LinuxQuestions.org - [SOLVED] [bash] assign boolean expression to variable

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - [bash] assign boolean expression to variable (https://www.linuxquestions.org/questions/programming-9/%5Bbash%5D-assign-boolean-expression-to-variable-747543/)

[bash] assign boolean expression to variable

I would like to assign true/false to $RUN in one line rather than using the clumsy if statement below.

Code:

: ${DELETE:=false}

: ${FORCE:=false}

: ${SKIP:=false}

RUN=true

if $DELETE && ! $FORCE && ! $SKIP && [ -z "$RUNDAY" ] ; then

    RUN=false

fi

I tried the following but it does not work:

Code:

RUN=! $DELETE || $FORCE || $SKIP || [ -n "$RUNDAY" ]

This should work:

Code:

! $DELETE || $FORCE || $SKIP || [ -n "$RUNDAY" ] && RUN=false

Thank you for your reponse, jlliagre.

With the variables set as follows

Code:

DELETE=false

FORCE=false

SKIP=true

RUN=true

Code:

! $DELETE || $FORCE || $SKIP || [ -n "$RUNDAY" ] && RUN=false

sets RUN to false

I would have expected bash to stop evaluating as soon as the any of the sub-expressions is found true.

bash does not use the strings "true" and "false" to denote logical values true and false. What does it use? Here's from the "Advanced Bash-Scripting Guide 6.05"

Code:

#!/bin/bash



#  Tip:

#  If you're unsure of how a certain condition would evaluate,

#+ test it in an if-test.



echo



echo "Testing \"0\""

if [ 0 ]      # zero

then

  echo "0 is true."

else          # Or else ...

  echo "0 is false."

fi            # 0 is true.



echo



echo "Testing \"1\""

if [ 1 ]      # one

then

  echo "1 is true."

else

  echo "1 is false."

fi            # 1 is true.



echo



echo "Testing \"-1\""

if [ -1 ]    # minus one

then

  echo "-1 is true."

else

  echo "-1 is false."

fi            # -1 is true.



echo



echo "Testing \"NULL\""

if [ ]        # NULL (empty condition)

then

  echo "NULL is true."

else

  echo "NULL is false."

fi            # NULL is false.



echo



echo "Testing \"xyz\""

if [ xyz ]    # string

then

  echo "Random string is true."

else

  echo "Random string is false."

fi            # Random string is true.



echo



echo "Testing \"\$xyz\""

if [ $xyz ]  # Tests if $xyz is null, but...

              # it's only an uninitialized variable.

then

  echo "Uninitialized variable is true."

else

  echo "Uninitialized variable is false."

fi            # Uninitialized variable is false.



echo



echo "Testing \"-n \$xyz\""

if [ -n "$xyz" ]            # More pedantically correct.

then

  echo "Uninitialized variable is true."

else

  echo "Uninitialized variable is false."

fi            # Uninitialized variable is false.



echo





xyz=          # Initialized, but set to null value.



echo "Testing \"-n \$xyz\""

if [ -n "$xyz" ]

then

  echo "Null variable is true."

else

  echo "Null variable is false."

fi            # Null variable is false.





echo





# When is "false" true?



echo "Testing \"false\""

if [ "false" ]              #  It seems that "false" is just a string.

then

  echo "\"false\" is true." #+ and it tests true.

else

  echo "\"false\" is false."

fi            # "false" is true.



echo



echo "Testing \"\$false\""  # Again, uninitialized variable.

if [ "$false" ]

then

  echo "\"\$false\" is true."

else

  echo "\"\$false\" is false."

fi            # "$false" is false.

              # Now, we get the expected result.



#  What would happen if we tested the uninitialized variable "$true"?



echo



exit 0

Quote:

Originally Posted by catkin (Post 3643406)

bash does not use the strings "true" and "false" to denote logical values true and false.

jlliagre's snippet executes one of the commands true or false based on the value of the variable. But I think it shoule be

Code:

$DELETE && ! $FORCE && ! $SKIP && [ -z "$RUNDAY" ] && RUN=false

Code:

! $DELETE || $FORCE || $SKIP || [ -n "$RUNDAY" ] || RUN=false

ntubski, you cracked it!

However, I take catkin's point that I am not doing things the bash way. Coming from other programming languages, I find 0 = true totally counter-intuitive though.

Quote:

Originally Posted by hashbang#! (Post 3644239)

ntubski, you cracked it!

However, I take catkin's point that I am not doing things the bash way. Coming from other programming languages, I find 0 = true totally counter-intuitive though.

It's more (not fully!) intuitive if you consider that it's primary use is to indicate whether a command ran OK. With that perspective, it makes sense to use 0 for OK, leaving 126 other numbers (max 7 bit range less 0 and 127 reserved) to be specific about the error.

That's our perspective as problem solvers; if it worked we don't care; if it failed we are interested! :)

Quote:

Originally Posted by catkin (Post 3644248)

That's our perspective as problem solvers; if it worked we don't care; if it failed we are interested! :)

at the beginning I was startled by this 0=true thing, but once you get used to know that 0 = OK and everything else is a DETAILED error code then you cant just go back to the other way. This is more useful and actually 1=true becomes more "counter-intuitive".

:D

Arithmetic Evaluation gives the more familiar 0=false C-like semantics:

Code:

DELETE=0

FORCE=0

SKIP=0

((RUN = !DELETE || FORCE || SKIP || ${#RUNDAY}))

ntubski, I like that a lot, particularly the ${#var}!

However, the following now returns an error:

Code:

$DELETE && rm -f $RUNFILE

bash: 1: command not found

Because 1 and 0 only mean true and false inside arithmetic evaluation:

Code:

((DELETE)) && rm -f $RUNFILE