converting shell while loop to for loop

farkus888 · 09-11-2007, 05:31 AM

I have a shell script that has a loop that passes through the lines in a file one at a time. the problem that I have is that the rsh commands inside the loop break it. some research shows that a for loop will not break with rsh commands like a while loop will. so my question is how can I best convert a loop of the form

Code:

while read line
do
stuff
done < file

to one of the form

Code:

for line in (lines of file)
do
stuff
done

this is a sh script on solaris 9. thanks in advance for your help.

colucix · 09-11-2007, 05:42 AM

A method could be: first set the input field separator as a newline to avoid problems with lines containing blanks, then...

Code:

#!/bin/sh
IFS="\n"
for line in `cat file`
do
  echo $line
done

You may also reset the IFS to the previous value (usually a blank space) by previously storing it in another (backup) variable.

Code:

IFS_PRE=$IFS

farkus888 · 09-11-2007, 06:38 PM

that sounds good until I run it and it breaks up my absolute filenames that are in the file we are iterating through on every n using the exact syntax you gave me. its breaking my absolute filenames into 3 or 4 garbled pieces of garbage.

osor · 09-11-2007, 06:49 PM

Quote:

Originally Posted by farkus888

that sounds good until I run it and it breaks up my absolute filenames that are in the file we are iterating through on every n using the exact syntax you gave me. its breaking my absolute filenames into 3 or 4 garbled pieces of garbage.

That’s because the IFS variable can’t be set using escape sequences (at least I don’t think so). You have to actually set the variable to a newline—e.g.:

Code:

IFS="$(echo)"

With the previous setting, IFS matches either a literal backslash or a literal “n” as the field separator.

farkus888 · 09-11-2007, 07:09 PM

yeah its now interpreting the backquoted section literally and breaking on each of the characters inside the quotes.

osor · 09-11-2007, 07:20 PM

Quote:

Originally Posted by farkus888

yeah its now interpreting the backquoted section literally and breaking on each of the characters inside the quotes.

Huh? Are you saying what I suggested does or doesn’t work?

Just FYI, on my machine this works as expected:

Code:

#!/bin/sh
IFS="$(echo)"
for line in `cat file`
do
  echo $line
done

This does not work (lines are broken on every literal lowercase “n”):

Code:

#!/bin/sh
IFS="\n"
for line in `cat file`
do
  echo $line
done

This happens to work, but I’m not sure if it’s portable:

Code:

#!/bin/sh
for line in `cat file`
do
  echo $line
done

chrism01 · 09-11-2007, 07:24 PM

The default IFS is (iirc) (any/all of) space, tab and newline, but, as mentioned above, you have to set them literally, not via escape sequence.
Also, use single quote marks eg for newline its
IFS='
'

Of course, if the src file is just a list of absolute filenames, 1 per line, there's no need to fiddle with the IFS anyway.

farkus888 · 09-11-2007, 07:46 PM

looks like chrism01 wins with ignore it and the default FS works. I have a feeling I'll get stuck on something else along the way to finishing this project but for now I am set, thanks.

colucix · 09-12-2007, 02:30 AM

Quote:

Originally Posted by osor

That’s because the IFS variable can’t be set using escape sequences

You're right, sorry! Indeed I tested on a file containing blank spaces and no n chars. I remember something about using octal codes, but not sure. I will check. Sorry again! Cheers