Comments on: Reading a file, line by line

By: rahul benegal

rahul benegal — Sun, 27 Dec 2009 06:52:55 +0000

Do note that “while read” rather unexpectedly trims spaces in the line read !!
You would have to do a:

while IFS=” read line to avoid that.

Another thing that once tripped me was if i incrementing a counter in the loop, it was lost outside. After much hunting, i discovered that `while read` starts a subshell.

If you are populating an array or hash or counter etc in a `while read` loop, the only option I’ve figured out is:

for line in $( cat file.txt ) …

thanks
rahul

By: Splitting Strings Natively with the Shell: Why

Splitting Strings Natively with the Shell: Why — Wed, 09 Dec 2009 23:18:48 +0000

[...] strings into tokens or “words”. I previously discussed how to do this with the IFS variable and promised a more in depth discussion. Today, I will make the case on WHY to use IFS to split [...]

By: Cris F.A. Johnson

Cris F.A. Johnson — Mon, 30 Nov 2009 08:56:12 +0000

“you need to enclose the RHS of the assignment in quotes”

You only need to quote the right-hand side when there is a literal whitespace character. (This is true for any Bourne-type shell.)

“the “print” command needs to be changed to “echo”.”

The print command should be changed to printf; echo is deprecated.

By: Brock Noland

Brock Noland — Sun, 29 Nov 2009 18:13:35 +0000

Actually, Jeff my test does not show you are correct. At least with BASH 3.1.17 and KSH 1993-12-28 r.

$ cat test.sh
#!/bin/bash
echo \$IFS = "'$IFS'"
oldIFS=$IFS
echo \$oldIFS = "'$oldIFS'"

$ ./test.sh
$IFS = '
'
$oldIFS = '
'

By: Brock Noland

Brock Noland — Sun, 29 Nov 2009 18:08:20 +0000

Seth, As I am sure you are aware, this works in BASH as well!

Alex,

First, point is excellent. I will update the post. However, using a subshell whether it be awk, cut, sed or something else is much, much slower.

By: Alex

Alex — Sat, 28 Nov 2009 13:52:44 +0000

Commands like this are pretty risky:
oldIFS=$IFS
IFS=$oldIFS
If IFS included the newline character (as it does by default), or the tab character, it won’t do any more after this command. For copying variables which might contain whitespace other than spaces, you need to enclose the RHS of the assignment in quotes:
oldIFS=”$IFS”
IFS=”$oldIFS”
But yes, manipulating IFS can be a nifty little trick from time to time. You can do the splitting with cut rather than awk, like this:
domain = $(echo $line | cut -d ‘|’ -f 1)
ip = $(echo $line | cut -d ‘|’ -f 2)
But in this case manipulating IFS is probably still quicker and even easier.

By: Seth

Seth — Wed, 25 Nov 2009 06:05:31 +0000

Good tip, but I thought this was _BASH_ cures cancer.

For the above to work as a bash script the “print” command needs to be changed to “echo”.

Also, it would be nice if read would accept a word delimiter as a parameter the way AWK does.