16.2. Redirecting Code Blocks

#!/bin/bash

if [ -z "$1" ]
then
  Filename=names.data  # Default, if no filename specified.
else
  Filename=$1
fi  
# Filename=${1:-names.data}
# can replace the above test (parameter substitution).

count=0

echo

while [ "$name" != Smith ]  # Why is variable $name in quotes?
do
  read name                 # Reads from $Filename, rather than stdin.
  echo $name
  let "count += 1"
done <"$Filename"           # Redirects stdin to file $Filename. 
#    ^^^^^^^^^^^^

echo; echo "$count names read"; echo

# Note that in some older shell scripting languages,
# the redirected loop would run as a subshell.
# Therefore, $count would return 0, the initialized value outside the loop.
# Bash and ksh avoid starting a subshell whenever possible,
# so that this script, for example, runs correctly.
# Thanks to Heiner Steven for pointing this out.

exit 0

#!/bin/bash

# This is an alternate form of the preceding script.

# Suggested by Heiner Steven
# as a workaround in those situations when a redirect loop
# runs as a subshell, and therefore variables inside the loop
# do not keep their values upon loop termination.


if [ -z "$1" ]
then
  Filename=names.data     # Default, if no filename specified.
else
  Filename=$1
fi  


exec 3<&0                 # Save stdin to file descriptor 3.
exec 0<"$Filename"        # Redirect standard input.

count=0
echo


while [ "$name" != Smith ]
do
  read name               # Reads from redirected stdin ($Filename).
  echo $name
  let "count += 1"
done <"$Filename"         # Loop reads from file $Filename. 
#    ^^^^^^^^^^^^


exec 0<&3                 # Restore old stdin.
exec 3<&-                 # Close temporary fd 3.

echo; echo "$count names read"; echo

exit 0

#!/bin/bash
# Same as previous example, but with "until" loop.

if [ -z "$1" ]
then
  Filename=names.data         # Default, if no filename specified.
else
  Filename=$1
fi  

# while [ "$name" != Smith ]
until [ "$name" = Smith ]     # Change  !=  to =.
do
  read name                   # Reads from $Filename, rather than stdin.
  echo $name
done <"$Filename"             # Redirects stdin to file $Filename. 
#    ^^^^^^^^^^^^

# Same results as with "while" loop in previous example.

exit 0

#!/bin/bash

if [ -z "$1" ]
then
  Filename=names.data          # Default, if no filename specified.
else
  Filename=$1
fi  

line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.
# Very contrived and kludgy, nevertheless shows that
# it's possible to redirect stdin within a "for" loop...
# if you're clever enough.
#
# More concise is     line_count=$(wc < "$Filename")


for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers.
# while [ "$name" != Smith ]   --   more complicated than a "while" loop   --
do
  read name                    # Reads from $Filename, rather than stdin.
  echo $name
  if [ "$name" = Smith ]       # Need all this extra baggage here.
  then
    break
  fi  
done <"$Filename"              # Redirects stdin to file $Filename. 
#    ^^^^^^^^^^^^

exit 0

#!/bin/bash

if [ -z "$1" ]
then
  Filename=names.data          # Default, if no filename specified.
else
  Filename=$1
fi  

Savefile=$Filename.new         # Filename to save results in.
FinalName=Jonah                # Name to terminate "read" on.

line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.


for name in `seq $line_count`
do
  read name
  echo "$name"
  if [ "$name" = "$FinalName" ]
  then
    break
  fi  
done < "$Filename" > "$Savefile"     # Redirects stdin to file $Filename,
#    ^^^^^^^^^^^^^^^^^^^^^^^^^^^       and saves it to backup file.

exit 0

#!/bin/bash

if [ -z "$1" ]
then
  Filename=names.data   # Default, if no filename specified.
else
  Filename=$1
fi  

TRUE=1

if [ "$TRUE" ]          # if true    and   if :   also work.
then
 read name
 echo $name
fi <"$Filename"
#  ^^^^^^^^^^^^

# Reads only first line of file.
# An "if/then" test has no way of iterating unless embedded in a loop.

exit 0