considered to be built-in functions, not operators (see -X - perldoc.perl.org).  The most widely used are just five listed in the table below:

Operator File condition
-d Is this a directory?
-e Does this file exist?
-r Is this file readable by the person running the script?
-s Returns the size of the file
-w Is this file writeable by the person running the script?
-x Is this file executable by the person running the script?

First we will discuss probably the most popular test among listed above the -e test. If you want to open the file for writing if the file does not already exist, you can first test to see if a file exists using the -e operator.

All other file-test operators have the same syntax as the -e operator used below. This is a unary operator that accept string as its only operand. The value of the string should contain the name of the file to be tested. It can be fully qualified name or relative name.

unless (-e "/home/nnb/.profile") {
   die ("file /home/nnb/.profile does not exist");
}
open (SYSPROF, "/home/nnb/.profile");

If the file exists, the -e operator returns true; otherwise, it returns false. Similar tests exist to test other file conditions. Here is another example:

unless (open(SYSIN, "</etc/hosts") {
   if (-e "/etc/hosts") {                 
      die ("File /etc/hosts exists, but cannot be opened for reading (wrong permissions?).\n"));
   } else {
      die ("File /etc/hosts does not exist.\n");
   }
}

If you use Windows and need specify full path using backslashes, please remember that they need to be doubles as backslash serves as a escape character in Perl literals. Actually you can use regular slash, but this is convenient solution only for people who get used to Unix.

You can even extend previous trick with || operator, but it becomes somewhat less comprehensible and although this is a Perl idiom this is probably a bad Perl idiom -- I recommend against using it:

open(SYSIN, "infile") && !(-e "infile") || die("Cannot open infile\n");

In Unix and Windows NT (unless you are always using root and administrator IDs to login ;-) before you can open a file for reading, you must have permission to read the file. The -r file-test operator tests whether you have permission to read a file.

$fname="testfile";
unless (open(SYSIN, "$fname")) {
  if (!(-e "file1")) {die ("File $fname does not exist.\n");}
  unless (-r "file1")) {die ("You are not allowed to read $fname.\n");
  die ("File1 cannot be opened. Reason unknown\n");
}

To check whether you have write permission on a file, use the -w file-test operator.

unless (-w "$fname") {
   print STDERR ("Can't write to $fname.\n");
}

The -x file-test operator is used mainly in Unix environment and it checks whether you have execute permission on the file (in other words, whether the system thinks this is an executable script, and whether you have permission to run it if it is), as illustrated here:

unless (-x "$fname") {
   print STDERR ("I can run $fname.\n");
}

The -s file-test operator returns the size of the file in bytes. This provides a more refined test for whether or not to open a file for writing: if the file exists but is empty, no information is lost if you overwrite the existing file.

$size = -s "outfile";
if ($size == 0) {
   print ("The file is empty.\n");
} else {
   print ("The file is $size bytes long.\n");
}

The file-test operators provide a way of retrieving information on a particular file. The most common file-test operators are

Full list of file test operators

Full list of file test operators contain more than a dozen entries as shown in the table below:

Operator Description
-b Is filename a block device?
-c Is filename a character device?
-d Is name a directory?
-e Does filename exist?
-f Is filename an ordinary file?
-g Does filename have its setgid bit set?
-k Does filename have its "sticky bit" set?
-l Is filename a symbolic link?
-o Is filename owned by the user?
-p Is name a named pipe?
-r Is filename a readable file?
-s Returns the size of the file
-t Does name represent a terminal?
-u Does filename have its setuid bit set? The effective userid in this case will be different from the user login ID
-w Is filename a writable file?
-x Is filename an executable file?
-z Is filename an empty file?
-A How long since filename accessed? Similar to -M
-B Is filename a binary file?
-C Returns number of days since filename's inode accessed
-M Returns number of days since filename modified: Start time minus file modification time, in days (fractions of day are returned for the current day, for example 0.13)
-O Is filename owned by the "real user" only? (See -u (setuid bit) comment)
-R Is filename readable by the "real user" only? (See -u (setuid bit) comment)
-S Is name a socket?
-T Is filename a text file?
-W Is filename writable by the "real user" only? (See -u (setuid bit) comment)
-X Is filename executable by the "real user" only? (See -u (setuid bit) comment)

stat function

The stat function accepts a filehandle or the name of the file. It returns a 13-element list giving the status info for a file. If name of the file is specified it can be either fully qualified or relative name. If no arguments are supplied $_  is used as a default argument. The function returns an empty list in case of failure. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks)= stat($filename);

Not all fields are supported on all filesystem types as this is a Unix-oriented structure.

Here are the meaning of the fields:

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions).
             The result is in octal form and contains five digits
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated 

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:

if (-x $file && (($d) = stat(_)) && $d < 0) {
   print "$file is executable NFS file\n";
}

(This works on machines only for which the device number is negative under NFS.)

In scalar context, stat() returns a Boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle

Recommended Links

Google matched content

Softpanorama Recommended

Top articles

Sites

Top articles

Sites

 



Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotesSomerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose BierceBernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOSProgramming Languages History : PL/1 : Simula 67 : C : History of GCC developmentScripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-MonthHow to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Haterís Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D


Copyright © 1996-2021 by Softpanorama Society. www.softpanorama.org was initially created as a service to the (now defunct) UN Sustainable Development Networking Programme (SDNP) without any remuneration. This document is an industrial compilation designed and created exclusively for educational use and is distributed under the Softpanorama Content License. Original materials copyright belong to respective owners. Quotes are made for educational purposes only in compliance with the fair use doctrine.

FAIR USE NOTICE This site contains copyrighted material the use of which has not always been specifically authorized by the copyright owner. We are making such material available to advance understanding of computer science, IT technology, economic, scientific, and social issues. We believe this constitutes a 'fair use' of any such copyrighted material as provided by section 107 of the US Copyright Law according to which such material can be distributed without profit exclusively for research and educational purposes.

This is a Spartan WHYFF (We Help You For Free) site written by people for whom English is not a native language. Grammar and spelling errors should be expected. The site contain some broken links as it develops like a living tree...

You can use PayPal to to buy a cup of coffee for authors of this site

Disclaimer:

The statements, views and opinions presented on this web page are those of the author (or referenced source) and are not endorsed by, nor do they necessarily reflect, the opinions of the Softpanorama society. We do not warrant the correctness of the information provided or its fitness for any purpose. The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be tracked by Google please disable Javascript for this site. This site is perfectly usable without Javascript.

Last modified: March, 12, 2019