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Double square bracket conditionals

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Arithmetic expressions Double square bracket conditionals Single sqare bracket conditionals
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The [[ ]] construct was introduced in ksh88 as a way to compensate for multiple shortcomings and limitations of the [ ] (test) solution.  Essentially it makes [ ] construct obsolete except for running a program to get a return code.  In turn double round brackets ((..)) construct made  The [[ ]] construct obsolete for integer comparisons.

There are two types of operators that can be used inside double square bracket construct:

 Paradoxically integer comparison operators are represented as strings ( -eq, -ne, -gt, etc)  while string comparison operators as delimiters ("=", "==", "!=", "<", ">", etc). 

The [[ ]] construct expects expression. In ksh delimiters [[ and ]] serve as single quotes so you do not have macro expansion inside: variable substitution and wildcard expansion aren't done within [[ and ]], making quoting less necessary. In bash this is less true :-). 

It can act as independent operator as it produces return code. So constructs like

[[ $string =∼ [aeiou] ]] && exit; 

are legitimate and actually pretty compact way to write if statements without else clause that contain a single statement in then block.

One of the [[ ]] construct warts is that it redefined == as a pattern matching operation, which anybody who programmed in C/C++/Java strongly resent. Latest bash version corrected that and allow using Perl-style =~ operator instead (I think ksh93 allow that too):

string="abba"
[[ $string =∼ [aeiou] ]]
echo $?
0

[[ $string =∼ h[sdfghjkl] ]]
echo $?
1

Like [ ] construct [[  ]] construct can be used as a separate statement that returns an exit status depending upon whether condition is true or not.  With && and || constructs discussed above this provides an alternative syntax for if-then and if-else constructs

if [[ -d  $HOME/$user ]] ; then echo " Home for user $user exists..."; fi

can be written simpler as

[[ -d  $HOME/$user ]] && echo " Home for user $user exists..."

There are several types of expressions that can be used inside [[  ... ]] construct:

One unpleasant reality (and probably the most common gotcha) of using legacy [[...]] integer comparison constructs is that if one of the variable is not initialized it produces syntax error. Various tricks are used to avoid this nasty macro substitution side effect, that came of a legacy of extremely week implementation of comparisons in Borne shell (there are way too many crazy things implemented in Borne shell, anyway ;-). 

There are two classic tricks to deal with this gotchas in old [[..]] construct as you will be dealing with scripts infested with those old constructs pretty often. They can be and often are used simultaneously:

Generally, it is better to initialize most variables explicitly. I know it is difficult as old habits die slowly, but this can be done. Here are the most common "legacy integer comparison operators":

-eq 
is equal to

if [[ "$a" -eq "$b" ]]

-ne
is not equal to

if [[ "$a" -ne "$b" ]]

-gt
is greater than

if [[ "$a" -gt "$b" ]]

-ge
is greater than or equal to

if [[ "$a" -ge "$b" ]]

-lt
is less than

if [[ "$a" -lt "$b" ]]

-le
is less than or equal to

if [[ "$a" -le "$b" ]]

String comparisons

String comparisons is all what left useful in this construct as for integer comparisons ((..)) construct is better and for file comparisons older [...] construct is equal. This topic is discussed at greater length at String Operations in Shell

Notes:

Operator True if...
str = pat
str == pat
str matches pat. Note that in case of "==" that's not what you logically expect, if you have some experience with C /C++/Java programming !!!
str != pat str does not match pat.
str1 < str2 str1 is less than str2 is collation order used
str1 > str2 str1 is greater than str2.
-n str str is not null (has length greater than 0).
-z str str is null (has length 0).
file1 -ef file2 file1 is another name for file2 (hard or symbolic link)

While we're cleaning up code we wrote in the last chapter, let's fix up the error handling in the highest script  The code for that script is:

filename=${1:?"filename missing."}
howmany=${2:-10}
sort -nr $filename | head -$howmany

Recall that if you omit the first argument (the filename), the shell prints the message highest: 1: filename missing. We can make this better by substituting a more standard "usage" message:

if [[ -z $1 ]]; then
    print 'usage: howmany filename [-N]'
else
    filename=$1
    howmany=${2:-10}
    sort -nr $filename | head -$howmany
fi

It is considered better programming style to enclose all of the code in the if-then-else, but such code can get confusing if you are writing a long script in which you need to check for errors and bail out at several points along the way. Therefore, a more usual style for shell programming is this:

if [[ -z $1 ]]; then
    print 'usage: howmany filename [-N]'
    return 1
fi
filename=$1
howmany=${2:-10}
sort -nr $filename | head -$howmany

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