3.14. I/O Redirection

   1 #!/bin/bash
   2 # Redirecting stdin using 'exec'.
   3 
   4 
   5 exec 6<&0   # Link file descriptor #6 with stdin.
   6 
   7 exec < data-file   # stdin replaced by file "data-file"
   8 
   9 read a1   # Reads first line of file "data-file".
  10 read a2   # Reads second line of file "data-file."
  11 
  12 echo
  13 echo "Following lines read from file."
  14 echo "-------------------------------"
  15 echo $a1
  16 echo $a2
  17 
  18 echo; echo; echo
  19 
  20 exec 0<&6 6<&-
  21 # Now restore stdin from fd #6, where it had been saved,
  22 # and close fd #6 ( 6<&- ) to free it for other processes to use.
  23 # <&6 6<&-    also works.
  24 
  25 echo -n "Enter data  "
  26 read b1  # Now "read" functions as expected, reading from normal stdin.
  27 echo "Input read from stdin."
  28 echo "----------------------"
  29 echo "b1 = $b1"
  30 
  31 echo
  32 
  33 exit

   1 #!/bin/bash
   2 
   3 if [ -z "$1" ]
   4 then
   5   Filename=names.data  # Default, if no filename specified.
   6 else
   7   Filename=$1
   8 fi  
   9 # Filename=${1:-names.data}  can replace the above test.
  10 # (parameter substitution)
  11 
  12 count=0
  13 
  14 echo
  15 
  16 while [ "$name" != Smith ]  # Why is variable $name in quotes?
  17 do
  18   read name         # Reads from $Filename, rather than stdin.
  19   echo $name
  20   let "count += 1"
  21 done <"$Filename"   # Redirects stdin to file $Filename. 
  22 
  23 echo; echo "$count names read"; echo
  24 
  25 # Note that in some older shell scripting languages,
  26 # the redirected loop would run as a subshell.
  27 # Therefore, $count would return 0, the initialized value outside the loop.
  28 # Bash and ksh avoid starting a subshell whenever possible,
  29 # so that this script, for example, runs correctly.
  30 # Thanks to Heiner Steven for pointing this out.
  31 
  32 exit 0

   1 #!/bin/bash
   2 
   3 # This is an alternate form of the preceding script.
   4 
   5 # Suggested by Heiner Steven
   6 # as a workaround in those situations when a redirect loop
   7 # runs as a subshell, and therefore variables inside the loop
   8 # do not keep their values upon loop termination.
   9 
  10 
  11 if [ -z "$1" ]
  12 then
  13   Filename=names.data  # Default, if no filename specified.
  14 else
  15   Filename=$1
  16 fi  
  17 
  18 
  19 exec 3<&0                 # Save stdin to file descriptor 3.
  20 exec 0<"$Filename"        # Redirect standard input.
  21 
  22 count=0
  23 echo
  24 
  25 
  26 while [ "$name" != Smith ]
  27 do
  28   read name         # Reads from redirected stdin ($Filename).
  29   echo $name
  30   let "count += 1"
  31 done <"$Filename"   # Loop reads from file $Filename. 
  32 
  33 
  34 exec 0<&3               # Restore old stdin.
  35 exec 3<&-               # Close temporary fd 3.
  36 
  37 echo; echo "$count names read"; echo
  38 
  39 exit 0

   1 #!/bin/bash
   2 # Same as previous example, but with "until" loop.
   3 
   4 if [ -z "$1" ]
   5 then
   6   Filename=names.data  # Default, if no filename specified.
   7 else
   8   Filename=$1
   9 fi  
  10 
  11 # while [ "$name" != Smith ]
  12 until [ "$name" = Smith ]     # Change  !=  to =.
  13 do
  14   read name         # Reads from $Filename, rather than stdin.
  15   echo $name
  16 done <"$Filename"   # Redirects stdin to file $Filename. 
  17 
  18 # Same results as with "while" loop in previous example.
  19 
  20 exit 0

   1 #!/bin/bash
   2 
   3 if [ -z "$1" ]
   4 then
   5   Filename=names.data  # Default, if no filename specified.
   6 else
   7   Filename=$1
   8 fi  
   9 
  10 line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.
  11 # Very contrived and kludgy, nevertheless shows that
  12 # it's possible to redirect stdin within a "for" loop...
  13 # if you're clever enough.
  14 #
  15 # More concise is     line_count=$(wc < "$Filename")
  16 
  17 
  18 for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers.
  19 # while [ "$name" != Smith ]   --   more complicated than a "while" loop   --
  20 do
  21   read name         # Reads from $Filename, rather than stdin.
  22   echo $name
  23   if [ "$name" = Smith ]   # Need all this extra baggage here.
  24   then
  25     break
  26   fi  
  27 done <"$Filename"   # Redirects stdin to file $Filename. 
  28 
  29 exit 0

   1 #!/bin/bash
   2 
   3 if [ -z "$1" ]
   4 then
   5   Filename=names.data  # Default, if no filename specified.
   6 else
   7   Filename=$1
   8 fi  
   9 
  10 TRUE=1
  11 
  12 if [ "$TRUE" ]   # if true    and   if :   also work.
  13 then
  14  read name
  15  echo $name
  16 fi <"$Filename"
  17 # Reads only first line of file.
  18 # An if/then test has no way of iterating unless embedded in a loop.
  19 
  20 exit 0

   1 #!/bin/bash
   2 # logevents.sh, by Stephane Chazelas.
   3 
   4 # Event logging to a file.
   5 # Must be run as root (for write access in /var/log).
   6 
   7 
   8 FD_DEBUG1=3
   9 FD_DEBUG2=4
  10 FD_DEBUG3=5
  11 
  12 # Uncomment a line below to activate script.
  13 # LOG_EVENTS=1
  14 # LOG_VARS=1
  15 
  16 
  17 log()  # Writes time and date to log file.
  18 {
  19 echo "$(date)  $*" >&7     # This *appends* the date to the file.
  20                            # See below.
  21 }
  22 
  23 
  24 
  25 case $LOG_LEVEL in
  26  1) exec 3>&2         4> /dev/null 5> /dev/null;;
  27  2) exec 3>&2         4>&2         5> /dev/null;;
  28  3) exec 3>&2         4>&2         5>&2;;
  29  *) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
  30 esac
  31 
  32 FD_LOGVARS=6
  33 if [[ $LOG_VARS ]]
  34 then exec 6>> /var/log/vars.log
  35 else exec 6> /dev/null               # Bury output.
  36 fi
  37 
  38 FD_LOGEVENTS=7
  39 if [[ $LOG_EVENTS ]]
  40 then
  41   # then exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
  42   # Above line will not work in Bash, version 2.04.
  43   exec 7>> /var/log/event.log        # Append to "event.log".
  44   log                                # Write time and date.
  45 else exec 7> /dev/null               # Bury output.
  46 fi
  47 
  48 echo "DEBUG3: beginning" >&${FD_DEBUG3}
  49 
  50 #command1 >&5 2>&4
  51 ls -l >&5 2>&4
  52 
  53 #command2 
  54 echo "Done"
  55 
  56 echo "sending mail" >&${FD_LOGEVENTS}   # Writes "sending mail" to fd #7.
  57 
  58 
  59 exit 0