Tyler Chris - Fedora Linux стр 47.

4.11.1.1. Redirection

$ cal 7 2006

July 2006

Su Mo Tu We Th Fr Sa

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30 31

$ cal 7 2006 >month.txt

$ cat month.txt

July 2006

Su Mo Tu We Th Fr Sa

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30 31

When you redirect output with > , the previous contents of the file are overwritten. To append (add) to the file, use >> :

$ cal3 2009>>month.txt

$ catmonth.txt

July 2006

Su Mo Tu We Th Fr Sa

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30 31

July 2006

Su Mo Tu We Th Fr Sa

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30 31

March 2009

Su Mo Tu We Th Fr Sa

1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16 17 18 19 20 21

22 23 24 25 26 27 28

29 30 31

Error messages are not sent to standard output, so you can still see the messages even when standard output is redirected:

$ cal17 2009 >month.txt

cal: illegal month value: use 1-12

To redirect error messages, place the file descriptor number ( 2 ) in front of the redirection symbol ( > or >> ):

$ cal 17 2009 2>errors

$ caterrors

cal: illegal month value: use 1-12

You can redirect both standard output and standard error:

$ cal 17 2009 >month.txt 2>errors

To redirect the input of a command, use the less-than sign ( < ) followed by the filename containing the data you wish to use as the input:

$ echo "2^8" >problem

$ bc <problem

256

bc is a calculator program. The first command places a numeric expression in the file problem ; the second line starts bc , using problem as the input. The output from bc is the solution of the expression: 256 .

Of course, you can redirect both input and output:

$ bc <problem >result

4.11.1.2. Piping

$ mount

/dev/mapper/main-root on / type ext3 (rw)

proc on /proc type proc (rw)

sysfs on /sys type sysfs (rw)

devpts on /dev/pts type devpts (rw,gid=5,mode=620)

/dev/hdc2 on /boot type ext3 (rw)

tmpfs on /dev/shm type tmpfs (rw)

/dev/mapper/main-home on /home type ext3 (rw)

none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)

/dev/sdb on /media/disk type vfat (rw,noexec,nosuid,nodev,shortname=winnt,uid=503)

$ mount | grep/dev/mapper

/dev/mapper/main-root on / type ext3 (rw)

/dev/mapper/main-home on /home type ext3 (rw)

In this example, the output of the mount command is used as the input to the grep command, which outputs only lines that match the specified pattern. A group of commands connected together with pipe symbols is known as a pipeline . You can extend a pipeline by connecting additional commands:

$ mount | grep /dev/mapper| sort

/dev/mapper/main-home on /home type ext3 (rw)

/dev/mapper/main-root on / type ext3 (rw)

The input to a pipeline and the output from a pipeline may be redirected:

$ cut -d: -f1 </etc/passwd|sort|head >output

$ catoutput

adm

apache

avahi

beaglidx

bin

chip

chris

daemon

dbus

distcache

However, it's essential that the input redirect take place at the start of the pipeline (at the command on the left) and that the output redirection take place at the end (at the command on the right). Consider this wrong example:

$ cut -d: -f1 </etc/passwd|sort >output|head

In this case, it's unclear whether the standard output of sort should be placed in the file output or used as the standard input to the head command. The result is undefined (which means don't do this! ).

4.11.2. How Does It Work?

the command is executed. If there is a redirection error (such as an invalid filename or a file permission problem), it will be reported by the shell and the command will not be executed:

$ cal >foo/bar/baz

bash: foo/bar/baz: No such file or directory

Note that the error message starts with bash, indicating that it was produced by the shell and not by the cal command.

$ nice "cal" >test.txt

$ cattest.txt

July 2006

Su Mo Tu We Th Fr Sa

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30 31

4.11.3. What About...

4.11.3.1. ...redirecting standard output and standard error to the same destination?

$ cal 17 2009 >/tmp/calresult 2>&1

Notice that the order of the redirections matters. The preceding command will redirect all output to /tmp/calresult , but this command will not redirect standard error:

$ cal 17 2009 2>&1 >/tmp/calresult

The 2>&1 redirection is evaluated first, so standard error is directed to the same destination as standard output (which, at that point, is the terminal); > /tmp/calresult then redirects standard output by itself.

This construct can also be used with piping:

$ cal 17 2009 2>&1 | head -2

This will feed both the standard output and the standard error from cal into the standard input of head .

4.11.3.2. ...redirecting to a device?

$ head -50 /etc/services >/dev/lp0