bash script string compare

Habitual · 02-07-2017, 05:08 PM

Quote:

Originally Posted by rknichols

In both the "[ ... ]" and "[[ ... ]]" constructs in bash, the ">" and "<" operators perform a string comparison. If you want numeric comparison you have to use "-gt" and "-lt".

Every day a refresher course! Want being the keyword, I guess.
Thank you.

ondoho · 02-08-2017, 12:50 AM

Quote:

Originally Posted by secomax

Code:

\>

i'm really bothered by this.
why escape the '>' in the first place?
why doesn't it throw an error?

MadeInGermany · 02-08-2017, 01:05 AM

Code:

[ "$val1" \> "$val2" ]

is a test command, and can be written as

Code:

test "$val1" \> "$val2"

Command arguments are subject to word splitting and expansion before execution (in the test command).
An unquoted > would be a redirection to a file, therefor the escape. You can also escape it with

Code:

test "$val1" ">" "$val2"

What counts is that the test sees 3 strings, where the 2nd it wants an operator.
This is different in the [[ ]] compound where the shell parser directly handles the operation without prior word splitting or expansion.

Code:

[[ $val1 > $val2 ]]

secomax · 02-08-2017, 01:36 AM

Does that means I have to quote every instance of the string that contains spaces ?

pan64 · 02-08-2017, 01:51 AM

Quote:

Originally Posted by ondoho

i'm really bothered by this.
why escape the '>' in the first place?
why doesn't it throw an error?

because syntactically correct, although it is just annoying and pointless.

Quote:

Does that means I have to quote every instance of the string that contains spaces ?

It is a really good idea, but there can be cases when you do not want to quote. So in general yes, but in special cases probably not.

secomax · 02-08-2017, 03:13 AM

If I remove the escape from it will give error
because the bash think it is an output redirection

Quote:

It is a really good idea, but there can be cases when you do not want to quote. So in general yes, but in special cases probably not.

Thanks for your help
I'm in love with bash scripts and bash tricks

pan64 · 02-08-2017, 03:23 AM

Quote:

Originally Posted by secomax

If I remove the escape from it will give error

Oh, yes, that is only working with [[ ]] (one more reason to use [[ ]] instead of [ ])

secomax · 02-08-2017, 04:53 AM

Really thanks a lot

r3sistance · 02-08-2017, 07:05 AM

another advantage to [[ ]] compared to [ ] is what if val1 or val2 were empty or null?

Code:

#cat strtest.sh
val1=
val2="somestring"

if [[ $val1 > $val2 ]]; then echo true; fi
#./strtest.sh
#

compared too

Code:

#cat strtest.sh
val1=
val2="somestring"

if [ $val1 \> $val2 ]; then echo true; fi
#./strtest.sh
./strtest.sh: line 4: [: >: unary operator expected

That is another behavioral difference to be aware of. This behavior will also be experienced if 'val1=""' too

MadeInGermany · 02-08-2017, 12:15 PM

Empty variables are another reason for the rule:
always quote variables in command arguments, including the [ ] test command.
Literal strings that contain a space must always be in quotes.
Examples

Code:

var=* # assignment, does not need to be quoted.
var="* *" # literal string with whitespace, needs quotes

var="two words"
if [[ $var > "two words" ]]; then
  echo "$var comes after 'two words' in the alphabet"
fi

var="one two words"
case $var in
( *"two words"* )
  echo "'two words' found in '$var'."
;;
esac

if [[ $var == *"two words"* ]]; then
  echo "'two words' found in '$var'."
fi

One additional note: $var in 'ticks' is normally not expanded by the shell, but here the 'ticks' are within "quotes" so loose their special meaning.

secomax · 02-10-2017, 06:52 AM

thanks for you all
it helped me

Habitual · 02-10-2017, 07:57 AM

You are welcome.

Go TeamLQ!