ServerBuddies Support Blog

input and output redirection

The following example combines input and output redirection. The file text.txt is first checked for spelling mistakes, and the output is redirected to an error log file:

Output of one command can be piped into another command virtually as many times as you want, just as long as these commands would normally read input from standard input and write output to the standard output. Sometimes they don’t, but then there may be special options that instruct these commands to behave according to the standard definition.

Make sure you don’t use names of existing files that you still need. Redirecting output to existing files will replace the content of those files.

Input redirection

You may want a file to be the input for a command that normally wouldn’t accept a file as an option. This redirecting of input is done using the “<” (less-than symbol) operator.

Below is an example of sending a file to somebody, using input redirection.

If the user buddies exists on the system, you don’t need to type the full address. If you want to reach somebody on the Internet, enter the fully qualified address as an argument to mail.

This reads a bit more difficult than the beginner’s cat file | mail someone, but it is of course a much more elegant way of using the available tools.

File protection with chmod

chmod 400 file To protect a file against accidental overwriting.
chmod 500 directory To protect yourself from accidentally removing, renaming or moving files from this directory.
chmod 600 file A private file only changeable by the user who entered this command.
chmod 644 file A publicly readable file that can only be changed by the issuing user.
chmod 660 file Users belonging to your group can change this file, others don’t have any access to it at all.
chmod 700 file Protects a file against any access from other users, while the issuing user still has full access.
chmod 755 directory For files that should be readable and executable by others, but only changeable by the issuing user.
chmod 775 file Standard file sharing mode for a group.
chmod 777 file Everybody can do everything to this file.

To list all open files, use:

To list all open Internet, x.25 (HP-UX), and UNIX domain files, use:

To list all open IPv4 network files in use by the process whose PID is 1234, use:

Presuming the UNIX dialect supports IPv6, to list only open IPv6 network files, use:

To list all files using any protocol on ports 513, 514, or 515 of host wonderland.cc.pur-due.edu, use:

To list all open files for login name “abe”, or user ID 1234, or process 456, or pro-cess 123, or process 789, use:

To list all open files on device /dev/hd4, use:

To find the process that has /u/abe/foo open, use:

To send a SIGHUP to the processes that have /u/abe/bar open, use:

To find any open file, including an open UNIX domain socket file, with the name /dev/log,use:

proc pseudo-filesystem in Linux

proc-pseudo-filesystem in Linux The Linux process pseudo-filesystem, the /proc directory. Every process on the system has a directory here with its name on it, inside of which lies many things — including an fd (”file descriptor”) subdirectory containing links to all files that the process has open. Even if a file has been removed from the filesystem, a copy of the data will be righ there:

To know where to go, you need to get the id of the process that has the file open, and the file descriptor. These you get with lsof, whose name means “list open files.” (It actually does a whole lot more than this and is so useful that almost every system has it installed. If yours isn’t one of them, you can grab the latest version straight from its author.)