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16.1. Using execAn exec <filename command redirects stdin to a file. From that point on, all stdin comes from that file, rather than its normal source (usually keyboard input). This provides a method of reading a file line by line and possibly parsing each line of input using sed and/or awk. Example 16-1. Redirecting stdin using 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 Similarly, an exec >filename command redirects stdout to a designated file. This sends all command output that would normally go to stdout to that file. Example 16-2. Redirecting stdout using 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 Example 16-3. Redirecting both stdin and stdout in the same script with exec #!/bin/bash # upperconv.sh # Converts a specified input file to uppercase. E_FILE_ACCESS=70 E_WRONG_ARGS=71 if [ ! -r "1ドル" ] # Is specified input file readable? then echo "Can't read from input file!" echo "Usage: 0ドル input-file output-file" exit $E_FILE_ACCESS fi # Will exit with same error #+ even if input file (1ドル) not specified (why?). if [ -z "2ドル" ] then echo "Need to specify output file." echo "Usage: 0ドル input-file output-file" exit $E_WRONG_ARGS fi exec 4<&0 exec < 1ドル # Will read from input file. exec 7>&1 exec > 2ドル # 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. # ----------------------------------------------- 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ドル\" written to \"2ドル\" as uppercase conversion." exit 0 I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem. Example 16-4. Avoiding a subshell #!/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. |
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