01-30-2009
The first number in this group of 4 is for the special bits. '0' means nothing, '1' is the "sticky bit", '2' the "set group ID", and '4' the "set user ID" bit. '4' is probably the most used of those, as it changes execution of a program so that it doesn't run as the calling UID, but as the one of the file owner (same for the SGID bit, only with the group instead of the user). The passwd utility most often has this one set, as it requires root privileges to change the password file, but any user should be able to run it.
The effects of the "sticky bit" vary between platforms, and whether it's set on a file or directory. For files it originally was intended to tell the kernel "Keep this file in memory after it ended" so that subsequent calls wouldn't need to reread it from disk (not working in Linux). For directories it means "only the owner of this file may manipulate it", eg. on the /tmp dir, where every user may write, but others shouldn't be able to delete or modify a file there.
The first character in an ls -l output (in your example 'd') specifies the file type. '-' is a regular file, 'd' is a directory, 'c' is a character device, ....
HTH
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write(1) General Commands Manual write(1)
Name
write - write message to another user
Syntax
write user [ttyname]
Description
The command copies lines from your terminal to that of another user. When first called, it sends the message
Message from yoursystem!yourname yourttyname...
The recipient of the message should write back at this point. Communication continues until an end of file is read from the terminal or an
interrupt is sent. At that point writes `EOT' on the other terminal and exits.
If you want to write to a user who is logged in more than once, the ttyname argument may be used to indicate the appropriate terminal name.
Permission to write may be denied or granted by use of the mesg command. At the outset writing is allowed. Certain commands, in particu-
lar and disallow messages in order to prevent messy output.
If the character `!' is found at the beginning of a line, calls the shell to execute the rest of the line as a command.
The following protocol is suggested for using when you first write to another user, wait for him to write back before starting to send.
Each party should end each message with a distinctive signal. The letter `o' is the convention for `over' which indicates that the message
is complete. The letters `oo' are the convention for `over and out' which is used when the conversation is about to be terminated.
Files
/etc/utmp to find user
/bin/sh to execute `!'
See Also
mail(1), mesg(1), who(1)
write(1)