Revisions to How can I declare and use Boolean variables in a shell script?

Improve formatting of cases - numbered list instead of code block. Unless I'm horribly mistaken, `var=` is equivalent to `var=""`.

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edited Apr 19 at 18:26

wjandrea

31.6k
9
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# Variable var not defined beforehand. Case 1
var=''  # Equivalent to var="".      # Case 2
var=                                 # Case 3
unset var                            # Case 4
var='<some valid command>'           # Case 5

Variable var not defined beforehand

var='' (equivalent to var="" or var=)

unset var

var='<some valid command>'

The last case (#5#4) is especially naughty because it will execute the command contained in the variable (which is why the condition evaluates to true for valid commands^{3, 4}).

# Variable var not defined beforehand. Case 1
var=''  # Equivalent to var="".      # Case 2
var=                                 # Case 3
unset var                            # Case 4
var='<some valid command>'           # Case 5

The last case (#5) is especially naughty because it will execute the command contained in the variable (which is why the condition evaluates to true for valid commands^{3, 4}).

Variable var not defined beforehand

var='' (equivalent to var="" or var=)

unset var

var='<some valid command>'

The last case (#4) is especially naughty because it will execute the command contained in the variable (which is why the condition evaluates to true for valid commands^{3, 4}).

Better distinguishing the variable name from the name of the type. It's confusing when the variable name is the same as the name of a type in some programming languages.

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edited Apr 8, 2021 at 13:43

einpoklum

126.7k
69
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bool=truemy_bool=true

if [ "$bool""$my_bool" = true ]

bool=truemy_bool=true

if [ "$bool""$my_bool" = true ]; then
if [ "$bool""$my_bool" = "true" ]; then

if [[ "$bool""$my_bool" = true ]]; then
if [[ "$bool""$my_bool" = "true" ]]; then
if [[ "$bool""$my_bool" == true ]]; then
if [[ "$bool""$my_bool" == "true" ]]; then

if test "$bool""$my_bool" = true; then
if test "$bool""$my_bool" = "true"; then

bool=true

if [ "$bool" = true ]

bool=true

if [ "$bool" = true ]; then
if [ "$bool" = "true" ]; then

if [[ "$bool" = true ]]; then
if [[ "$bool" = "true" ]]; then
if [[ "$bool" == true ]]; then
if [[ "$bool" == "true" ]]; then

if test "$bool" = true; then
if test "$bool" = "true"; then

my_bool=true

if [ "$my_bool" = true ]

my_bool=true

if [ "$my_bool" = true ]; then
if [ "$my_bool" = "true" ]; then

if [[ "$my_bool" = true ]]; then
if [[ "$my_bool" = "true" ]]; then
if [[ "$my_bool" == true ]]; then
if [[ "$my_bool" == "true" ]]; then

if test "$my_bool" = true; then
if test "$my_bool" = "true"; then

Changed Grammar mistakes and comment positioning

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edited Aug 17, 2020 at 3:36

tgdavies

11.2k
4
38
42

var='echo this text will be displayed when the condition is evaluated'
if $var; then
  echo 'Muahahaha!'
fi

# Outputs:
# Thisthis text will be displayed when the condition is evaluated
# Muahahaha!

var='echo this text will be displayed when the condition is evaluated'
if $var; then
  echo 'Muahahaha!'
fi

# Outputs:
# This text will be displayed when the condition is evaluated
# Muahahaha!

var='echo this text will be displayed when the condition is evaluated'
if $var; then
  echo 'Muahahaha!'
fi

# Outputs:
# this text will be displayed when the condition is evaluated
# Muahahaha!