In various guides and scripts I come across people tend to use different syntax of if statements. What's the difference and what are best practices? I believe all the following statements, and many more variants, will return true:
bar="foo"
if [ "foo" = "foo" ]
if [[ "foo" == $bar ]]
if [ "foo" = "$bar" ]
if [[ "foo" = "$bar" ]]
if [[ "foo" -eq $bar ]]
As I understand it
= expects strings
-eq expects integers
"$bar" is for literal matches, i.e. z* may expand but "z*" will literally match the wildcard char.
The difference between [] and [[]] is that in the latter word splitting and path name expansion are not done, but are in the former.
Plus [[]] allows the additional operators :
&& (AND), || (OR), > (String1 lexically greater than String2), < (String1 lexically less than String2)
The == comparison operator behaves differently within a double-brackets test than within single brackets.
[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
[ $a == z* ] # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
Check out http://tldp.org/LDP/abs/html/comparison-ops.html for more info
= and == do exactly the same thing (i.e. they both do pattern matching within [[ ]]) except that some implementations of [ don't support ==. Basically, the only reason to use == is that it's familiar from other languages. Also, [ does support string comparisons with < and >, you just have to escape them or they'll be parsed as I/O redirections: [ string1 \< string2 ] for example.[ is a bash builtin, [[ is a bash keyword. Best practice is to always use [[ if the script doesn't have to be compatible with other shells (e.g., if it starts with #!/bin/bash), and use [ only for compatibility with the Bourne shell. See http://mywiki.wooledge.org/BashFAQ/031.
I recommend case/esac.
case "$foo" in
"bar" ) echo "bar";;
*) echo "not equal";;
esac
No fretting about different syntax.
