[20201116]bash shell IO重定向.txt

--//bash shell的文件句柄缺省0,1,2指向标准输入，标准输出，标准错误，简单探究IO 重定向问题。
--//测试例子来源：Wiley Shell Scripting.pdf

$ ls -l /dev/std*
lrwxrwxrwx 1 root root 15 2019-12-23 23:46:09 /dev/stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 2019-12-23 23:46:09 /dev/stdin -> /proc/self/fd/0
lrwxrwxrwx 1 root root 15 2019-12-23 23:46:09 /dev/stdout -> /proc/self/fd/1

1.测试1：
$ ls -l /proc/self/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-11-16 09:56:42 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-16 09:56:42 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-16 09:56:42 2 -> /dev/pts/4
lr-x------ 1 oracle oinstall 64 2020-11-16 09:56:42 3 -> /proc/50312/fd

2.测试2：
$ ls -l /proc/self/fd > /tmp/ls-output.txt
$ cat /tmp/ls-output.txt
total 0
lrwx------ 1 oracle oinstall 64 2020-11-16 09:57:01 0 -> /dev/pts/4
l-wx------ 1 oracle oinstall 64 2020-11-16 09:57:01 1 -> /tmp/ls-output.txt
lrwx------ 1 oracle oinstall 64 2020-11-16 09:57:01 2 -> /dev/pts/4
lr-x------ 1 oracle oinstall 64 2020-11-16 09:57:01 3 -> /proc/50313/fd
--//标准输出1定向到/tmp/ls-output.txt。

3.测试3:
$ ls -l /proc/self/fd >| /tmp/ls-output.txt 2>| /tmp/ls-err.txt
$ cat /tmp/ls-output.txt
total 0
lrwx------ 1 oracle oinstall 64 2020-11-16 09:57:57 0 -> /dev/pts/4
l-wx------ 1 oracle oinstall 64 2020-11-16 09:57:57 1 -> /tmp/ls-output.txt
l-wx------ 1 oracle oinstall 64 2020-11-16 09:57:57 2 -> /tmp/ls-err.txt
lr-x------ 1 oracle oinstall 64 2020-11-16 09:57:57 3 -> /proc/50317/fd
--//标准输出1定向到/tmp/ls-output.txt,标准错误2定向到/tmp/ls-err.txt

4.测试4:
$ ls -l /proc/self/fd /nosuchfile >| /tmp/ls-output.txt 2>| /tmp/ls-err.txt

$ cat /tmp/ls-output.txt
/proc/self/fd:
total 0
lrwx------ 1 oracle oinstall 64 2020-11-16 09:59:18 0 -> /dev/pts/4
l-wx------ 1 oracle oinstall 64 2020-11-16 09:59:18 1 -> /tmp/ls-output.txt
l-wx------ 1 oracle oinstall 64 2020-11-16 09:59:18 2 -> /tmp/ls-err.txt
lr-x------ 1 oracle oinstall 64 2020-11-16 09:59:18 3 -> /proc/50335/fd

$ cat /tmp/ls-err.txt
ls: /nosuchfile: No such file or directory

5.测试5:
 $ ls -l /proc/self/fd /nosuchfile < /etc/hosts   >| /tmp/ls-output.txt 2>| /tmp/ls-err.txt
$ cat /tmp/ls-output.txt
/proc/self/fd:
total 0
lr-x------ 1 oracle oinstall 64 2020-11-16 10:00:20 0 -> /etc/hosts
l-wx------ 1 oracle oinstall 64 2020-11-16 10:00:20 1 -> /tmp/ls-output.txt
l-wx------ 1 oracle oinstall 64 2020-11-16 10:00:20 2 -> /tmp/ls-err.txt
lr-x------ 1 oracle oinstall 64 2020-11-16 10:00:20 3 -> /proc/50365/fd
--//标准输出1定向到/tmp/ls-output.txt,标准错误2定向到/tmp/ls-err.txt
--//标准输入0定向到/etc/hosts

$ cat /tmp/ls-err.txt
ls: /nosuchfile: No such file or directory

6.测试6 exec：

$ echo $$
38978
$ ps -fp 38978
UID        PID  PPID  C STIME TTY          TIME CMD
oracle   38978 38977  0 Apr27 pts/4    00:00:01 -bash
--//进程号38978就是登录运行的bash shell。

$ ls -l /proc/$$/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 2 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-02 09:26:00 255 -> /dev/pts/4

The open files are 0 (stdin), 1 (stdout), and 2 (stderr). 255 is a little trick that bash uses to keep a
copy of these for when they are redirected. This is specific to bash. Other shells will act the same for
these tests, but will not have file descriptor 255.

7.测试7 exec>：
$ exec 3> /tmp/testing
$ exec 3> /tmp/testing
$ ls -l /proc/$$/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 2 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-02 09:26:00 255 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 3 -> /tmp/testing

8.测试8 exec<：

$ exec 4< /tmp/testing
$ ls -l /proc/$$/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 2 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-02 09:26:00 255 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 3 -> /tmp/testing
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 4 -> /tmp/testing

8.测试9：

$ exec 3<&-
$ ls -l /proc/$$/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 2 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-02 09:26:00 255 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 4 -> /tmp/testing

$ exec 4<&-
$ ls -l /proc/$$/fd
total 0
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 0 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 1 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-04-27 11:27:43 2 -> /dev/pts/4
lrwx------ 1 oracle oinstall 64 2020-11-02 09:26:00 255 -> /dev/pts/4