Advanced Bash-Scripting Guide
# Redirecting only stderr to a pipe.
exec 3>&1 # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls').
# ^^^^ ^^^^ exec 3>&- # Now close it for the remainder of the script.
# Thanks, S.C.
For a more detailed introduction to I/O redirection see Appendix F.
20.1. Using exec
An 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 20-1. 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.
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exec N > filename
affects the entire script or current shell. Redirection in the PID of the script or shell
from that point on has changed. However . . .
N > filename
affects only the newly-forked process, not the entire script or shell.
Thank you, Ahmed Darwish, for pointing this out.
Example 20-2. 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 20-3. 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" ] # Is specified input file readable?
then
echo "Can't read from input file!"
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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 redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem.
Example 20-4. 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 "------------------------"
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