system redirects

1. Lesson 8
• Flow of data
• redirects
• Input and output
• pipe
• tee

2. Work Flow

3. Input and output redirection
bash shell, and the other shells employed by Linux allow the user to specify what
should happen to the input and output of programs that run.
shell creates new processes as follows:
1. shell reads the commands to be executed
2. shell opens the files which will be needed (open(2) for files, pipe(2) for streams)
3. shell creates the new processes which will be executed (fork(2) each process)
4. Each new process is connected to the appropriate plumbing (dup2(2) each redirection)
5. Control of processes is passed to programs the user asked for (execve(2) each
command)

4. shell processes input and output

5. Standard Streams

6. Input and output
Normal Flow of Data:

7. Output Redirects
Simple output redirection. Creates/overwrites file:

8. Output Redirects
stderr output redirection. Creates/overwrites file.

9. Output Redirects
Combined out & err redirection. Creates/overwrites files.
File names must be different!

10. Output Redirects
Combined out & err redirection. Creates/overwrites files.
Only one file name, used for both output streams

11. Redirecting Standard Output

12. Standard output redirection (>, >>)
Standard output is the terminal, by default.
The following redirections are possible:
[jack@foo jack]$ uptime
5:09pm up 8:26, 6 users, load average: 0.22, 0.20, 0.20
No redirection
Redirection to a file (“>” means “to”). If the file already exists, it is overwritten.
[jack@foo jack]$ uptime > Uptime-file
[jack@foo jack]$ ls -l Uptime-file
-rw-rw-r-- 1 jack jack 62 Apr 8 17:09 Uptime-file
[jack@foo jack]$ cat Uptime-file
5:09pm up 8:26, 6 users, load average: 0.13, 0.18, 0.19
Redirection appending to a file (“>>” means “to the end of”)
[jack@foo jack]$ uptime >> Uptime-file
[jack@foo jack]$ cat Uptime-file
5:09pm up 8:26, 6 users, load average: 0.13, 0.18, 0.19
5:10pm up 8:27, 6 users, load average: 0.24, 0.21, 0.20
Redirecting to a program. Here the output of uptime is being piped as the input to wc.
jack@foo jack]$ uptime | wc
1 10 62

13. Standard error redirection (2>, 2>>, 2>&1)
[jack@foo jack]$ tar cf sysconfig.tar /etc/sysconfig
tar: Removing leading `/' from member names
tar: /etc/sysconfig/rhn/systemid: Cannot open: Permission denied
tar: /etc/sysconfig/rhn/up2date-keyring.gpg: Cannot open: Permission
denied
tar: /etc/sysconfig/iptables: Cannot open: Permission denied
tar: /etc/sysconfig/static-routes: Cannot open: Permission denied
tar: Error exit delayed from previous errors
Output sent to stderr:
•Error messages (command 2>/dev/null)
•Warning messages (command 2>> warning.log)
•Debugging and progress indicators (command 2>&1 | less )
If stderr is not redirected, error output is displayed on the terminal.

14. Standard error redirection (2>, 2>>, 2>&1)
The notation 2>&1 means “stderr(2) to copy of stdout(1)”.
The following all send standard error output to the same destination as standard output:
• command >file 2>&1
• command 2>file 1>&2
• command &> file
• command >& file (this is not the preferred method)
• command 2>&1 | anothercommand
discard the output and the errors from the commands, sending them to /dev/null.
[jack@foo jack]$ tar vcf pam.tar /etc/sysconfig >/dev/null 2>&1
[jack@foo jack]$ nosuchcommandorfilename 2>/dev/null 1>&2
[jack@foo jack]$ ls -la /root 2>/dev/null 1>&2
[jack@foo jack]$ updatedb >& /dev/null &
[jack@foo jack]$ killall -HUP inetd sendmail &> /dev/null
[jack@foo jack]$ killall -HUP inetd sendmail &> /dev/null
commands send standard error output to the same destination standard output (a program).
[jack@foo jack]$ bash -x /etc/init.d/network restart 2>&1 | less
[jack@foo jack]$ tar -c / 2>&1 | less
[jack@foo jack]$ strace mailq 2>&1 | less

15. Output Redirects Summary
> file
capture stdout to file; overwrites; > is equivalent to 1>
2> file
capture stderr to file; overwrites
> file 2> file2
capture stdout to file; capture stderr to file2; overwrites
>> file
capture stdout to file; appends; >> is equivalent to 1>>
2>> file
capture stderr to file; appends
>> file 2>> file2
capture stdout to file; capture stderr to file2; appends
> file 2>&1
capture stdout to file; capture stderr to file; overwrites
>> file 2>&1
capture stdout to file; capture stderr to file; appends

16. Input Redirects
Simple input redirection

17. Input Redirects
• Input redirection isn’t common anymore - now cmds can handle their
own file I/O
• Input and output redirection can be combined:
cat < who.all > cat.who.all
cat < who.all 2> cat.who.all.err
cat < who.all > cat.who.all.all 2>&1

18. Redirecting Standard Input

19. Standard input redirection (<, <<EOF, |)
[jack@foo jack]$ wc
Hello. I typed this line. I typed and typed. I wondered, as I typed, how many words I was typing. Tis
wondrous that as I typed, bash piped.
<Ctrl+D>
4 28 143
Standard input is terminal by default. Following redirections are possible:
Read input from the terminal:
Read input from file with <. standard input anonymous: wc does not know file name which it is reading
from.
[jack@foo jack]$ wc < /etc/passwd
46 62 2010
Read input from a program using the pipe redirection. Here cat is piping its output to wc.
[jack@foo jack]$ cat /etc/passwd | wc
46 62 2010
Read input from script or cmdline. Operator << is traditionally followed by EOF, but any letters can be used.
When a line is found which consists only of EOF, that's the end of the input. This is called a “here
document”.
[jack@foo jack]$ wc << EOF
> Hello
> There are
> Four lines
> I think
> EOF
4 7 35

20. Pipes
The output of who is piped into the input of wc -l
This produces a count of the current user sessions

21. Piping Output to Next Command

22. Pipes
Pipeline – Ability to join input and output of processes together.
To set up a pipe use pipe symbol | between cmds that generates output.
Generally, data piped is text, but its possible to join far too many processes together.
Pipes can be chained as needed, and can also be combined with redirection:
who | fgrep bob | wc -l > bob.sessions
It’s even possible to intermix pipes and redirection! Just keep streams straight:
who 2> who.errors | fgrep bob 2>&1 | wc -l

23. The tee Command

24. tee – divert a copy to a file
tee is like a teepiece for pipes (or T piece).
tee takes one argument, a filename, and will feed all input from stdin to file,
simultaneously feeding output to stdout.
tee forks its input stream, sending one copy to file on disk, and another
copy to stdout
By default tee is like redirection > and overwrites contents in the files specified.
To apend use tee -a.