topic Re: If [ ] and [[ ]] in Operating System - HP-UX

If [ ] and [[ ]]

Ryan Clerk — Tue, 25 Jan 2005 14:53:39 GMT

Hi,

I noticed that sometimes
"if [ $A = $B ]" is used and other times
"if [[ $A = $B ]]" is used. Is there a difference or is it just a personal choice?

TIA,
Ryan

Re: If [ ] and [[ ]]

A. Clay Stephenson — Tue, 25 Jan 2005 14:55:57 GMT

In more modern shells (Posix, ksh, ...) that support the "[[ ... ]]" syntax for if the difference is that "[[ .. ]]" is an internal (to the shell) test whereas "[ .. ]" invokes the external test command. The double brackets are thus more efficient.

Re: If [ ] and [[ ]]

Sundar_7 — Tue, 25 Jan 2005 14:59:09 GMT

And you cannot use boolean expressions with [[. You will need to use [ to use boolean operators

# S=1
# [[ $S -eq 10 -o $S -gt 10 ]]
ksh: syntax error: `-o' unexpected
# [ $S -eq 10 -o $S -gt 10 ]
#

Re: If [ ] and [[ ]]

A. Clay Stephenson — Tue, 25 Jan 2005 14:59:31 GMT

I should also add that in addition the syntax changes a bit as well --- especially the boolean operators.

e.g. (and)

if [ ${A} = ${B} -a ${A} = ${C} ]

is written

if [[ ${A} = ${B} && ${A} = ${C} ]].

Similarly, -o (or) becomes ||.

Re: If [ ] and [[ ]]

MAUCCI_2 — Wed, 26 Jan 2005 15:53:36 GMT

Another noticeable difference is that you can do pattern matching under [[ ]]

> if [[ "toto" = toto* ]]; then echo equals; else echo "not equal"; fi

equals

> if [ "toto" = toto* ]; then echo equals; else echo "not equal"; fi

not equal

http://www.kornshell.com/doc/faq.html

Q10. What is the difference between [...] and [[...]]?
A10. The [[...]] is processed as part of the shell grammar
whereas [...] is processed like any other command.
Operators and operands are detected when the command is
read, not after expansions are performed. The shell does not
do word splitting or pathname generation inside [[...]].
This allows patterns to be specified for string matching
purposes.