16.2. Redirecting Code Blocks

#!/bin/bash
# redir2.sh
if [ -z "1ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
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
exit 0
# Note that in some older shell scripting languages,
#+ the redirected loop would run as a subshell.
# Therefore, $count would return 0, the initialized value outside the loop.
# Bash and ksh avoid starting a subshell *whenever possible*,
#+ so that this script, for example, runs correctly.
# (Thanks to Heiner Steven for pointing this out.)
# However . . .
# Bash *can* sometimes start a subshell in a *redirected* "while" loop.
abc=hi
echo -e "1\n2\n3" | while read l
 do abc="$l"
 echo $abc
 done
echo $abc
# (Thanks, Bruno de Oliveira Schneider, for demonstrating this
#+ with the above snippet of code.)

#!/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ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
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 # Loop reads from file $Filename
 #+ because of line 20.
# The original version of this script terminated the "while" loop with
#+ done <"$Filename" 
# Exercise:
# Why is this unnecessary?
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ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
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ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
fi 
line_count=`wc $Filename | awk '{ print 1ドル }'`
# Number of lines in target file.
#
# Very contrived and kludgy, nevertheless shows that
#+ it's possible to redirect stdin within a "for" loop...
#+ if you're clever enough.
#
# More concise is line_count=$(wc -l < "$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ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
fi 
Savefile=$Filename.new # Filename to save results in.
FinalName=Jonah # Name to terminate "read" on.
line_count=`wc $Filename | awk '{ print 1ドル }'` # 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ドル" ]
then
 Filename=names.data # Default, if no filename specified.
else
 Filename=1ドル
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

Aristotle
Belisarius
Capablanca
Euler
Goethe
Hamurabi
Jonah
Laplace
Maroczy
Purcell
Schmidt
Semmelweiss
Smith
Turing
Venn
Wilson
Znosko-Borowski
# This is a data file for
#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".