20.1. 使用 exec
exec <filename 命令重定向stdin到一个文件。从那时起，所有stdin都来自该文件，而不是其正常来源（通常是键盘输入）。这提供了一种逐行读取文件的方法，并可能使用 sed 和/或 awk 解析每行输入。
示例 20-1. 重定向stdin使用 exec
#!/bin/bash
# Redirecting stdin using 'exec'.
exec 6<&0 # Link file descriptor #6 with stdin.
# Saves stdin.
exec < data-file # stdin replaced by file "data-file"
read a1 # Reads first line of file "data-file".
read a2 # Reads second line of file "data-file."
echo
echo "Following lines read from file."
echo "-------------------------------"
echo $a1
echo $a2
echo; echo; echo
exec 0<&6 6<&-
# Now restore stdin from fd #6, where it had been saved,
#+ and close fd #6 ( 6<&- ) to free it for other processes to use.
#
# <&6 6<&- also works.
echo -n "Enter data "
read b1 # Now "read" functions as expected, reading from normal stdin.
echo "Input read from stdin."
echo "----------------------"
echo "b1 = $b1"
echo
exit 0 |
类似地，exec >filename 命令重定向stdout到一个指定的文件。这会将通常发送到stdout的所有命令输出发送到该文件。
 | exec N > filename 影响整个脚本或当前 shell。从那时起，脚本或 shell 的 PID 中的重定向已更改。然而 . . . N > filename 仅影响新 fork 的进程，而不影响整个脚本或 shell。 感谢 Ahmed Darwish 指出这一点。 |
示例 20-2. 重定向stdout使用 exec
#!/bin/bash
# reassign-stdout.sh
LOGFILE=logfile.txt
exec 6>&1 # Link file descriptor #6 with stdout.
# Saves stdout.
exec > $LOGFILE # stdout replaced with file "logfile.txt".
# ----------------------------------------------------------- #
# All output from commands in this block sent to file $LOGFILE.
echo -n "Logfile: "
date
echo "-------------------------------------"
echo
echo "Output of \"ls -al\" command"
echo
ls -al
echo; echo
echo "Output of \"df\" command"
echo
df
# ----------------------------------------------------------- #
exec 1>&6 6>&- # Restore stdout and close file descriptor #6.
echo
echo "== stdout now restored to default == "
echo
ls -al
echo
exit 0 |
示例 20-3. 同时重定向stdin和stdout在同一脚本中使用 exec
#!/bin/bash
# upperconv.sh
# Converts a specified input file to uppercase.
E_FILE_ACCESS=70
E_WRONG_ARGS=71
if [ ! -r "$1" ] # Is specified input file readable?
then
echo "Can't read from input file!"
echo "Usage: $0 input-file output-file"
exit $E_FILE_ACCESS
fi # Will exit with same error
#+ even if input file ($1) not specified (why?).
if [ -z "$2" ]
then
echo "Need to specify output file."
echo "Usage: $0 input-file output-file"
exit $E_WRONG_ARGS
fi
exec 4<&0
exec < $1 # Will read from input file.
exec 7>&1
exec > $2 # Will write to output file.
# Assumes output file writable (add check?).
# -----------------------------------------------
cat - | tr a-z A-Z # Uppercase conversion.
# ^^^^^ # Reads from stdin.
# ^^^^^^^^^^ # Writes to stdout.
# However, both stdin and stdout were redirected.
# Note that the 'cat' can be omitted.
# -----------------------------------------------
exec 1>&7 7>&- # Restore stout.
exec 0<&4 4<&- # Restore stdin.
# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."
exit 0 |
I/O 重定向是避免可怕的 子 shell 中不可访问的变量 问题的巧妙方法。
示例 20-4. 避免子 shell
#!/bin/bash
# avoid-subshell.sh
# Suggested by Matthew Walker.
Lines=0
echo
cat myfile.txt | while read line;
do {
echo $line
(( Lines++ )); # Incremented values of this variable
#+ inaccessible outside loop.
# Subshell problem.
}
done
echo "Number of lines read = $Lines" # 0
# Wrong!
echo "------------------------"
exec 3<> myfile.txt
while read line <&3
do {
echo "$line"
(( Lines++ )); # Incremented values of this variable
#+ accessible outside loop.
# No subshell, no problem.
}
done
exec 3>&-
echo "Number of lines read = $Lines" # 8
echo
exit 0
# Lines below not seen by script.
$ cat myfile.txt
Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8. |