What is the difference between the GNU Makefile variable assignments and

Question

Can anybody give a clear explanation of how variable assignment really works in Makefiles    What is the difference between     VARIABLE   value  VARIABLE    value  VARIABLE    value  VARIABLE    value   I have read the section in GNU Make s manual  but it still doesn t make sense to me

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Using   causes the variable to be assigned a value   If the variable already had a value  it is replaced   This value will be expanded when it is used   For example   HELLO   world HELLO WORLD     HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    This echoes  hello world   echo   HELLO WORLD    Using    is similar to using     However  instead of the value being expanded when it is used  it is expanded during the assignment   For example   HELLO   world HELLO WORLD      HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    Still echoes  world world   echo   HELLO WORLD   HELLO WORLD      HELLO  world     This echoes  hello world   echo   HELLO WORLD    Using    assigns the variable a value iff the variable was not previously assigned   If the variable was previously assigned a blank value  VAR    it is still considered set I think   Otherwise  functions exactly like     Using    is like using    but instead of replacing the value  the value is appended to the current one  with a space in between   If the variable was previously set with     it is expanded I think   The resulting value is expanded when it is used I think   For example   HELLO WORLD   hello HELLO WORLD    world     This echoes  hello world   echo   HELLO WORLD    If something like HELLO WORLD     HELLO WORLD  world  were used  recursion would result  which would most likely end the execution of your Makefile   If A      A    B  were used  the result would not be the exact same as using    because B is expanded with    whereas    would not cause B to be expanded

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When you use VARIABLE   value  if value is actually a reference to another variable  then the value is only determined when VARIABLE is used  This is best illustrated with an example   VAL   foo VARIABLE     VAL  VAL   bar    VARIABLE and VAL will both evaluate to  bar    When you use VARIABLE    value  you get the value of value as it is now  For example   VAL   foo VARIABLE      VAL  VAL   bar    VAL will evaluate to  bar   but VARIABLE will evaluate to  foo    Using VARIABLE    val means that you only set the value of VARIABLE if VARIABLE is not set already  If it s not set already  the setting of the value is deferred until VARIABLE is used  as in example 1    VARIABLE    value just appends value to VARIABLE  The actual value of value is determined as it was when it was initially set  using either   or

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Lazy Set  VARIABLE   value   Normal setting of a variable  but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used  not the one it had when it was declared  Immediate Set  VARIABLE    value   Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time   Lazy Set If Absent  VARIABLE    value   Setting of a variable only if it doesn t have a value  value is always evaluated when VARIABLE is accessed  It is equivalent to  ifeq    origin FOO   undefined    FOO   bar endif   See the documentation for more details   Append  VARIABLE    value   Appending the supplied value to the existing value  or setting to that value if the variable didn t exist

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The most upvoted answer can be improved  Let me refer to GNU Make manual  quot Setting variables quot  and  quot Flavors quot   and add some comments  Recursively expanded variables  The value you specify is installed verbatim  if it contains references to other variables  these references are expanded whenever this variable is substituted  in the course of expanding some other string   When this happens  it is called recursive expansion   foo     bar   The catch  foo will be expanded to the value of   bar  each time foo is evaluated  possibly resulting in different values  Surely you cannot call it  quot lazy quot   This can surprise you if executed on midnight    This variable is haunted  WHEN     shell date -I   something      touch   WHEN  flag    If this is executed on 00 00 00 000    WHEN  will have a different value  something-else-later  something     test -f   WHEN  flag    echo  quot Boo  quot   Simply expanded variable VARIABLE    value VARIABLE     value   Variables defined with          or           are simply expanded variables    Simply expanded variables are defined by lines using          or                  Both forms are equivalent in GNU make  however only the           form is described by the POSIX standard       2012    The value of a simply expanded variable is scanned once and for all  expanding any references to other variables and functions  when the variable is defined   Not much to add  It s evaluated immediately  including recursive expansion of  well  recursively expanded variables  The catch  If VARIABLE refers to ANOTHER VARIABLE  VARIABLE      ANOTHER VARIABLE -yohoho  and ANOTHER VARIABLE is not defined before this assignment  ANOTHER VARIABLE will expand to an empty value  Assign if not set FOO    bar  is equivalent to ifeq    origin FOO   undefined  FOO   bar endif  where   origin FOO  equals to undefined only if the variable was not set at all  The catch  if FOO was set to an empty string  either in makefiles  shell environment  or command line overrides  it will not be assigned bar  Appending VAR    bar  Appending   When the variable in question has not been defined before           acts just like normal          it defines a recursively-expanded variable  However  when there is a previous definition  exactly what          does depends on what flavor of variable you defined originally   So  this will print foo bar  VAR   foo       a mile of code VAR      BAR  BAR   bar   info   VAR    but this will print foo  VAR    foo       a mile of code VAR      BAR  BAR   bar   info   VAR    The catch is that    behaves differently depending on what type of variable VAR was assigned before  Multiline values The syntax to assign multiline value to a variable is  define VAR NAME    line line endef  or define VAR NAME   line line endef  Assignment operator can be omitted  then it creates a recursively-expanded variable  define VAR NAME line line endef  The last newline before endef is removed  Bonus  the shell assignment operator           HASH    printf   043   is the same as HASH      shell printf   043    Don t use it    shell  call is more readable  and the usage of both in a makefiles is highly discouraged  At least    shell  follows Joel s advice and makes wrong code look obviously wrong

User · Answer

Using   causes the variable to be assigned a value   If the variable already had a value  it is replaced   This value will be expanded when it is used   For example   HELLO   world HELLO WORLD     HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    This echoes  hello world   echo   HELLO WORLD    Using    is similar to using     However  instead of the value being expanded when it is used  it is expanded during the assignment   For example   HELLO   world HELLO WORLD      HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    Still echoes  world world   echo   HELLO WORLD   HELLO WORLD      HELLO  world     This echoes  hello world   echo   HELLO WORLD    Using    assigns the variable a value iff the variable was not previously assigned   If the variable was previously assigned a blank value  VAR    it is still considered set I think   Otherwise  functions exactly like     Using    is like using    but instead of replacing the value  the value is appended to the current one  with a space in between   If the variable was previously set with     it is expanded I think   The resulting value is expanded when it is used I think   For example   HELLO WORLD   hello HELLO WORLD    world     This echoes  hello world   echo   HELLO WORLD    If something like HELLO WORLD     HELLO WORLD  world  were used  recursion would result  which would most likely end the execution of your Makefile   If A      A    B  were used  the result would not be the exact same as using    because B is expanded with    whereas    would not cause B to be expanded

User · Answer

In the above answers  it is important to understand what is meant by  values are expanded at declaration use time   Giving a value like   c does not entail any expansion  It is only when this string is used by a command that it will maybe trigger some globbing  Similarly  a value like   wildcard   c  or   shell ls   c  does not entail any expansion and is completely evaluated at definition time even if we used    in the variable definition   Try the following Makefile in directory where you have some C files   VAR1     c VAR2      c VAR3     wildcard   c  VAR4      wildcard   c  VAR5     shell ls   c  VAR6      shell ls   c   all       touch foo c      echo  now VAR1       VAR1       ls   VAR1       echo  now VAR2       VAR2       ls   VAR2       echo  now VAR3       VAR3       ls   VAR3       echo  now VAR4       VAR4       ls   VAR4       echo  now VAR5       VAR5       ls   VAR5       echo  now VAR6       VAR6       ls   VAR6      rm -v foo c   Running make will trigger a rule that creates an extra  empty  C file  called foo c but none of the 6 variables has foo c in its value

User · Answer

Using   causes the variable to be assigned a value   If the variable already had a value  it is replaced   This value will be expanded when it is used   For example   HELLO   world HELLO WORLD     HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    This echoes  hello world   echo   HELLO WORLD    Using    is similar to using     However  instead of the value being expanded when it is used  it is expanded during the assignment   For example   HELLO   world HELLO WORLD      HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    Still echoes  world world   echo   HELLO WORLD   HELLO WORLD      HELLO  world     This echoes  hello world   echo   HELLO WORLD    Using    assigns the variable a value iff the variable was not previously assigned   If the variable was previously assigned a blank value  VAR    it is still considered set I think   Otherwise  functions exactly like     Using    is like using    but instead of replacing the value  the value is appended to the current one  with a space in between   If the variable was previously set with     it is expanded I think   The resulting value is expanded when it is used I think   For example   HELLO WORLD   hello HELLO WORLD    world     This echoes  hello world   echo   HELLO WORLD    If something like HELLO WORLD     HELLO WORLD  world  were used  recursion would result  which would most likely end the execution of your Makefile   If A      A    B  were used  the result would not be the exact same as using    because B is expanded with    whereas    would not cause B to be expanded

User · Answer

I suggest you do some experiments using  make   Here is a simple demo  showing the difference between   and         Filename  Makefile   x    foo y      x  bar x    later  a   foo b     a  bar a   later  test       echo x -   x       echo y -   y       echo a -   a       echo b -   b    make test prints   x - later y - foo bar a - later b - later bar   Check more elaborate explanation here

User · Answer

In the above answers  it is important to understand what is meant by  values are expanded at declaration use time   Giving a value like   c does not entail any expansion  It is only when this string is used by a command that it will maybe trigger some globbing  Similarly  a value like   wildcard   c  or   shell ls   c  does not entail any expansion and is completely evaluated at definition time even if we used    in the variable definition   Try the following Makefile in directory where you have some C files   VAR1     c VAR2      c VAR3     wildcard   c  VAR4      wildcard   c  VAR5     shell ls   c  VAR6      shell ls   c   all       touch foo c      echo  now VAR1       VAR1       ls   VAR1       echo  now VAR2       VAR2       ls   VAR2       echo  now VAR3       VAR3       ls   VAR3       echo  now VAR4       VAR4       ls   VAR4       echo  now VAR5       VAR5       ls   VAR5       echo  now VAR6       VAR6       ls   VAR6      rm -v foo c   Running make will trigger a rule that creates an extra  empty  C file  called foo c but none of the 6 variables has foo c in its value

User · Answer

The most upvoted answer can be improved  Let me refer to GNU Make manual  quot Setting variables quot  and  quot Flavors quot   and add some comments  Recursively expanded variables  The value you specify is installed verbatim  if it contains references to other variables  these references are expanded whenever this variable is substituted  in the course of expanding some other string   When this happens  it is called recursive expansion   foo     bar   The catch  foo will be expanded to the value of   bar  each time foo is evaluated  possibly resulting in different values  Surely you cannot call it  quot lazy quot   This can surprise you if executed on midnight    This variable is haunted  WHEN     shell date -I   something      touch   WHEN  flag    If this is executed on 00 00 00 000    WHEN  will have a different value  something-else-later  something     test -f   WHEN  flag    echo  quot Boo  quot   Simply expanded variable VARIABLE    value VARIABLE     value   Variables defined with          or           are simply expanded variables    Simply expanded variables are defined by lines using          or                  Both forms are equivalent in GNU make  however only the           form is described by the POSIX standard       2012    The value of a simply expanded variable is scanned once and for all  expanding any references to other variables and functions  when the variable is defined   Not much to add  It s evaluated immediately  including recursive expansion of  well  recursively expanded variables  The catch  If VARIABLE refers to ANOTHER VARIABLE  VARIABLE      ANOTHER VARIABLE -yohoho  and ANOTHER VARIABLE is not defined before this assignment  ANOTHER VARIABLE will expand to an empty value  Assign if not set FOO    bar  is equivalent to ifeq    origin FOO   undefined  FOO   bar endif  where   origin FOO  equals to undefined only if the variable was not set at all  The catch  if FOO was set to an empty string  either in makefiles  shell environment  or command line overrides  it will not be assigned bar  Appending VAR    bar  Appending   When the variable in question has not been defined before           acts just like normal          it defines a recursively-expanded variable  However  when there is a previous definition  exactly what          does depends on what flavor of variable you defined originally   So  this will print foo bar  VAR   foo       a mile of code VAR      BAR  BAR   bar   info   VAR    but this will print foo  VAR    foo       a mile of code VAR      BAR  BAR   bar   info   VAR    The catch is that    behaves differently depending on what type of variable VAR was assigned before  Multiline values The syntax to assign multiline value to a variable is  define VAR NAME    line line endef  or define VAR NAME   line line endef  Assignment operator can be omitted  then it creates a recursively-expanded variable  define VAR NAME line line endef  The last newline before endef is removed  Bonus  the shell assignment operator           HASH    printf   043   is the same as HASH      shell printf   043    Don t use it    shell  call is more readable  and the usage of both in a makefiles is highly discouraged  At least    shell  follows Joel s advice and makes wrong code look obviously wrong

User · Answer

Lazy Set  VARIABLE   value   Normal setting of a variable  but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used  not the one it had when it was declared  Immediate Set  VARIABLE    value   Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time   Lazy Set If Absent  VARIABLE    value   Setting of a variable only if it doesn t have a value  value is always evaluated when VARIABLE is accessed  It is equivalent to  ifeq    origin FOO   undefined    FOO   bar endif   See the documentation for more details   Append  VARIABLE    value   Appending the supplied value to the existing value  or setting to that value if the variable didn t exist

User · Answer

When you use VARIABLE   value  if value is actually a reference to another variable  then the value is only determined when VARIABLE is used  This is best illustrated with an example   VAL   foo VARIABLE     VAL  VAL   bar    VARIABLE and VAL will both evaluate to  bar    When you use VARIABLE    value  you get the value of value as it is now  For example   VAL   foo VARIABLE      VAL  VAL   bar    VAL will evaluate to  bar   but VARIABLE will evaluate to  foo    Using VARIABLE    val means that you only set the value of VARIABLE if VARIABLE is not set already  If it s not set already  the setting of the value is deferred until VARIABLE is used  as in example 1    VARIABLE    value just appends value to VARIABLE  The actual value of value is determined as it was when it was initially set  using either   or

User · Answer

When you use VARIABLE   value  if value is actually a reference to another variable  then the value is only determined when VARIABLE is used  This is best illustrated with an example   VAL   foo VARIABLE     VAL  VAL   bar    VARIABLE and VAL will both evaluate to  bar    When you use VARIABLE    value  you get the value of value as it is now  For example   VAL   foo VARIABLE      VAL  VAL   bar    VAL will evaluate to  bar   but VARIABLE will evaluate to  foo    Using VARIABLE    val means that you only set the value of VARIABLE if VARIABLE is not set already  If it s not set already  the setting of the value is deferred until VARIABLE is used  as in example 1    VARIABLE    value just appends value to VARIABLE  The actual value of value is determined as it was when it was initially set  using either   or

User · Answer

I suggest you do some experiments using  make   Here is a simple demo  showing the difference between   and         Filename  Makefile   x    foo y      x  bar x    later  a   foo b     a  bar a   later  test       echo x -   x       echo y -   y       echo a -   a       echo b -   b    make test prints   x - later y - foo bar a - later b - later bar   Check more elaborate explanation here

User · Answer

Lazy Set  VARIABLE   value   Normal setting of a variable  but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used  not the one it had when it was declared  Immediate Set  VARIABLE    value   Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time   Lazy Set If Absent  VARIABLE    value   Setting of a variable only if it doesn t have a value  value is always evaluated when VARIABLE is accessed  It is equivalent to  ifeq    origin FOO   undefined    FOO   bar endif   See the documentation for more details   Append  VARIABLE    value   Appending the supplied value to the existing value  or setting to that value if the variable didn t exist

User · Answer

Using   causes the variable to be assigned a value   If the variable already had a value  it is replaced   This value will be expanded when it is used   For example   HELLO   world HELLO WORLD     HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    This echoes  hello world   echo   HELLO WORLD    Using    is similar to using     However  instead of the value being expanded when it is used  it is expanded during the assignment   For example   HELLO   world HELLO WORLD      HELLO  world     This echoes  world world   echo   HELLO WORLD   HELLO   hello    Still echoes  world world   echo   HELLO WORLD   HELLO WORLD      HELLO  world     This echoes  hello world   echo   HELLO WORLD    Using    assigns the variable a value iff the variable was not previously assigned   If the variable was previously assigned a blank value  VAR    it is still considered set I think   Otherwise  functions exactly like     Using    is like using    but instead of replacing the value  the value is appended to the current one  with a space in between   If the variable was previously set with     it is expanded I think   The resulting value is expanded when it is used I think   For example   HELLO WORLD   hello HELLO WORLD    world     This echoes  hello world   echo   HELLO WORLD    If something like HELLO WORLD     HELLO WORLD  world  were used  recursion would result  which would most likely end the execution of your Makefile   If A      A    B  were used  the result would not be the exact same as using    because B is expanded with    whereas    would not cause B to be expanded

User · Answer

Lazy Set  VARIABLE   value   Normal setting of a variable  but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used  not the one it had when it was declared  Immediate Set  VARIABLE    value   Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time   Lazy Set If Absent  VARIABLE    value   Setting of a variable only if it doesn t have a value  value is always evaluated when VARIABLE is accessed  It is equivalent to  ifeq    origin FOO   undefined    FOO   bar endif   See the documentation for more details   Append  VARIABLE    value   Appending the supplied value to the existing value  or setting to that value if the variable didn t exist

User · Answer

When you use VARIABLE   value  if value is actually a reference to another variable  then the value is only determined when VARIABLE is used  This is best illustrated with an example   VAL   foo VARIABLE     VAL  VAL   bar    VARIABLE and VAL will both evaluate to  bar    When you use VARIABLE    value  you get the value of value as it is now  For example   VAL   foo VARIABLE      VAL  VAL   bar    VAL will evaluate to  bar   but VARIABLE will evaluate to  foo    Using VARIABLE    val means that you only set the value of VARIABLE if VARIABLE is not set already  If it s not set already  the setting of the value is deferred until VARIABLE is used  as in example 1    VARIABLE    value just appends value to VARIABLE  The actual value of value is determined as it was when it was initially set  using either   or

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