Shell Programming and Scripting

Hi,

First post, so I hope someone can help me with this weirdness

I have a number files with some rows of information I want to extract, at the same time I want to add to a string some details from the file. I have found two different ways of looping over rows in a file, but one method doesn't let me concatenarte strings and I don't know why....

The first method sets the IFS environmental variable. I don't really favour this method, but it works..

Code:

#!/bin/bash -
OLDIFS=$IFS
IFS='
'

STE="cat"
  
for FILE in $( ls *-df | cut -f 4 -d / ); do
  N=0
  for LINE in $( cat $FILE ); do
    N=$((N+1))
    STE="$STE""dog"
  done
done
echo "$STE" && exit

and produces:
catdogdogdogdogdogdogdogdogdogdogdogdogdogdogdogdo gdogdogdogdogdogdogdogdogdogdogdogdog


Method 2 uses "read" but prevents the concatenation of the string:

Code:

#!/bin/bash -
STE="cat"
  
for FILE in $( ls *-df | cut -f 4 -d / ); do
  N=0
  cat "$FILE" | while read LINE ; do
    N=$((N+1))
    STE="$STE""dog"
  done
done
echo "$STE" && exit

and produces:
cat

Why is there a difference in the two methods? I would rather not be changing environmental variables and a simple loop is my preferred choice.

Thanks for your help, as this has been driving me crazy...
Stephen

The difference lies in the use of a pipe if you are using bash. This would work in ksh

Code:

cat "$FILE" | while read LINE ; do
    N=$((N+1))
    STE="$STE""dog"
  done

In bash (contrary to ksh) the last command in the pipe does not run in the current process so the variable STE loses its value once it finishes.

S.

So if I want to use BASH (the company standard), the only option is to change the IFS variable? And then change it back again?

Thanks,
LITW

As Scrutinizer mentioned, with bash the commands after the pipe run in a subshell and variables in a subshell are not visible outside the block of code in the subshell and they are not accessible to the parent process.

You can avoid the use of a pipe (and useless use of cat) like:

Code:

STE="cat"
  
for FILE in $( ls *-df | cut -f 4 -d / ); do
  N=0
  while read LINE ; do
    N=$((N+1))
    STE="$STE""dog"
  done < "$FILE"
done
echo "$STE" && exit

Thanks to you both, I am really glad I joined this forum now

Works like a charm. A bit of a follow up question though, is it bad practise to change the IFS variable like in the first example?

Thanks again,
LITW

If you don't read lines (filenames) with spaces it isn't necessary to change the IFS.

Regards