使用exec

1 #!/bin/bash
2 # 使用‘exec‘重定向stdin. 
3
4
5 exec 6<&0 # 将文件描述符#6与stdin链接起来. 
6 # 保存stdin. 
7
8 exec < data-file # stdin被文件"data-file"所代替. 
9
10 read a1 # 读取文件"data-file"的第一行. 
11 read a2 # 读取文件"data-file"的第二行. 
12
13 echo
14 echo "Following lines read from file."
15 echo "-------------------------------"
16 echo $a1
17 echo $a2
18
19 echo; echo; echo
20
21 exec 0<&6 6<&-22 #  现在将stdin从fd #6中恢复, 因为刚才我们把stdin重定向到#6了, 
23 #+ 然后关闭fd #6 ( 6<&- ), 好让这个描述符继续被其他进程所使用. 
24 #
25 # <&6 6<&-  这么做也可以. 
26
27 echo -n "Enter data "
28 read b1 # 现在"read"已经恢复正常了, 就是能够正常的从stdin中读取.
29 echo "Input read from stdin."
30 echo "----------------------"
31 echo "b1 = $b1"
32
33 echo
34
35 exit 0

1 #!/bin/bash
2 # reassign-stdout.sh
3
4 LOGFILE=logfile.txt
5
6 exec 6>&1 # 将fd #6与stdout链接起来. 
7 # 保存stdout. 
8
9 exec > $LOGFILE # stdout就被文件"logfile.txt"所代替了. 
10
11 # ----------------------------------------------------------- #
12 # 在这块中所有命令的输出都会发送到文件$LOGFILE中. 
13
14 echo -n "Logfile: "
15 date
16 echo "-------------------------------------"
17 echo
18
19 echo "Output of \"ls -al\" command"
20 echo
21 ls -al
22 echo; echo
23 echo "Output of \"df\" command"
24 echo
25 df
26
27 # ----------------------------------------------------------- #
28
29 exec 1>&6 6>&- # 恢复stdout, 然后关闭文件描述符#6. 
30
31 echo
32 echo "== stdout now restored to default == "
33 echo
34 ls -al
35 echo
36
37 exit 0

1 #!/bin/bash
2 # upperconv.sh
3 # 将一个指定的输入文件转换为大写. 
4
5 E_FILE_ACCESS=70
6 E_WRONG_ARGS=71
7
8 if [ ! -r "$1" ] # 判断指定的输入文件是否可读? 
9 then
10 echo "Can‘t read from input file!"
11 echo "Usage: $0 input-file output-file"
12 exit $E_FILE_ACCESS
13 fi #  即使输入文件($1)没被指定
14 #+ 也还是会以相同的错误退出(为什么?). 
15
16 if [ -z "$2" ]
17 then
18 echo "Need to specify output file."
19 echo "Usage: $0 input-file output-file"
20 exit $E_WRONG_ARGS
21 fi
22
23
24 exec 4<&0
25 exec < $1 # 将会从输入文件中读取. 
26
27 exec 7>&1
28 exec > $2 # 将写到输出文件中. 
29 # 假设输出文件是可写的(添加检查?). 
30
31 # -----------------------------------------------32 cat - | tr a-z A-Z # 转换为大写. 
33 # ^^^^^ # 从stdin中读取. 
34 # ^^^^^^^^^^ # 写到stdout上. 
35 # 然而, stdin和stdout都被重定向了. 
36 # -----------------------------------------------37
38 exec 1>&7 7>&- # 恢复stout.
39 exec 0<&4 4<&- # 恢复stdin.
40
41 # 恢复之后, 下边这行代码将会如预期的一样打印到stdout上. 
42 echo "File \"$1\" written to \"$2\" as uppercase conversion."
43
44 exit 0

1 #!/bin/bash
2 # avoid-subshell.sh
3 # 由Matthew Walker所提出的建议. 
4
5 Lines=0
6
7 echo
8
9 cat myfile.txt | while read line; # (译者注: 管道会产生子shell)
10 do {
11 echo $line
12 (( Lines++ )); #  增加这个变量的值
13 #+ 但是外部循环却不能访问. 
14 #  子shell问题. 
15 }
16 done
17
18 echo "Number of lines read = $Lines" # 0
19 # 错误!
20
21 echo "------------------------"
22
23
24 exec 3<> myfile.txt
25 while read line <&3
26 do {
27 echo "$line"
28 (( Lines++ )); #  增加这个变量的值
29 #+ 现在外部循环就可以访问了. 
30 #  没有子shell, 现在就没问题了. 
31 }
32 done
33 exec 3>&-34
35 echo "Number of lines read = $Lines" # 8
36
37 echo
38
39 exit 0