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

Question

I tried to declare a Boolean variable in a shell script using the following syntax   variable  false  variable  true   Is this correct  Also  if I wanted to update that variable would I use the same syntax  Finally  is the following syntax for using Boolean variables as expressions correct   if    variable    if     variable

User · Answer

POSIX  Portable Operating System Interface   I miss here the key point  which is portability  That s why my header has POSIX in itself   Essentially  all of the voted answers are correct  with the exception they are Bash-specific too much   Basically  I only wish to add more information about portability        and   brackets like in     var    true   are not necessary  and you can omit them and use the test command directly   test   var    true  amp  amp  yourCodeIfTrue    yourCodeIfFalse   Important note  I no longer recommend this as it s being slowly deprecated and more difficult to combine multiple statements  Imagine what those words true and false mean to the shell  test it yourself   echo     true       0    echo     false       1    But using quotes   echo      true        bash   true   syntax error  operand expected  error token is   true    sh  dash   sh  1  arithmetic expression  expecting primary    true      The same goes for   echo      false       The shell can t interpret it other than a string  I hope you are getting the idea of how good it is using proper keyword without quotes   But no one said it in previous answers  What does this mean  Well  several things    You should get used to the Boolean keywords are actually treated like numbers  that is true   0 and false   1  remember all non-zero values are treated like false  Since they are treated as numbers  you should treat them like that too  i e  if you define variable say   var bool true echo   var bool      true    you can create an opposite value of it with   var bool     1 -  var bool       same as        var bool    echo   var bool     1     As you can see for yourself  the shell does print true string for the first time you use it  but since then  it all works via number 0 representing trueor 1 representing false  respectively      Finally  what you should do with all that information   First  one good habit would be assigning 0 instead of true  1 instead of false  Second good habit would be to test if the variable is   isn t equal to zero   if     var bool  -eq 0    then      yourCodeIfTrue else      yourCodeIfFalse fi

User · Answer

There seems to be some misunderstanding here about the Bash builtin true  and more specifically  about how Bash expands and interprets expressions inside brackets   The code in miku s answer has absolutely nothing to do with the Bash builtin true  nor  bin true  nor any other flavor of the true command  In this case  true is nothing more than a simple character string  and no call to the true command builtin is ever made  neither by the variable assignment  nor by the evaluation of the conditional expression   The following code is functionally identical to the code in the miku s answer   the world is flat yeah if     the world is flat    yeah    then     echo  Be careful not to fall off   fi   The only difference here is that the four characters being compared are  y    e    a   and  h  instead of  t    r    u   and  e   That s it  There s no attempt made to call a command or builtin named yeah  nor is there  in miku s example  any sort of special handling going on when Bash parses the token true  It s just a string  and a completely arbitrary one at that   Update  2014-02-19   After following the link in miku s answer  now I see where some of the confusion is coming from  Miku s answer uses single brackets  but the code snippet he links to does not use brackets  It s just   the world is flat true if  the world is flat  then   echo  Be careful not to fall off   fi   Both code snippets will behave the same way  but the brackets completely change what s going on under the hood    Here s what Bash is doing in each case   No brackets    Expand the variable  the world is flat to the string  true   Attempt to parse the string  true  as a command  Find and run the true command  either a builtin or  bin true  depending on the Bash version   Compare the exit code of the true command  which is always 0  with 0  Recall that in most shells  an exit code of 0 indicates success and anything else indicates failure  Since the exit code was 0  success   execute the if statement s then clause   Brackets    Expand the variable  the world is flat to the string  true   Parse the now-fully-expanded conditional expression  which is of the form string1   string2  The   operator is bash s string comparison operator  So    Do a string comparison on  true  and  true   Yep  the two strings were the same  so the value of the conditional is true  Execute the if statement s then clause    The no-brackets code works  because the true command returns an exit code of 0  which indicates success  The bracketed code works  because the value of  the world is flat is identical to the string literal true on the right side of the     Just to drive the point home  consider the following two snippets of code    This code  if run with root privileges  will reboot your computer   var reboot if  var  then   echo  Muahahaha  You are going down   fi   This code just prints  Nice try   The reboot command is not called   var reboot if    var    then   echo  Nice try   fi   Update  2014-04-14  To answer the question in the comments regarding the difference between   and     AFAIK  there is no difference  The    operator is a Bash-specific synonym for    and as far as I ve seen  they work exactly the same in all contexts   Note  however  that I m specifically talking about the   and    string comparison operators used in either     or       tests  I m not suggesting that   and    are interchangeable everywhere in bash   For example  you obviously can t do variable assignment with     such as var   foo   well technically you can do this  but the value of var will be   foo   because Bash isn t seeing an    operator here  it s seeing an    assignment  operator  followed by the literal value   foo   which just becomes   foo     Also  although   and    are interchangeable  you should keep in mind that how those tests work does depend on whether you re using it inside     or        and also on whether or not the operands are quoted  You can read more about that in Advanced Bash Scripting Guide  7 3 Other Comparison Operators  scroll down to the discussion of   and

User · Answer

Revised Answer  Feb 12  2014   the world is flat true      do something interesting    if     the world is flat    true     then     echo  Be careful not to fall off   fi     Original Answer  Caveats  https   stackoverflow com a 21210966 89391  the world is flat true      do something interesting    if  the world is flat   then     echo  Be careful not to fall off   fi   From  Using boolean variables in Bash  The reason the original answer is included here is because the comments before the revision on Feb 12  2014 pertain only to the original answer  and many of the comments are wrong when associated with the revised answer  For example  Dennis Williamson s comment about bash builtin true on Jun 2  2010 only applies to the original answer  not the revised

User · Answer

Here is a simple example which works for me   temp1 true temp2 false  if     temp1    true          temp2    true   then     echo  Do something    else     echo  Do something else   fi

User · Answer

Here is an implementation of a short handed if true     Function to test if a variable is set to  true   if               1       true     amp  amp  return 0          1       True     amp  amp  return 0          1       Yes     amp  amp  return 0     return 1     Example 1  my boolean true   if   my boolean   amp  amp        echo  True Is True             echo  False Is False      Example 2    my boolean false    if   my boolean   amp  amp  echo  Not True is True

User · Answer

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  lt set gt    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  amp  amp  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   amp  amp   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

User · Answer

Instead of faking a Boolean and leaving a trap for future readers  why not just use a better value than true and false   For example   build state success if something-horrible  then   build state failed fi  if      build state     success     then   echo go home  you are done else   echo your head is on fire  run around in circles fi

User · Answer

Alternative - use a function  is ok        is ok    return 1   is ok  amp  amp  echo  It s OK     echo  Something s wrong    Defining the function is less intuitive  but checking its return value is very easy

User · Answer

You can use shFlags   It gives you the option to define  DEFINE bool  Example   DEFINE bool big menu  true   Include  advanced  options in the menu listing      From the command line you can define   sh script sh --bigmenu sh script sh --nobigmenu   False

User · Answer

TL DR bool true  if    quot  bool quot    true     Issues with Miku s  original  answer I do not recommend the accepted answer1  Its syntax is pretty  but it has some flaws  Say we have the following condition  if  var  then   echo  Muahahaha   fi  In the following cases2  this condition will evaluate to true and execute the nested command    Variable var not defined beforehand  Case 1 var       Equivalent to var  quot  quot          Case 2 var                                    Case 3 unset var                              Case 4 var   lt some valid command gt               Case 5  Typically you only want your condition to evaluate to true when your  quot Boolean quot  variable  var in this example  is explicitly set to true  All the other cases are dangerously misleading  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 commands3  4   Here is a harmless example  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   Quoting your variables is safer  e g  if  quot  var quot   then  In the above cases  you should get a warning that the command is not found  But we can still do better  see my recommendations at the bottom   Also see Mike Holt s explanation of Miku s original answer  Issues with Hbar s answer This approach also has unexpected behavior  var false if    var    then   echo  quot This won t print  var is false  quot  fi    Outputs    This won t print  var is false   You would expect the above condition to evaluate to false  thus never executing the nested statement  Surprise  Quoting the value   quot false quot    quoting the variable   quot  var quot    or using test or    instead of    do not make a difference  What I do recommend  Here are ways I recommend you check your  quot Booleans quot   They work as expected  bool true  if    quot  bool quot    true    then if    quot  bool quot     quot true quot     then  if     quot  bool quot    true     then if     quot  bool quot     quot true quot      then if     quot  bool quot     true     then if     quot  bool quot      quot true quot      then  if test  quot  bool quot    true  then if test  quot  bool quot     quot true quot   then  They re all pretty much equivalent  You ll have to type a few more keystrokes than the approaches in the other answers5  but your code will be more defensive   Footnotes  Miku s answer has since been edited and no longer contains  known  flaws  Not an exhaustive list  A valid command in this context means a command that exists  It doesn t matter if the command is used correctly or incorrectly  E g  man woman would still be considered a valid command  even if no such man page exists  For invalid  non-existent  commands  Bash will simply complain that the command wasn t found  If you care about length  the first recommendation is the shortest

User · Answer

Long story short  There are no Booleans in Bash The true and false commands Bash does have Boolean expressions in terms of comparison and conditions  That said  what you can declare and compare in Bash are strings and numbers  That s it  Wherever you see true or false in Bash  it s either a string or a command builtin which is only used for its exit code  This syntax    if true  then      is essentially    if COMMAND  then      where the command is true  The condition is true whenever the command returns exit code 0  true and false are Bash builtins and sometimes also standalone programs that do nothing but returning the corresponding exit code   Conditions in if  then  fi When using square brackets or the test command  you rely on the exit code of that construct  Keep in mind that     and       are also just commands builtins like any other  So     if    1    1     then echo yes  fi  corresponds to if COMMAND  then echo yes  fi  and the COMMAND here is    with the parameters 1    1    The if  then  fi construct is just syntactic sugar  You can always just run the commands separated by a double ampersand for the same effect     1    1     amp  amp  echo yes  When using true and false in these testing constructs you are actually only passing the string  quot true quot  or  quot false quot  to the testing command  Here is an example  Believe it or not but those conditions are all yielding the same result  if    false     then     if     quot false quot      then     if    true     then     if     quot true quot      then      TL DR  always compare against strings or numbers To make this clear to future readers  I would recommend always using quotes around true and false  DO if     quot   var  quot      quot true quot      then     if     quot   var  quot      quot false quot      then     if     quot   var  quot      quot yes quot      then     if     quot   var  quot      quot USE FEATURE X quot      then     if    -n  quot   var -  quot      then echo  quot var is not empty quot       DON T   Always use double square brackets in bash  if          then       This is not as clear or searchable as -n if     quot   var  quot      then       Creates impression of Booleans if     quot   var  quot     true     then        -eq  is for numbers and doesn t read as easy as      if     quot   var  quot  -eq  quot true quot      then      Maybe   Creates impression of Booleans    It can be used for strict checking of dangerous operations    This condition is false for anything but the literal string  quot true quot   if     quot   var  quot      quot true quot      then

User · Answer

This is a speed test about different ways to test  Boolean  values in Bash      bin bash rounds 100000  b true   For true  b false for false type -a true time for i in   seq  rounds   do command  b  done time for i in   seq  rounds   do  b  done time for i in   seq  rounds   do     b     true    done time for i in   seq  rounds   do test   b     true  done time for i in   seq  rounds   do     b    true     done  b x    Or any non-null string for true  b    for false time for i in   seq  rounds   do     b     done time for i in   seq  rounds   do     b     done  b 1   Or any non-zero integer for true  b 0 for false time for i in   seq  rounds   do   b    done   It would print something like  true is a shell builtin true is  bin true  real    0m0 815s user    0m0 767s sys     0m0 029s  real    0m0 562s user    0m0 509s sys     0m0 022s  real    0m0 829s user    0m0 782s sys     0m0 008s  real    0m0 782s user    0m0 730s sys     0m0 015s  real    0m0 402s user    0m0 391s sys     0m0 006s  real    0m0 668s user    0m0 633s sys     0m0 008s  real    0m0 344s user    0m0 311s sys     0m0 016s  real    0m0 367s user    0m0 347s sys     0m0 017s

User · Answer

Bill Parker is getting voted down  because his definitions are reversed from the normal code convention  Normally  true is defined as 0 and false is defined as nonzero  1 will work for false  as will 9999 and -1  The same with function return values - 0 is success and anything nonzero is failure  Sorry  I don t have the street credibility yet to vote or to reply to him directly   Bash recommends using double brackets now as a habit instead of single brackets  and the link Mike Holt gave explains the differences in how they work  7 3  Other Comparison Operators  For one thing  -eq is a numerical operator  so having the code        NOTE     This gives error message       The world is flat 0  if      The world is flat   -eq true    then   will issue an error statement  expecting an integer expression  This applies to either parameter  as neither is an integer value  Yet  if we put double brackets around it  it will not issue an error statement  but it will yield a wrong value  well  in 50  of the possible permutations   It will evaluate to   0 -eq true     success  but also to   0 -eq false     success  which is wrong  hmmm     what about that builtin being a numerical value           NOTE     This gives wrong output       The world is flat true  if       The world is flat   -eq true     then   There are other permutations of the conditional which will give wrong output as well  Basically  anything  other than the error condition listed above  that sets a variable to a numerical value and compares it to a true false builtin  or sets a variable to a true false builtin and compares it to a numerical value  Also  anything that sets a variable to a true false builtin and does a comparison using -eq  So avoid -eq for Boolean comparisons and avoid using numerical values for Boolean comparisons  Here s a summary of the permutations that will give invalid results     With variable set as an integer and evaluating to true false       This will issue error warning and not run        The world is flat 0  if      The world is flat   -eq true    then    With variable set as an integer and evaluating to true false       These statements will not evaluate properly        The world is flat 0  if      The world is flat   -eq true    then   if       The world is flat   -eq true     then   if      The world is flat     true    then   if       The world is flat     true     then   if      The world is flat      true    then   if       The world is flat      true     then     With variable set as an true false builtin and evaluating to true false       These statements will not evaluate properly        The world is flat true  if       The world is flat   -eq true     then   if      The world is flat     0    then   if       The world is flat     0     then   if      The world is flat      0    then   if       The world is flat      0     then   So  now to what works  Use true false builtins for both your comparison and your evaluations  as Mike Hunt noted  don t enclose them in quotes   Then use either or single or double equal sign    or     and either single or double brackets      or         Personally  I like the double equals sign  because it reminds me of logical comparisons in other programming languages  and double quotes just because I like typing  So these work     With variable set as an integer and evaluating to true false       These statements will work properly          The world is flat true false  if      The world is flat     true    then   if       The world is flat     true     then   if      The world is flat     true    then   if       The world is flat      true     then   There you have it

User · Answer

My findings and suggestion differ a bit from the other posts  I found that I could use  booleans  basically as one would in any  regular  language  without the  hoop jumping  suggested     There isn t any need for    or explicit string comparisons    I tried multiple Linux distributions  I tested Bash  Dash  and BusyBox  The results were always the same  I m not sure what the original top voted posts are talking about  Maybe times have changed and that s all there is to it   If you set a variable to true  it subsequently evaluates as an  affirmative  within a conditional  Set it to false  and it evaluates to a  negative   Very straightforward  The only caveat  is that an undefined variable also evaluates like true  It would be nice if it did the opposite  as it would in most languages   but that s the trick - you just need to explicitly initialize your booleans to true or false   Why does it work this way   That answer is two fold  A  true false in a shell really means  no error  vs  error   i e  0 vs anything else   B  true false are not values - but rather statements in shell scripting   Regarding the second point  executing true or false on a line by itself sets the return value for the block you re in to that value  i e  false is a declaration of  error encountered   where true  clears  that   Using it with an assignment to a variable  returns  that into the variable  An undefined variable  evaluates like true in a conditional because that equally represents 0 or  no error encountered      See the example Bash lines and results below  Test it yourself if you want to confirm        bin sh    Not yet defined    echo  when set to   myBool   if   myBool   then echo  it evaluates to true   else echo  it evaluates to false   fi   myBool true echo  when set to   myBool   if   myBool   then echo  it evaluates to true   else echo  it evaluates to false   fi   myBool false echo  when set to   myBool   if   myBool   then echo  it evaluates to true   else echo  it evaluates to false   fi    Yields  when set to it evaluates to true when set to true it evaluates to true when set to false it evaluates to false

User · Answer

Here is an improvement on miku s original answer that addresses Dennis Williamson s concerns about the case where the variable is not set   the world is flat true  if   the world is flat -false    then     echo  Be careful not to fall off   fi   And to test if the variable is false   if     the world is flat -false    then     echo  Be careful not to fall off   fi   About other cases with a nasty content in the variable  this is a problem with any external input fed to a program   Any external input must be validated before trusting it  But that validation has to be done just once  when that input is received   It doesn t have to impact the performance of the program by doing it on every use of the variable like Dennis Williamson suggests

User · Answer

Regarding syntax  this is a simple methodology that I use  by example  to consistently and sanely manage Boolean logic     Tests var  var    var    var 0 var 1 var  abc  var abc  if    -n    var        then     echo  true  fi if    -z    var        then     echo  false  fi    Results   var           false   var           false   var           false   var 0         true   var 1         true   var  abc      true   var abc       true   If the variable is never declared the answer is    false  So  a simple way to set a variable to true  using this syntax methodology  would be  var 1  conversely  var      Reference   -n   True if the length of var string is non-zero   -z   True if the length of var string is zero

User · Answer

How can I declare and use Boolean variables in a shell script    Unlike many other programming languages  Bash does not segregate its variables by  type    1   So the answer is pretty clear  There isn t any Boolean variable in Bash   However   Using a declare statement  we can limit the value assignment to variables  2      bin bash declare -ir BOOL  0 1    Remember BOOL can t be unset till this shell terminates readonly false   BOOL 0   readonly true   BOOL 1      Same as declare -ir false 0 true 1   true    amp  amp  echo  True    false    amp  amp  echo  False     true    amp  amp  echo  Not True     false    amp  amp  echo  Not false    The r option in declare and readonly is used to state explicitly that the variables are readonly  I hope the purpose is clear

User · Answer

In many programming languages  the Boolean type is  or is implemented as  a subtype of integer  where true behaves like 1 and false behaves like 0   Boolean in C  Boolean in Python  Boolean in Java   Mathematically  Boolean algebra resembles integer arithmetic modulo 2  Therefore  if a language doesn t provide native Boolean type  the most natural and efficient solution is to use integers  This works with almost any language  For example  in Bash you can do    val 1    val    amp  amp  echo  quot true quot     echo  quot false quot  true   val 0    val    amp  amp  echo  quot true quot     echo  quot false quot  false  man bash     expression   The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION  If the value of the expression is non-zero  the return status is 0  otherwise the return status is 1  This is exactly equivalent to let  quot expression quot

User · Answer

Bash really confuses the issue with the likes of                 etc   All treading on each others  code spaces  I guess this is mostly historical  where Bash had to pretend to be sh occasionally   Most of the time  I can just pick a method and stick with it  In this instance  I tend to declare  preferably in a common library file I can include with   in my actual script s     TRUE 1  FALSE 0   I can then use the           arithmetic operator to test thusly   testvar  FALSE  if    -d   does directory exist     then     testvar  TRUE  fi  if    testvar    TRUE     then       Do stuff because the directory does exist fi    You do have to be disciplined  Your testvar must either be set to  TRUE or  FALSE at all times  In           comparators  you don t need the preceding    which makes it more readable  I can use           because  TRUE 1 and  FALSE 0  i e  numeric values  The downside is having to use a   occasionally   testvar  TRUE   which is not so pretty    It s not a perfect solution  but it covers every case I need of such a test

User · Answer

I found the existing answers confusing   Personally  I just want to have something which looks and works like C   This snippet works many times a day in production   snapshotEvents true  if   snapshotEvents  then       Do stuff if true fi   and to keep everyone happy  I tested   snapshotEvents false  if    snapshotEvents  then       Do stuff if false fi   Which also worked fine   The  snapshotEvents evaluates the contents of value of the variable   So you need the     You don t really need the parentheses  I just find them helpful    Tested on  GNU Bash  version 4 1 11 2 -release Bash Guide for Beginners  Machtelt Garrels  v1 11  2008

User · Answer

My receipe to  my own  idiocy    setting ---------------- commonMode false if     something     COMMON      then     commonMode true fi    using ---------------- if  commonMode  then     echo  YES  Common Mode  else     echo  NO  no Common Mode  fi   commonMode  amp  amp  echo  commonMode is ON           commonMode    echo  commonMode is OFF xxxxxx

User · Answer

Use arithmetic expressions      bin bash  false 0 true 1    false    amp  amp  echo false   true    amp  amp  echo true    false    amp  amp  echo not false    true    amp  amp  echo not true   Output      true     not false

[bash] How can I declare and use Boolean variables in a shell script?

POSIX (Portable Operating System Interface)

Finally, what you should do with all that information

Examples related to bash

Examples related to shell

Examples related to scripting

Examples related to boolean

Examples related to sh