LS: unix command for listing files and directories

ls is a command to list files and directories in Unix and unix like systems.

if it is invoked without any arguments then will list files and folders in current working directory in bare format. In this bare format its difficult to distinguish files and folders and other parameters.

FORMAT: ls [Options] [Filename]

Following are the most common options to be used with "ls".

-a :  Include all files even which are starting with "."(Hidden files in Unix ), by default they are ignored.

-l  : Long format, lists permissions in unix format, size, owner , date modified etc.

-F : Appends a character to reveal the type of file e.g. * for executables '/' for directory
-R : list recursively all the directories and files

-d : list directories instead of content

-t : sort by modification time

-h : Print size in human readable format like 1M, 4G ( Use with -l)

Example: 

For listing files and directories with their parameters like size permissions and all.

> ls -l 

For listing in as ls -l along with size in human readable format..

>ls -h -l

Note: ls can also be used in windows environment with cygwin installation.

CACLS command for windows changing mode permission

It is a command line utility which displays and modify Access Control Lists (ACLs) in windows environment(Microsoft windows NT) of files and folders. An Access Control List is a list of permissions for securable object, such as a file or folder, that controls who can access it.

 

XCACLS.exe : It's a updated version for cacls for windows 2000 and later.

 

CACLS filename [/T] [/E] [/C] [/G user:perm] [/P user:perm [...]] [/D user [...]]

 

Options:

filename: Name of the file to be modify ( * wildcard for all files )

/T            Changes ACLs of specified files in the current directory and all subdirectories.

/E            Edit ACL instead of replacing it.

/C            Continue on access denied errors.

/G user:perm  Grant specified user access rights.

/P user:perm  Replace specified user's access rights.

/D user       Deny specified user access.--

 

Perm can be:

N  None

R  Read

W  Write

C  Change (write)

F  Full control

 

Examples:

1: to replace ACL with Full access permission to current directory and all subdirectories for user "test"

>cacls * /T /P test:F

2. To edit ACL for particular user "test" with Change access for abc.text file without changing other permission.

> cacls abc.text /E test:C

 

Also look for chmod a linux command for changing file permissions.

 

unlink in Perl: to delete the file

Unlink : this command permanently deletes a list of file from system.
Return: returns number of files successfully deleted. on failure sets value of "$!"  and returns false. It will not tell you the name of file which it couldn't delete.
Do not try deleting directory instead use rmdir command.

unlink("file1.txt");
unlink "a.txt", "b.txt";
unlink @list;
unlink glob "*.bak";

chmod in perl: setting file permissions

Changes the permission of the specified list of files. First argument should be numerical octal value. Returns number of files successfully changed.

usage is as follow:

$result = chmod 0777, "file1.txt","file2.txt";
chmod 0755, @filelist;
$mode  = 0655; chmod $mode, @filelist;

$mode = "0655"; chmod oct($mode), @filelist;
$mode = "0655"; chmod $mode, @filelist; # Not a good usage Should avoid, instead use above method

How to decide mode octal value:
mode is of 4 octal digits e.g. 0754, equivalent to (userID)(user)(group)(Other)
0755: u: full, GO: read and execute
0777: all : full permission
0555: all : read and execute

 symbolic representation of file permissions:

Representation	Class	Description
u	User	Owner
g	Group	Members of file group
o	Others	Neither owner nor group
a	All	Everyone

Octal notation:

Octal	System	Description
0	---	No permission
1	--x	execute
2	-w-	write
3	-wx	Write and execute
4	r--	Read
5	r-x	Read and execute
6	rw-	Read and write
7	rwx	Read, write and execute

setting mode permission from command line

type following command in command prompt.
chmod {mode} filename, file2, file3

In C language its used as:
int chmod(const char *File_path, mode_t mode_value);

How to decide mode octal value:
mode is of 4 octal digits e.g. 0754, equivalent to (userID)(user)(group)(Other)

0755: u: full, GO: read and execute

0777: all : full permission

0555: all : read and execute


 symbolic representation of file permissions:

Representation	Class	Description
u	User	Owner
g	Group	Members of file group
o	Others	Neither owner nor group
a	All	Everyone

Octal notation:

Octal	System	Description
0	---	No permission
1	--x	execute
2	-w-	write
3	-wx	Write and execute
4	r--	Read
5	r-x	Read and execute
6	rw-	Read and write
7	rwx	Read, write and execute

These octal values are decided based on binary position of R W X.
R W X
0  0  0
0  0  1 : execute
0  1  0 : Write
1  0  0 : Read

Ex: changing permission for files in whole directory recursively as read, write, execute
chmod -R -v 777 ./*

reference: chmod

Replacing ^M character at end of line using VI editor ( ^H, ^G and so on..)

Some time these characters are placed when you convert file from DOS format to Linux format. These characters can be viewed only with Vi editor or either typing "cat -v" command for that file in command prompt. These characters are called control characters.

These characters can be deleted using ":%s/.$//g" command from VI prompt.
To replace character use it like ":%s/.$/replace/g" Press Ctrl Q+M, and this will character will come up. In some other format it can be typed using ctrl V+M.

But before doing all this do ":set ff=dos", to change file format to DOS.

At last a simple trick for Gvim on Windows XP:
:%s/\r/\r/g
This replaces unix carriage return by windows CRLF.

following is the list of Ctrl characters:

Hex Name
0x00 NUL
0x01 SOH, Control-A
0x02 STX, Control-B
0x03 ETX, Control-C
0x04 EOT, Control-D
0x05 ENQ, Control-E
0x06 ACK, Control-F
0x07 BEL, Control-G
0x08 BS, backspace, Control-H
0x09 HT, tab, Control-I
0x0a LF, line feed, newline, Control-J
0x0b VT, Control-K
0x0c FF, form feed, NP, Control-L
0x0d CR, carriage return, Control-M
0x0e SO, Control-N
0x0f SI, Control-O
0x10 DLE, Control-P
0x11 DC1, XON, Control-Q
0x12 DC2, Control-R
0x13 DC3, XOFF, Control-S
0x14 DC4, Control-T
0x15 NAK, Control-U
0x16 SYN, Control-V
0x17 ETB, Control-W
0x18 CAN, Control-X
0x19 EM, Control-Y
0x1a SUB, Control-Z

Getting Information about file

File Checking
Existence:

if( -e $file) { print "File exists \n";}
else { print "File doesn't exist\n"; }

File exists with size 0(zero) : -z
File exists with size non-zero : -s

Readable/writable/Executable

File Readable : -rFile Writable : -w
File Executable : -x

Text/Binary

Text : -T
Binary : -B