done < $1?

[Joe Allen]

1: #!/bin/sh
   2: 
   3: # Demonstrate reading a file line-by-line, using I/O
   4: # redirection in a compound command
   5: # Also test variable setting inside an implicit subshell.
   6: # Test this under sh and ksh and compare the output.
   7: 
   8: line="TEST"
   9: save=
  10: 
  11: if [ -z "$1" ]; then
  12:    echo "Usage: $0 filename"
  13: else
  14:    if [ -r $1 ]; then
  15:       while read line; do
  16:          echo "$line"
  17:          save=$line
  18:       done < $1
  19:    fi
  20: fi
  21: echo "End value of \$line is $line"
  22: echo "End value of \$save is $save"

I am confused with "done < $1". If taking away "< $1", would the code still work properly? Could anyone please explain this to me? Thanks.

[Jeanne Boyarsky]

Joe,
In UNIX, the idiom to read a file is:

while read line; do
# code to do work
done < fileToRead

In your example, the code has abstracted "fileToRead" into a variable that is passed at the command line. That's what $1 is. The program won't work without < $1 because UNIX won't know what lines to read.

[Joe Allen]

Hi Jeanne,

It is nice to see you are in this forum too. Thank you so much for the explanation.

Joe