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what is output redirection?

I am reading a UNIX book and in one of the chapters it mentioned using echo and output redirection, could you explain what output redirection is?

It was similar to this:
echo "sample text" >> file

This is probably very simple but I'm not familiar with it.

That could be, for instance, when you run, say the 'ls' command.
Say you have a directory with a lot of .conf files. You can run 'ls -l *.conf' to see them. Then, if you wanted to, you could redirect the output of 'ls' to another text file, like 'ls -l *.conf > conf_list.txt', such that the conf_list.txt file is created from the output of ls.

Example:

Code:

bs@bsubuntu:/etc$ ls -l *.conf

-rw-r--r--  1 root root  2188 2005-03-31 07:13 adduser.conf

-rw-r--r--  1 root root  2673 2005-03-01 16:08 debconf.conf

-rw-r--r--  1 root root  392 2004-10-26 09:52 deluser.conf

-rw-r--r--  1 root root  689 2005-02-28 08:19 discover.conf

-rw-r--r--  1 root root  6414 2005-03-19 00:11 evms.conf

-rw-r--r--  1 root root  354 2004-08-12 21:23 fdmount.conf

-rw-r--r--  1 root root  1100 2005-03-29 08:38 gksu.conf

-rw-r--r--  1 root root  3688 2005-04-02 13:33 hdparm.conf

-rw-r--r--  1 root root    26 1995-09-25 23:20 host.conf

-rw-r--r--  1 root root  1270 2005-03-31 07:13 inetd.conf

-rw-r--r--  1 root root  6947 2005-03-25 17:37 irssi.conf

-rw-r--r--  1 root root  179 2005-03-31 07:18 kernel-img.conf

-rw-r--r--  1 root root    15 2005-03-31 13:23 ld.so.conf

-rw-r--r--  1 root root  3243 2004-12-21 21:41 lftp.conf

-rw-r--r--  1 root root    19 2005-02-23 06:39 libao.conf

-rw-r--r--  1 root root  599 2004-08-12 19:14 logrotate.conf

-rw-r--r--  1 root root 10815 2004-11-09 06:25 ltrace.conf

-rw-r--r--  1 root root  268 2005-04-04 09:42 mplayerplug-in.conf

-rw-r--r--  1 root root  465 1999-03-11 10:48 nsswitch.conf

-rw-r--r--  1 root root  1741 2005-04-18 14:07 ntp.conf

-rw-r--r--  1 root root  534 2004-10-26 09:39 pam.conf

-rw-r--r--  1 root root  7649 2005-03-31 13:26 pnm2ppa.conf

-rw-r--r--  1 root root  328 2005-03-31 07:13 popularity-contest.conf

-rw-r--r--  1 root root  2554 2005-01-25 15:03 reportbug.conf

-rw-r--r--  1 root root    70 2005-05-16 17:10 resolv.conf

-rw-r--r--  1 root root    23 2004-10-26 11:36 scrollkeeper.conf

-rw-r--r--  1 root root 65682 2005-04-06 17:02 sensors.conf

-rw-r--r--  1 root root  424 2004-12-08 04:36 sysctl.conf

-rw-r--r--  1 root root  1664 2005-03-21 05:06 syslog.conf

-rw-r--r--  1 root root  1260 2004-10-28 13:50 ucf.conf

-rw-r--r--  1 root root  645 2005-02-01 11:10 updatedb.conf

-rw-r--r--  1 root root  4622 2004-10-28 18:57 vnc.conf

-rw-r--r--  1 root root  2690 2005-04-04 18:33 xcdroast.conf

bs@bsubuntu:/etc$ ls -l *.conf > /home/bs/conf_list.txt

bs@bsubuntu:/etc$ cat /home/bs/conf_list.txt

-rw-r--r--  1 root root  2188 2005-03-31 07:13 adduser.conf

-rw-r--r--  1 root root  2673 2005-03-01 16:08 debconf.conf

-rw-r--r--  1 root root  392 2004-10-26 09:52 deluser.conf

-rw-r--r--  1 root root  689 2005-02-28 08:19 discover.conf

-rw-r--r--  1 root root  6414 2005-03-19 00:11 evms.conf

-rw-r--r--  1 root root  354 2004-08-12 21:23 fdmount.conf

-rw-r--r--  1 root root  1100 2005-03-29 08:38 gksu.conf

-rw-r--r--  1 root root  3688 2005-04-02 13:33 hdparm.conf

-rw-r--r--  1 root root    26 1995-09-25 23:20 host.conf

-rw-r--r--  1 root root  1270 2005-03-31 07:13 inetd.conf

-rw-r--r--  1 root root  6947 2005-03-25 17:37 irssi.conf

-rw-r--r--  1 root root  179 2005-03-31 07:18 kernel-img.conf

-rw-r--r--  1 root root    15 2005-03-31 13:23 ld.so.conf

-rw-r--r--  1 root root  3243 2004-12-21 21:41 lftp.conf

-rw-r--r--  1 root root    19 2005-02-23 06:39 libao.conf

-rw-r--r--  1 root root  599 2004-08-12 19:14 logrotate.conf

-rw-r--r--  1 root root 10815 2004-11-09 06:25 ltrace.conf

-rw-r--r--  1 root root  268 2005-04-04 09:42 mplayerplug-in.conf

-rw-r--r--  1 root root  465 1999-03-11 10:48 nsswitch.conf

-rw-r--r--  1 root root  1741 2005-04-18 14:07 ntp.conf

-rw-r--r--  1 root root  534 2004-10-26 09:39 pam.conf

-rw-r--r--  1 root root  7649 2005-03-31 13:26 pnm2ppa.conf

-rw-r--r--  1 root root  328 2005-03-31 07:13 popularity-contest.conf

-rw-r--r--  1 root root  2554 2005-01-25 15:03 reportbug.conf

-rw-r--r--  1 root root    70 2005-05-16 17:10 resolv.conf

-rw-r--r--  1 root root    23 2004-10-26 11:36 scrollkeeper.conf

-rw-r--r--  1 root root 65682 2005-04-06 17:02 sensors.conf

-rw-r--r--  1 root root  424 2004-12-08 04:36 sysctl.conf

-rw-r--r--  1 root root  1664 2005-03-21 05:06 syslog.conf

-rw-r--r--  1 root root  1260 2004-10-28 13:50 ucf.conf

-rw-r--r--  1 root root  645 2005-02-01 11:10 updatedb.conf

-rw-r--r--  1 root root  4622 2004-10-28 18:57 vnc.conf

-rw-r--r--  1 root root  2690 2005-04-04 18:33 xcdroast.conf

bs@bsubuntu:/etc$

Or it could be like this:

You can run ps -A and get output on the screen. Or you could run ps -A and redirect the ouput to grep:

Code:

bs@bsubuntu:/etc$ ps -A

  PID TTY          TIME CMD

    1 ?        00:00:00 init

    2 ?        00:00:00 ksoftirqd/0

    3 ?        00:00:00 events/0

    4 ?        00:00:00 khelper

  22 ?        00:00:00 kacpid

  94 ?        00:00:00 kblockd/0

  132 ?        00:00:00 pdflush

  133 ?        00:00:00 pdflush

  135 ?        00:00:00 aio/0

  134 ?        00:00:00 kswapd0

  722 ?        00:00:00 kseriod

 1096 ?        00:00:00 kjournald

 1124 ?        00:00:00 udevd

 2249 ?        00:00:00 khpsbpkt

 4650 ?        00:00:00 knodemgrd_0

 4783 ?        00:00:00 khubd

 6030 ?        00:00:00 dhclient3

 6386 ?        00:00:00 syslogd

 6401 ?        00:00:00 dd

 6403 ?        00:00:00 klogd

 6436 ?        00:00:00 gdm

 6447 ?        00:00:00 gdm

 6484 ?        00:02:34 Xorg

 6487 ?        00:00:02 cupsd

 7005 ?        00:00:00 acpid

 7071 ?        00:00:00 dbus-daemon-1

 7086 ?        00:00:07 hald

 7099 ?        00:00:00 inetd

 7237 ?        00:00:00 master

 7266 ?        00:00:00 qmgr

 7431 ?        00:00:00 sshd

 7448 ?        00:00:00 ntpd

 7475 ?        00:00:00 atd

 7486 ?        00:00:00 cron

 7508 tty1    00:00:00 getty

 7509 tty2    00:00:00 getty

 7510 tty3    00:00:00 getty

 7511 tty4    00:00:00 getty

 7512 tty5    00:00:00 getty

 7513 tty6    00:00:00 getty

 7610 ?        00:00:00 x-session-manag

 7655 ?        00:00:00 ssh-agent

 7658 ?        00:00:00 dbus-launch

 7659 ?        00:00:00 dbus-daemon-1

 7661 ?        00:00:01 gconfd-2

 7664 ?        00:00:00 gnome-keyring-d

 7666 ?        00:00:00 esd

 7668 ?        00:00:00 bonobo-activati

 7670 ?        00:00:01 gnome-settings-

 7673 ?        00:00:00 gam_server

 7681 ?        00:00:00 xscreensaver

 7705 ?        00:00:00 gnome-smproxy

 7707 ?        00:00:06 metacity

 7709 ?        00:00:00 gnome-volume-ma

 7711 ?        00:00:04 nautilus

 7713 ?        00:00:02 gnome-panel

 7715 ?        00:00:02 gnome-cups-icon

 7717 ?        00:00:03 gnome-terminal

 7719 ?        00:01:04 gkrellm

 7721 ?        00:00:02 gaim

 7723 ?        00:00:00 update-notifier

 7727 ?        00:00:00 evolution-alarm

 7733 ?        00:00:00 gnome-pty-helpe

 7734 pts/0    00:00:00 bash

 7746 ?        00:00:03 wnck-applet

 7752 ?        00:00:00 evolution-excha

 7756 ?        00:00:00 gnome-vfs-daemo

 7763 ?        00:00:00 evolution-data-

 7765 ?        00:00:00 trashapplet

 7775 ?        00:00:00 mapping-daemon

 7798 ?        00:00:00 notification-ar

 7800 ?        00:00:00 mixer_applet2

 9044 ?        00:00:02 xchat

12261 ?        00:01:34 firefox-bin

12389 ?        00:00:01 java_vm

13088 ?        00:00:00 pickup

13108 pts/0    00:00:00 ps

bs@bsubuntu:/etc$

bs@bsubuntu:/etc$ ps -A | grep gnome

 7664 ?        00:00:00 gnome-keyring-d

 7670 ?        00:00:01 gnome-settings-

 7705 ?        00:00:00 gnome-smproxy

 7709 ?        00:00:00 gnome-volume-ma

 7713 ?        00:00:02 gnome-panel

 7715 ?        00:00:02 gnome-cups-icon

 7717 ?        00:00:04 gnome-terminal

 7733 ?        00:00:00 gnome-pty-helpe

 7756 ?        00:00:00 gnome-vfs-daemo

bs@bsubuntu:/etc$

Be carefull with redirecting output to printers though. A fellow student of mine mindlessly executed something like "cat book.pdf > printer".
By the time he'd realized his stupid mistake and stopped the printer, he'd allready wasted a great deal of his printing quota for that month.

I am taking your question to literally mean that you are asking for an understanding about what redirection is and does. If I am incorrect and you already know this information, then please accept my apology.

In UNIX, Linux, etc the shell (does not matter which one) automatically opens three data streams for you. The first stream, stdin (standard in) accepts data from the keyboard and passes it to a program. The second stream stdout (standard out) takes text data from a program and displays it in a screen window. The third stream stderr (standard error) also takes text data from a program and also displays it in a screen window. These are the default behaviors for any program running from a shell.

The operating system allows you to "redirect" any one, or all, of the data streams to either obtain data from something other than the keyboard, or to output text to something other than the screen window. This is accomplished by giving the shell special instructions when you execute a command. "<" causes the program's stdin to be redirected to the new source instead of the keyboard. ">" causes the program's stdout to be redirected to the new target instead of the screen window. "2>" causes the program's stderr to be redirected like the stdout redirection. ">>" causes the program's stdout to also be redirected to a new target, but causes the new information to be a continuation of whatever was originally in the target output.

Your example "echo "sample text" >> file" simply means using the program "echo" (a utility to simply echo the contents of the first argument passed to it out to stdout) the text "sample text" is sent to a file called "file", and the output text is concatenated onto the end of the file if it preexisted. If it did not preexist, then a new file called "file" will be created with the contents of a single text line of "sample text".

note the difference btw
>
and
>>