Question on using "[[ ]]" in lieu of IF Construct

sunpraveen · #1 (**permalink**) 02-06-2009

Unix gurus,

I have a piece of code as below.

Code:

[[ -n ${ORACLE_SID} ]] && INST="${ORACLE_SID}" || INST="${TWO_TASK}"

I know that the above code can be used in lieu of an IF construct. I also know that the above code can be extended for the "true" condition to include more than one command (as below):

Code:

[[ -n ${ORACLE_SID} ]] && INST="${ORACLE_SID}" && echo "INST=${INST}" || INST="${TWO_TASK}"

But I am unable to extend the "false" condition to include more than one command.

Desired Output using "[[ ]]" instead of IF construct:

Code:

if the variable ORACLE_SID is set, then
command 1;
command 2;
else
command 1;
command 2;

How can I achieve it? Any tips or suggestions?

TIA,

Regards,

Praveen

otheus · #2 (**permalink**) 02-06-2009

Actually, it's not the ""[[ ]]" construct nor the "[ ]" construct, but the && construct that does the trick. There is no standard way, however, of doing negation in the UNIX shell using the && || constructs. One can, however, do something like this:

Code:

not() { 
  "$@"
  if [ $? = 0 ]; then return 1; else return 0; fi
}

# not true || echo false
false
# not false && echo true
true

You can also group things together between { }:

Code:

# not true || { echo false; date; echo really false; false; } || echo Total failure.
false
really false
Total failure.
# not true || { echo false; echo really false; true; } && echo not a total failure.
false
really false
not a total failure.

Does that help?

You should be able now to do:

Code:

command_A && { command_A_worked; command_A_worked_report; true; } || { command_A_failed; command_A_failed_report; false; }

cfajohnson · #3 (**permalink**) 02-09-2009

Quote:

Originally Posted by otheus

There is no standard way, however, of doing negation in the UNIX shell using the && || constructs. One can, however, do something like this:

Code:

not() { 
  "$@"
  if [ $? = 0 ]; then return 1; else return 0; fi
}

# not true || echo false
false
# not false && echo true
true

What's wrong with the negation operator?
Code:
if ! some_command
then
  echo command failed
fi

ddreggors · #4 (**permalink**) 02-07-2009

Code:

TRUE="FALSE"
[[ $TRUE = "TRUE" ]] && echo "true" || (echo "false"; echo "false2")

This outputs:
false
flase2

Now we change this to:

Code:

TRUE="TRUE"
[[ $TRUE = "TRUE" ]] && echo "true" || (echo "false"; echo "false2")

and this ouputs:
true

cfajohnson · #5 (**permalink**) 02-07-2009

Quote:

Originally Posted by sunpraveen

Unix gurus,

I have a piece of code as below.

Code:

[[ -n ${ORACLE_SID} ]] && INST="${ORACLE_SID}" || INST="${TWO_TASK}"

The [[ ... ]] syntax is non-standard. In almost all cases, you can use the standard [ ... ].

Quote:

I know that the above code can be used in lieu of an IF construct. I also know that the above code can be extended for the "true" condition to include more than one command (as below):

Code:

[[ -n ${ORACLE_SID} ]] && INST="${ORACLE_SID}" && echo "INST=${INST}" || INST="${TWO_TASK}"

But I am unable to extend the "false" condition to include more than one command.

Code:
[ -n "$ORACLE_SID" ] && {
 INST=$ORACLE_SID"
 echo "INST=$INST"
} || {
 INST=$TWO_TASK
 echo "INST=$INST"
}
However, you should note that, in the general case, that is not the same as:
Code:
if [ -n "$ORACLE_SID" ]
then
 : do whatever
else
 : do something else
fi
A simplified example:
Code:
[ -n "$qwerty" ] && false || echo "FAIL!"
"FAIL!" will always be printed. The command after || is executed if either of the preceding commands fails.

ddreggors · #6 (**permalink**) 02-07-2009

Quote:

Originally Posted by cfajohnson

The [[ ... ]] syntax is non-standard. In almost all cases, you can use the standard [ ... ].

OK, then we can still use this. Only change needed was to double quote the variable $TRUE and remove the extra brackets.

Code:

[ "$TRUE" = "TRUE" ] && echo "true" || (echo "false"; echo "false2")

sunpraveen · #7 (**permalink**) 02-09-2009

Thanks everyone, for your replies. It was very helpful.

Regards,

Praveen

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