redir2.sh

Shall高级编程

#!/bin/bash
# redir2.sh

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

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 PIPED "while-read" loop,
#+ as distinct from 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.
#  And, thanks, Brian Onn, for correcting an annotation error.