Processes use file descriptors (FDs) to perform input/output operations. The first 3 FDs - STDIN, STDOUT, and STDERR - are used by default for keyboard, screen, and screen respectively. Standard FDs can be redirected using operators like <, >, 2> to redirect input, output, and errors. Pipelines connect the STDOUT of one command to the STDIN of another to pass output as input between commands.

1. Chapter 1Chapter 1
Data StreamData Stream
Ref. Pge. 15

2. File Descriptor (FD)File Descriptor (FD)
●
Processes use File Descriptors toProcesses use File Descriptors to
input or output (I/O) data with systeminput or output (I/O) data with system
●
Each process has 256 FDsEach process has 256 FDs
●
The first 3 FDs are standardThe first 3 FDs are standard
0 0 : Standard Input (STDIN): Standard Input (STDIN)
1 1 : Standard Output (STDOUT): Standard Output (STDOUT)
2 2 : Standard Error Output (STDERR): Standard Error Output (STDERR)

3. Standard FDStandard FD
●
By default, each standard FD isBy default, each standard FD is
connected to an I/O device:connected to an I/O device:
STDIN STDIN : Keyboard: Keyboard
STDOUT STDOUT : Screen: Screen
STDERR STDERR : Screen: Screen
processFD0
FD2
FD1
keyboard screen

4. IO RedirectionIO Redirection
●
Standard FD can be changed inStandard FD can be changed in
command line:command line:
STDIN STDIN : < file: < file
STDOUT STDOUT : > file: > file
STDERR STDERR : 2> file: 2> file
processFD0
FD2
FD1
keyboard screen

5. IO Redirection ExampleIO Redirection Example
●
Command default:Command default:
$ mail -s test root$ mail -s test root
this is a test mail.this is a test mail.
please skip.please skip.
^d^d

6. IO Redirection ExampleIO Redirection Example
●
Change the STDIN by usingChange the STDIN by using << ::
$ mail -s test root < /etc/passwd$ mail -s test root < /etc/passwd

7. IO Redirection ExampleIO Redirection Example
●
Change the STDOUT by usingChange the STDOUT by using >> ::
$ cat /etc/passwd > std.out$ cat /etc/passwd > std.out

8. IO Redirection ExampleIO Redirection Example
●
Change the STDERR by usingChange the STDERR by using 2>2> ::
$ cat /etc/password 2> std.err$ cat /etc/password 2> std.err

9. DeviceDevice /dev/null/dev/null
●
Keep the STDERR only:Keep the STDERR only:
$ find /etc > /dev/null$ find /etc > /dev/null

10. Output AppendingOutput Appending
●
Keep the existing content in targetKeep the existing content in target
file by usingfile by using >>>> ::
$ find /etc >/dev/null 2>> std.out$ find /etc >/dev/null 2>> std.out

11. HERE DocumentHERE Document
●
Multiple line input by usingMultiple line input by using << TAG<< TAG ::
$ mail -s test root << .$ mail -s test root << .
this is a test mail.this is a test mail.
please skip.please skip.
..

12. Output CombinationOutput Combination
●
Save STDOUT and STDERR to a sameSave STDOUT and STDERR to a same
file by usingfile by using 2>&12>&1 ::
$ cat std.out std.none > std.both 2>&1$ cat std.out std.none > std.both 2>&1
OrOr
$ cat std.out std.none &> std.both$ cat std.out std.none &> std.both
●
Note:Note:
Order is important!Order is important! cmd1
FD2
FD1
FD0

13. Pipe LinePipe Line
●
Connect STDOUT of left command toConnect STDOUT of left command to
STDIN of right command:STDIN of right command:
$ cmd1 | cmd2$ cmd1 | cmd2
cmd1
FD2
FD1
FD0
screen
cmd2
FD2
FD1
FD0
Ref. Pge. 17

14. Pipe LinePipe Line
●
Combine STDERR into STDOUT inCombine STDERR into STDOUT in
pipe line:pipe line:
$ cmd1 2>&1 | cmd2$ cmd1 2>&1 | cmd2
cmd1
FD2
FD1
FD0
screen
cmd2
FD2
FD1
FD0

15. Output SplittingOutput Splitting
●
Tap an output copy to a file by usingTap an output copy to a file by using
commandcommand teetee ::
$ cmd1 | tee file | cmd2$ cmd1 | tee file | cmd2
$ cmd1 | tee -a file | cmd2$ cmd1 | tee -a file | cmd2
cmd1
FD2
FD1
FD0
tee file

16. XargsXargs
●
Change the STDIN to be as argumentChange the STDIN to be as argument
by usingby using xargsxargs ::
$ cmd1 | xargs cmd2$ cmd1 | xargs cmd2
cmd1
FD2
FD1
FD0
cmd2 --opt args...
FD0