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16.1. 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 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" ]     # 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.
# -----------------------------------------------

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 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.

 
 
  Published under the terms of the GNU General Public License Design by Interspire