Comments on: Bash Shell Functions Tutorial with 6 Practical Examples

By: Bash Shell Expansion Tutorial: 6 Examples of Expanding Expressions within Braces

Bash Shell Expansion Tutorial: 6 Examples of Expanding Expressions within Braces — Mon, 07 Jun 2010 06:03:49 +0000

[...] refer to our earlier article on bash shell functions for additional [...]

By: Zartan

Zartan — Mon, 26 Apr 2010 20:21:15 +0000

Sigh. I forgot to redirect dd’s statistics on stderr. Try this:

dd if=/dev/urandom bs=$(expr 4 \* $len) count=1 2> /dev/null | tr -cd ‘[:graph:]‘ | head -c $len

By: Zartan

Zartan — Mon, 26 Apr 2010 20:19:02 +0000

I suspect the author is using /dev/urandom instead of /dev/random because cat will try to fill an entire stdio buffer before passing it on to tr. Even a 4K buffer would easily empty /dev/random’s entropy pool, then hang waiting for more. Using “cat -u” would work better, but still would read too much.

Better to limit the amount read by using dd. Since tr is discarding characters outside the printable ASCII character set, get four times as many as needed:

len=${1:-12}
if [ "0$len" -le 1 ]
then
len=12
fi
dd if=/dev/urandom bs=$(expr 4 \* $len) count=1 | tr -cd ‘[:graph:]‘ | head -c $len
}

By: Chris F.A. Johnson

Chris F.A. Johnson — Mon, 26 Apr 2010 00:04:50 +0000

@gus3, why would anyone use syntax that is more verbose and less useful than the standard syntax, which works perfectly well in bash — and all other Bourne-type shells?

By: The Hulk

The Hulk — Sun, 25 Apr 2010 21:13:44 +0000

This is one of the greatest sites on the web (for Linux-heads).

By: don taber

don taber — Sat, 24 Apr 2010 14:23:45 +0000

My favorite is a floating point version of expr that can handle math & trig functions. It fires off Perl for each instance so it is not something you’d use where speed is a concern.

fpexpr ()
{
echo “$*” | perl -ne ‘use Math::Trig; print eval($_),”\n” ;’
}

By: gus3

gus3 — Fri, 23 Apr 2010 21:18:22 +0000

@Chris Johnson: Bash-only syntax is to be expected in an article titled, “Bash Shell Functions…”

@Alpha: Using /dev/urandom does not deplete the entropy pool for services which need real randomness (such as SSL/TLS encryption negotiation). “Strong password” does not automatically equal “entropy”. An example would be taking the phrase “I Love Lucy” and turning it into “;L0v3Loos3e”. It is a strong password, with all four character types (upper- and lower-case, digits, and punctuation), but it is hardly random.

By: Rubem

Rubem — Thu, 22 Apr 2010 18:18:22 +0000

I think it’s great article.
Please. I wondering why anyone talk about PAM autentication? Publish something about it. at a same quality of present article.

By: Alpha

Alpha — Wed, 21 Apr 2010 19:39:18 +0000

re: Example 3. Bash Function to generate random password

I was wondering why using /dev/random instead of /dev/urandom
FAILS — although the former seems more secure,
according to:
http://en.wikipedia.org/wiki//dev/random

Thanks kindly for your reply.

By: Chris F.A. Johnson

Chris F.A. Johnson — Wed, 21 Apr 2010 14:17:27 +0000

The syntax you use to define functions is a bash-only hybrid. The standard syntax (which will work in all Bourne-type shells) is:

function_name() COMPOUND_COMMAND [optional redirection]

The KornShell syntax (which does not work in all shells) is:

function function_name COMPOUND_COMMAND [optional redirection]