Chapter VI Linux System Administration
Redirection and piping technology
Outline
In the shell program, the most commonly used FD (file descriptor) about three, namely:
0 is a file descriptor, standard input (stdin)
1 is a file descriptor, standard output (stdout)
2 is a file descriptor, standard error (stderr)
A plurality Linux command appropriately combined together, make it work, so that we can more efficiently process data on a daily basis. To do this, we must engage in order to understand the principles of redirection of input and output redirection.
Briefly, input redirection refers to the import file to the command, and output redirection refers to the original data to be output to the screen information is written to the specified file. In the daily study and work, compared to the input redirection, we use a higher frequency output redirection, output redirection so in turn divided into two different techniques to redirect the standard output and error output redirection, as well as empty write and append write modes.
When to use redirection
1. When the screen output of information is important on it and we need to keep him down when
2. Background of the executive program ⾏, dry place that he does not want interference screen when normal output;
Example 3. ⾏ command system, for example, the timing results Perform the task, he can survive when desired;
4. Perform ⼀ some commands, we already know he error message may appear, when he discards want;
The error message when outputs need to correct information;
[root@RHCE7 ~]# find >/root/syj.txt 2>/root/syjerror.txt
6.1 Redirection Example
6.1.1 standard output redirection
[root@Redhat7 ~]# ifconfig >/root/syjjjj.txt
Empty original file
6.1.2 standard output redirection (added)
[root@Redhat7 ~]# ifconfig >/root/syjjjj.txt
In the original file to append
6.1.3 correct errors and redirect the output files are written to different
[root@Redhat7 etc]# find /etc 1>/root/syj.txt 2>/root/jjj.txt
6.1.4 redirected to the null device
[root@Redhat7 etc]# ls /root >ab 2>/dev/null
6.2 input redirection
Input redirection is <.
[root@Redhat7 ~]# grep root </etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
MySQL table structure input
[root@Redhat7 ~]# mysql -uroot -p123 <bbs.sql
6.3 using a redirect script
[root@Redhat7 ~]# cat ping.sh
ping -c1 192.168.27.140 &>/dev/null
if [ $? -eq 0 ]; then
echo "192.168.27.140 is up"
else
echo "192.168.69.113 is down"
be
[root@Redhat7 ~]# sh ping.sh
192.168.27.140 is up
6.4 Redirection summary
6.5 Process Pipeline Technology
Pipeline operator "|" symbol connecting the left and right commands, the output of the left standard commands, commands to the right of the standard input START.
6.5.1 Example pipeline technology - Sorting
[root@Redhat7 ~]# sort -t ":" -k3 -n /etc/passwd|head
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
Examples of techniques 6.5.2 Pipeline - CPU Statistics Top 5
[root@Redhat7 ~]# ps aux --sort=-%cpu | head -6
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
gdm 2443 0.3 4.1 1543032 77708 ? Sl 21:45 0:02 gnome-shell --mode=gdm
root 1 0.1 0.2 123312 3880 ? Ss 21:45 0:00 /usr/lib/systemd/systemd --switched-root --system --deserialize 21
root 4 0.1 0.0 0 0 ? S 21:45 0:00 [kworker/0:0]
root 281 0.1 0.0 0 0 ? S 21:45 0:01 [kworker/1:1]
root 2 0.0 0.0 0 0 ? S 21:45 0:00 [kthreadd]
6.5.3 Pipeline Technology practice tee
tee command to redirect data to a file, on the other hand may also provide a copy of the data as a subsequent redirection command stdin. Simply means that the data is redirected to the given file and screen.
Tee command content directly output as input to another command
[root@Redhat7 ~]# ls /iso7.2 | tee /output.txt | wc -l
15
[root@Redhat7 ~]# cat /output.txt
addons
EFI
EULA
GPL
images
isolinux
LiveOS
media.repo
Packages
release-notes
Repodt
RPM-GPG-KEY-redhat-beta
RPM-GPG-KEY-redhat-release
syjhct
TRANS.TBL
Tee and> the same effect, but the tee will be output to the screen
[root@Redhat7 ~]# date >syj.txt
[root@Redhat7 ~]# date | tee syj.txt
August 20, 2019 21:34:16 CST Tuesday
