Bash: Problems testing multiple statements with logical and (-a or &&)

AncientPC · 04-13-2008, 05:59 PM

I know the first and second statements evaluate correctly by themselves, but I would like to combine the two into one line (using -a or &&) as opposed to having nested if statements.

Code:

#!/bin/bash

if [[ ! -z $1 ] && [[ $1 =~ [0-9] ]]]; then
	echo "$1 is a number"
else
	echo "invalid arguments"
	exit 1
fi
exit 0

On a side note, what are the boolean values for true and false in bash?

Code:

if [ 0 -a 1 ]

returns true

Code:

if [ 0 -a 0 ]

returns true

Code:

if [ false -a false ]

returns true

Code:

if [ 0 && 1 ]

gives me a syntax error

angrybanana · 04-13-2008, 08:40 PM

Code:

[[ -n $1 && $1 =~ [0-9] ]]

bash variables for true and false are "true" and "false". Just don't use them in [] or [[]] since then it's just defaulting to the equivalent of [ -n "true" ].

Also, if you're doing bash, always use the double brackets '[['

Here's some more info [[]] vs []

AncientPC · 04-13-2008, 09:58 PM

Quote:

Originally Posted by angrybanana

Code:

[[ -n $1 && $1 =~ [0-9] ]]

bash variables for true and false are "true" and "false". Just don't use them in [] or [[]] since then it's just defaulting to the equivalent of [ -n "true" ].

Also, if you're doing bash, always use the double brackets '[['

Here's some more info [[]] vs []

Thanks, that helped a lot. I remember going over [] vs [[]] before but it slipped my mind. I have to relearn bash every few months.

On a side note, while I was looking up -n I ran into this (http://tldp.org/LDP/abs/html/compari...s.html#STRTEST):

Quote:

As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x -o "x$a" = "x$b" ] (the "x's" cancel out).

The final code:

Code:

#!/bin/bash

max=1000
if [[ -n "$1" && $1 =~ [0-9] ]]; then
	max=$1
else
	echo "Invalid arguments"
	exit 1
fi

ssh-add ~/.ssh/csservers
machines=`cat ~/bin/cshosts.txt`
count=1
for m in $machines; do
	if [[ $count -le $max && `ping -s 1 -c 1 $m>/dev/null; echo $?` -eq 0 ]]; then
		if [ "$(ssh $m who)" == "" ]; then
			echo "$m"
			let "count+=1"
		fi
	fi
done

exit 0

ntubski · 04-13-2008, 11:32 PM

I think this

Code:

if [[ $count -le $max && `ping -s 1 -c 1 $m>/dev/null; echo $?` -eq 0 ]]; then

could be simplified to

Code:

 if [[ $count -le $max ]] && ping -s1 -c1 $m >/dev/null ; then

AncientPC · 04-14-2008, 12:16 AM

Quote:

Originally Posted by ntubski

I think this

Code:

if [[ $count -le $max && `ping -s 1 -c 1 $m>/dev/null; echo $?` -eq 0 ]]; then

could be simplified to

Code:

 if [[ $count -le $max ]] && ping -s1 -c1 $m >/dev/null ; then

Yes that works, thank you very much.