Customizing bash's Internal Field Separator (IFS)

[Lars Noodén]

I'm running into some weirdness trying to explicitly set the special variable in bash, the Internal Field Separator to be a space, a new line and a tab. This is in GNU bash, version 3.2.39(1)-release (x86_64-pc-linux-gnu)

I would like to be able to explicitly set the Internal Field Separator to be a space, a new line and a tab. I would expect the backslash sequences to be interpreted, but they seem not to be. Despite many HOWTOs, Tutorials, and guides only setup 'one' below successfully assigns all three.

# one
export IFS=`/bin/echo -ne " \t\n"`
echo "$IFS" | cat -vte

These others below fail, usually by making the letters 't' and 'n' into field separators. That is not the behavior I expect, I would expect the backslash sequences to be interpreted properly:

# two
export IFS=" \t\n"
echo "$IFS" | cat -vte

# three
export IFS=$" \t\n"
echo "$IFS" | cat -vte

# four
export IFS=$' \t\n'
echo "$IFS" | cat -vte

Here is the problem in a different form:

$ FOO=$'a\n a';echo $FOO
a a
$ FOO=$"a\n a";echo $FOO
a\n a
$ FOO="a\n a";echo $FOO
a\n a
$ FOO='a\n a';echo $FOO
a\n a

[geirha]

Seems to be working for me.

$ IFS=$' \t\n' FOO=$'a b\tc\nd-e'; echo $FOO
a b c d-e
$ IFS=' ' FOO=$'a b\tc\nd-e'; echo $FOO
a b	c
d-e
$ IFS=$'\t' FOO=$'a b\tc\nd-e'; echo $FOO
a b c
d-e
$ IFS=b FOO=$'a b\tc\nd-e'; echo $FOO
a  	c
d-e

[colo]

Also note that bash's default IFS already consists of " \t\n".

[Lars Noodén]

@geirha: on which platform and for which version of bash is it working for you?

I am finding that it does not work on

GNU bash, version 3.2.39(1)-release (x86_64-pc-linux-gnu)

2.6.24-18-generic #1 SMP Wed May 28 19:28:38 UTC 2008 x86_64 GNU/Linux

$ lsb_release -rcd
Description: Ubuntu 8.04
Release: 8.04
Codename: hardy

[Lars Noodén]

Also note that bash's default IFS already consists of " \t\n".

Yes. However, for scripts, I wish to explicitly set the IFS, but without having to backtick output from echo.

[geirha]

@geirha: on which platform and for which version of bash is it working for you?

I am finding that it does not work on

GNU bash, version 3.2.39(1)-release (x86_64-pc-linux-gnu)

2.6.24-18-generic #1 SMP Wed May 28 19:28:38 UTC 2008 x86_64 GNU/Linux

$ lsb_release -rcd
Description: Ubuntu 8.04
Release: 8.04
Codename: hardy

GNU bash, version 3.2.39(1)-release (i486-pc-linux-gnu)

2.6.24-18-386 #1 Wed May 28 19:30:01 UTC 2008 i686 GNU/Linux

Ubuntu 8.04

[Lars Noodén]

GNU bash, version 3.2.39(1)-release (i486-pc-linux-gnu)

2.6.24-18-386 #1 Wed May 28 19:30:01 UTC 2008 i686 GNU/Linux

Ubuntu 8.04

Hmm. I'm finding that it does work for a different architecture:

$ uname -a ;lsb_release -rc
Linux oin 2.6.22-14-generic #1 SMP Tue Feb 12 07:42:25 UTC 2008 i686 GNU/Linux
Release: 7.10
Codename: gutsy
maintenance@oin:~$ uname -a ;lsb_release -rc;bash --version
Linux oin 2.6.22-14-generic #1 SMP Tue Feb 12 07:42:25 UTC 2008 i686 GNU/Linux
Release: 7.10
Codename: gutsy
GNU bash, version 3.2.25(1)-release (i486-pc-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

[Lars Noodén]

Thanks for helping to walk through the problem.

The failure to interpret backslash escapes happens when starting the shell script with just sh
#!/bin/sh

/bin/sh is really dash, not bash
https://launchpad.net/ubuntu/+spec/dash-as-bin-sh

So, changing the first line to the following gets rid of the problem:
#!/bin/bash


I guess that not interpreting backslash escapes is supposed to be normal behavior for dash?

[ghostdog74]

for hassle free delimiter parsing, use awk.

[HalPomeranz]

Btw, this syntax works:

#!/bin/bash

IFS=$' \t\n'