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Active reading [<https://en.wiktionary.org/wiki/Boolean#Noun> <http://en.wikipedia.org/wiki/Bash_%28Unix_shell%29>].
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Peter Mortensen
Peter Mortensen
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Long ago, when all we had was sh, booleansBooleans where handled by relying on a convention of the test program where test returns a false exit status if run with nowithout any arguments. This

This allows one to think of a variable that is unset as false and variable set to any value as true. TodayToday, test is builtin to bashtest is a builtin to Bash and is commonly known by its one character-character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unsetUnset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test, but has a nicer syntax: variablesvariables with spaces do not need to be quoted,quoted; one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusingThis stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, Booleans where handled by relying on a convention of the test program where test returns a false exit status if run without any arguments.

This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is a builtin to Bash and is commonly known by its one-character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then
    echo 'Is true'
else
    echo 'Is false'
fi

# Unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test, but has a nicer syntax: variables with spaces do not need to be quoted; one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then
    echo 'Flag up, count bigger than 1'
else
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

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Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusingThis stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

incorporated dolmen's point
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Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [:

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

Long ago, when all we had was sh, booleans where handled by relying on a convention of the test program where test returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to bash and is commonly known by its one character alias [ (or an executable to use in shells lacking it, as dolmen notes):

FLAG="up or <set>"

if [ "$FLAG" ] ; then 
    echo 'Is true'
else 
    echo 'Is false'
fi

# unset FLAG
#    also works
FLAG=

if [ "$FLAG" ] ; then
    echo 'Continues true'
else
    echo 'Turned false'
fi

Because of quoting conventions, script writers prefer to use the compound command [[ that mimics test but has nicer syntax: variables with spaces do not need to be quoted, one can use && and || as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.

For example, to determine if FLAG is set and COUNT is a number greater than 1:

FLAG="u p"
COUNT=3

if [[ $FLAG  && $COUNT -gt '1' ]] ; then 
    echo 'Flag up, count bigger than 1'
else 
    echo 'Nope'
fi

This stuff can get confusing when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.

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  • 443
  • 4
  • 7