shell script; equate two strings

jovie · 12-18-2009, 06:35 AM

Hi

Can someone give the the correct syntax to check if two strings are the same in a shell script?

Below is the script I have so far and below that, what I get when I run it while turning the sreensaver on/off. At the moment the if statement evaluates as true whatever value is in $run.

Many thanks for any help

Code:

#!/bin/sh

count=0				# Initialise a counter
while [ $count = 0 ]		# Set up a loop control
do				# Begin the loop

sleep 1

#is the screensaver running
run=$(gnome-screensaver-command -q)

echo $run

if [ '$run'=="The screensaver is inactive The screensaver is not inhibited" ]
then
	echo "not running"
else
	echo "running"
fi

done				# End of loop

Quote:

[ ~]$ /tmp/test.sh
The screensaver is inactive The screensaver is not inhibited
not running
The screensaver is inactive The screensaver is not inhibited
not running
The screensaver is active The screensaver is not inhibited
not running
The screensaver is active The screensaver is not inhibited
not running
The screensaver is inactive The screensaver is not inhibited
not running
^C
[ ~]$

ghostdog74 · 12-18-2009, 07:12 AM

Code:

str1="string"
str2="string"
case "$str1" in
  $str2 ) echo "same" ;;
  *) echo "not same";;
esac

pixellany · 12-18-2009, 07:29 AM

In the test statement ([...]), use "=" for string comparison----not "=="

You never change the loop counter---how does the loop exit?

ntubski · 12-18-2009, 11:25 AM

Quote:

Originally Posted by pixellany

In the test statement ([...]), use "=" for string comparison----not "=="

Also you need to seperate the "=" with spaces, and use double quotes when substituting variable values:

Code:

[ "$run" = "The screensaver is inactive The screensaver is not inhibited" ]

catkin · 12-18-2009, 11:57 AM

Quote:

Originally Posted by pixellany

In the test statement ([...]), use "=" for string comparison----not "=="

It's very rare that you write anything I disagree with, pixellany, but in this case, AFAIK, = and == are equivalent with == being the bash operator for which bash allows = to be used as a synonym, in the interests of POSIX compliance. The GNU Bash Reference seems to support my view where it says in 6.4 Bash Conditional Expressions: "string1 == string2 True if the strings are equal. ‘=’ may be used in place of ‘==’ for strict posix compliance".

ntubski · 12-18-2009, 01:03 PM

Quote:

Originally Posted by catkin

AFAIK, = and == are equivalent with == being the bash operator for which bash allows = to be used as a synonym, in the interests of POSIX compliance.

Yeah, bash lets you get away with using either (even in POSIX mode), but a more strict shell (eg dash) might not accept ==.

pixellany · 12-18-2009, 10:56 PM

I had a reason for saying "=", but now I have forgotten.....

. The real distinction is between "-eq" and "="------"-eq" won't work with strings.

konsolebox · 12-19-2009, 01:59 AM

Quote:

Originally Posted by pixellany

I had a reason for saying "=", but now I have forgotten.....

.

i hope you don't mind. that's probably because if you use '[[' in bash, the second string is parsed as a pattern. maybe if we are comparing 2 plain strings, '=' in common sense should act faster.

based on my tests, in bash, '[[' does things faster than '['. maybe it's because arguments in '[[' are immediately parsed, rather than being interpreted first just like a command then passed to the '[' command.

for example, try this code in bash:

Code:

a='string with spaces and $0me special chars'
[[ $a = 'any' ]]  # will this produce an error?

now let's compare the 2 types of test commands using 'time'

Code:

a='a long string with spaces . . . . . . . . . .'
time for (( i = 1; i <= 50000; i++ )); do [ "$a" = 'any' ]; done
time for (( i = 1; i <= 50000; i++ )); do test "$a" = 'any'; done  # just similar to '['
time for (( i = 1; i <= 50000; i++ )); do [[ $a = 'any' ]]; done

which of them do you think should be the fastest? it's probably '[['

catkin · 12-19-2009, 03:14 AM

Quote:

Originally Posted by konsolebox

which of them do you think should be the fastest? it's probably '[['

It was, ~40% less time than the worst case "test" version.

Code:

c:~$ a='a long string with spaces . . . . . . . . . .'
c:~$ time for (( i = 1; i <= 50000; i++ )); do [ "$a" = 'any' ]; done

real    0m1.235s
user    0m1.190s
sys     0m0.035s
c:~$ time for (( i = 1; i <= 50000; i++ )); do test "$a" = 'any'; done  # just similar to '['

real    0m1.183s
user    0m1.139s
sys     0m0.042s
c:~$ time for (( i = 1; i <= 50000; i++ )); do [[ $a = 'any' ]]; done

real    0m0.743s
user    0m0.724s
sys     0m0.019s

konsolebox · 12-19-2009, 03:44 AM

Quote:

Originally Posted by catkin

It was, ~40% less time than the worst case "test" version.

i didn't compare the results by percentage before. that's pretty big

.

i shouldn't have doubted what test command i should use before.

catkin · 12-19-2009, 04:15 AM

Quote:

Originally Posted by konsolebox

i didn't compare the results by percentage before. that's pretty big

.

i shouldn't have doubted what test command i should use before.

It sounds more dramatic as "the test version takes 66% longer than the [[ ]] version"

Lies, damn lies and statistics!

konsolebox · 12-19-2009, 04:28 AM

Quote:

Originally Posted by catkin

It sounds more dramatic as "the test version takes 66% longer than the [[ ]] version"

Lies, damn lies and statistics!

lies? ... err.. who lied?

catkin · 12-19-2009, 08:12 AM

Quote:

Originally Posted by konsolebox

lies? ... err.. who lied?

Nobody. I had used statistics to give two very different but valid results based on the same data -- 40% and 66% -- thus showing how statisticians can choose amongst valid results, the better to support one argument or another. This phenomenon is referenced in the old witticism, Lies, damned lies, and statistics. Sorry for not giving the link the first time.

jovie · 12-21-2009, 08:11 AM

Thanks for all the help.

My main mistake was not putting $run in quotes when I echo'd it. There was a new line character in there which I couldn't see so even when the syntax was right the strings didn't match.

Quote:

[]$ run=$(gnome-screensaver-command -q)
[]$ echo $run
The screensaver is inactive The screensaver is not inhibited
[]$ echo "$run"
The screensaver is inactive
The screensaver is not inhibited
[]$