Jam/MR - Make(1) Redux

The Jam/MR Executable

USAGE

jam [ -a ] [ -n ] [ -v ] 
    [ -d debug ] 
    [ -f jambase ] 
    [ -j jobs ] 
    [ -o actionsfile ] 
    [ -s var=value ] 
    [ -t target ] 
    [ target ... ]

DESCRIPTION

Jam is a program construction tool, like make(1).

Jam recursively builds target files from source files, using dependency information and updating actions expressed in the Jambase file, which is written in jam's own interpreted language. The default Jambase is compiled into jam and provides a boilerplate for common use, relying on a user-provide file "Jamfile" to enumerate actual targets and sources.

The Jambase is described in the Jambase Reference and the document Using Jamfiles and Jambase.

OPTIONS

If target is provided on the command line, jam builds target; otherwise jam builds the target 'all'.

Jam may be invoked with the following options:

-a Build all targets anyway, even if they are up-to-date.
-d n Enable cummulative debugging levels from 1 to n. Interesting values are:

1
Show actions (the default)
2
Show "quiet" actions and display all action text
3
Show dependency analysis, and target/source timestamps/paths
4
Show shell arguments
5
Show rule invocations and variable expansions
6
Show directory/header file/archive scans
7
Show variable settings
8
Show variable fetches
9
Show variable manipulation, scanner tokens

-d +n Enable debugging level n.
-d 0 Turn off all debugging levels. Only errors are not suppressed.
-f jambase Read jambase instead of using the built-in Jambase. Only one -f flag is permitted, but the jambase may explicitly include other files.
-j n Run up to n shell commands concurrently (UNIX and NT only). The default is 1.
-n Don't actually execute the updating actions, but do everything else. This changes the debug level default to -d2.
-o file Write the updating actions to the specified file instead of running them (or outputting them, as on the Mac).
-s var=value Set the variable var to value, overriding both internal variables and variables imported from the environment.
-t target Rebuild target and everything that depends on it, even if it is up-to-date.
-v Print the version of jam and exit.

OPERATION

Jam has four phases of operation: start-up, parsing, binding, and updating.

Start-up

Upon start-up, jam imports environment variable settings into jam variables. Environment variables are split at blanks with each word becoming an element in the variable's list of values. Environment variables whose names end in PATH are split at $(SPLITPATH) characters (e.g., ":" for Unix).

To set a variable's value on the command line, overriding the variable's environment value, use the -s option. To see variable assignments made during jam's execution, use the -d+7 option.

Parsing

In the parsing phase, jam reads and parses the Jambase file, by default the built-in one. It is written in the jam language. See Language below. The last action of the Jambase is to read (via the "include" rule) a user-provided file called "Jamfile".

Collectively, the purpose of the Jambase and the Jamfile is to name built target and source files, construct the dependency graph among them, and associate build actions with targets. The Jambase defines boilerplate rules and variable assignments, and the Jamfile uses these to specify the actual relationship among the target and source files. See the Jambase Reference and the document Using Jamfiles and Jambase for information.

Binding

Binding

After parsing, jam recursively descends the dependency graph and binds every file target with a location in the filesystem. If jam detects a circular dependency in the graph, it issues a warning.

File target names are given as absolute or relative path names in the filesystem. If the path name is absolute, it is bound as is. If the path name is relative, it is normally bound as is, and thus relative to the current directory. This can be modified by the settings of the $(SEARCH) and $(LOCATE) variables, which enable jam to find and build targets spread across a directory tree. See SEARCH and LOCATE Variables below.

Update Determination

After binding each target, jam determines whether the target needs updating, and if so marks the target for the updating phase. A target is normally so marked if it is missing, it is older than any of its sources, or any of its sources are marked for updating. This behavior can be modified by the application of special built-in rules, ALWAYS, LEAVES, NOCARE, NOTFILE, NOUPDATE, and TEMPORARY. See Modifying Binding below.

Header File Scanning

During the binding phase, jam also performs header file scanning, where it looks inside source files for the implicit dependencies on other files caused by C's #include syntax. This is controlled by the special variables $(HDRSCAN) and $(HDRRULE). The result of the scan is formed into a rule invocation, with the scanned file as the target and the found included file names as the sources. Note that this is the only case where rules are invoked outside the parsing phase. See HDRSCAN and HDRRULE Variables below.

Updating

After binding, jam again recursively descends the dependency graph, this time executing the update actions for each target marked for update during the binding phase. If a target's updating actions fail, then all other targets which depend on that target are skipped.

The -j flag instructs jam to build more than one target at a time. If there are multiple actions on a single target, they are run sequentially.

LANGUAGE

Overview

Jam has an interpreted, procedural language. Statements in jam are rule (procedure) definitions, rule invocations, flow-of-control structures, variable assignments, and sundry language support.

Lexical Features

Jam treats its input files as whitespace-separated tokens, with two exceptions: double quotes (") can enclose whitespace to embed it into a token, and everything between the matching curly braces ({}) in the definition of a rule action is treated as a single string. A backslash (\) can escape a double quote, or any single whitespace character.

Jam requires whitespace (blanks, tabs, or newlines) to surround all tokens, including the colon (:) and semicolon (;) tokens.

Jam keywords (an mentioned in this document) are reserved and generally must be quoted with double quotes (") to be used as arbitrary tokens, such as variable or target names.

Targets

The essential jam data entity is a target. Built targets are files to be updated. Source targets are the files used in updating built targets. Built targets and source targets are collectively referred to as file targets, and frequently built targets are source targets for other built targets. Pseudotargets are symbols which represent dependencies on other targets, but which are not themselves associated with any real file.

A file target's identifier is generally the file's name, which can be absolutely rooted, relative to the directory of jam's invocation, or simply local (no directory). Most often it is the last case, and the actual file path is bound using the $(SEARCH) and $(LOCATE) special variables. See SEARCH and LOCATE Variables below. A local filename is optionally qualified with grist, a string value used to assure uniqueness. A file target with an identifier of the form file(member) is a library member (usually an ar(1) archive on UNIX).

Rules

The basic jam language entity is called a rule. A rule is defined in two parts: the procedure and the actions. The procedure is a body of jam statements to be run when the rule is invoked; the actions are the OS shell commands to execute when updating the built targets of the rule.

Rules can return values, which can be expanded into a list with "[ rule args ... ]". A rule's value is the value of its last statement, though only the following statements have values: 'if' (value of the leg chosen), 'switch' (value of the case chosen), set (value of the resulting variable), and 'return' (value of its arguments). Note that 'return' doesn't actually cause a return, i.e., is a no-op unless it is the last statement of the last block executed within rule body.

The jam statements for defining and invoking rules are as follows:

rule rulename { statements }
Define a rule's procedure, replacing any previous definition.

actions [ modifiers ] rulename { commands }
Define a rule's updating actions, replacing any previous definition.

rulename field1 : field2 : ... : fieldN ;
Invoke a rule.

on target rulename field1 : field2 : ... : fieldN ;
Invoke a rule under the influence of target's specific variables..

[ rulename field1 : field2 : ... : fieldN ] [ on target rulename field1 : field2 : ... : fieldN ]
Used as an argument, expands to the return value of the rule invoked.

A rule is invoked with values in field1 through fieldN. They may be referenced in the procedure's statements as $(1) through $(N) (9 max), and the first two only may be referenced in the action's commands as $(1) and $(2). $(<) and $(>) are synonymous with $(1) and $(2).

Rules fall into two categories: updating rules (with actions), and pure procedure rules (without actions). Updating rules treat arguments $(1) and $(2) as built targets and sources, respectively, while pure procedure rules can take arbitrary arguments.

When an updating rule is invoked, its updating actions are added to those associated with its built targets ($(1)) before the rule's procedure is run. Later, to build the targets in the updating phase, commands are passed to the OS command shell, with $(1) and $(2) replaced by bound versions of the target names. See Binding above.

Action Modifiers

The following action modifiers are understood:

actions bind vars
$(vars) will be replaced with bound values.
actions existing
$(>) includes only source targets currently existing.
actions ignore
The return status of the commands is ignored.
actions piecemeal
commands are repeatedly invoked with a subset of $(>) small enough to fit in the command buffer on this OS.
actions quietly
The action is not echoed to the standard output.
actions together
The $(>) from multiple invocations of the same action on the same built target are glommed together.
actions updated
$(>) includes only source targets themselves marked for updating.

Built-in Rules

Jam has eleven built-in rules, all of which are pure procedure rules without updating actions. They are in three groups: the first builds the dependency graph; the second modifies it; and the third are just utility rules.

Dependency Building

DEPENDS targets1 : targets2 ;
Builds a direct dependency: makes each of targets1 depend on each of targets2. Generally, targets1 will be rebuilt if targets2 are themselves rebuilt are or are newer than targets1.
INCLUDES targets1 : targets2 ;
Builds a sibling dependency: makes any target that depends on any of targets1 also depend on each of targets2. This reflects the dependencies that arise when one source file includes another: the object built from the source file depends both on the original and included source file, but the two sources files don't depend on each other. For example:
DEPENDS foo.o : foo.c ; INCLUDES foo.c : foo.h ;
"foo.o" depends on "foo.c" and "foo.h" in this example.

Modifying Binding

The six rules ALWAYS, LEAVES, NOCARE, NOTFILE, NOUPDATE, and TEMPORARY modify the dependency graph so that jam treats the targets differently during its target binding phase. See Binding above. Normally, jam updates a target if it is missing, if its filesystem modification time is older than any of its dependencies (recursively), or if any of its dependencies are being updated. This basic behavior can be changed by invoking the following rules:

ALWAYS targets ;
Causes targets to be rebuilt regardless of whether they are up-to-date (they must still be in the dependency graph). This is used for the clean and uninstall targets, as they have no dependencies and would otherwise appear never to need building. It is best applied to targets that are also NOTFILE targets, but it can also be used to force a real file to be updated as well.
LEAVES targets ;
Makes each of targets depend only on its leaf sources, and not on any intermediate targets. This makes it immune to its dependencies being updated, as the "leaf" dependencies are those without their own dependencies and without updating actions. This allows a target to be updated only if original source files change.
NOCARE targets ;
Causes jam to ignore targets that neither can be found nor have updating actions to build them. Normally for such targets jam issues a warning and then skips other targets that depend on these missing targets. The HdrRule in Jambase uses NOCARE on the header file names found during header file scanning, to let jam know that the included files may not exist. For example, if a #include is within an #ifdef, the included file may not actually be around.
NOTFILE targets ;
Marks targets as pseudotargets and not real files. No timestamp is checked, and so the actions on such a target are only executed if the target's dependencies are updated, or if the target is also marked with ALWAYS. The default jam target "all" is a pseudotarget. In Jambase, NOTFILE is used to define several addition convenient pseudotargets.
NOUPDATE targets ;
Causes the timestamps on targets to be ignored. This has two effects: first, once the target has been created it will never be updated; second, manually updating target will not cause other targets to be updated. In Jambase, for example, this rule is applied to directories by the MkDir rule, because MkDir only cares that the target directory exists, not when it has last been updated.
TEMPORARY targets ;
Marks targets as temporary, allowing them to be removed after other targets that depend upon them have been updated. If a TEMPORARY target is missing, jam uses the timestamp of the target's parent. Jambase uses TEMPORARY to mark object files that are archived in a library after they are built, so that they can be deleted after they are archived.

Utility Rules

The two rules ECHO and EXIT are utility rules, used only in jam's parsing phase.

ECHO args ;
Blurts out the message args to stdout.
EXIT args ;
Blurts out the message args to stdout and then exits with a failure status.
"Echo", "echo", "Exit", and "exit" are accepted as aliases for ECHO and EXIT, since it is hard to tell that these are built-in rules and not part of the language, like "include".

The GLOB rule does filename globbing.

GLOB directories : patterns
Using the same wildcards as for the patterns in the switch statement). It is invoked by being used as an argument to a rule invocation inside of `[ ]`. For example: "FILES = [ GLOB dir1 dir2 : *.c *.h ]" will set the value of A to the list of C source and header files in dir1 or dir2.

Flow-of-Control

Jam has several simple flow-of-control statements:

for var in list { statements }
Executes statements for each element in list, setting the variable var to the element value.
if cond { statements } [ else statements ]
Does the obvious; the else clause is optional. cond is built of:

a true if any a element is a non-zero-length string
a = b list a matches list b string-for-string
a != b list a does not match list b
a < b a[i] string is less than b[i] string, where i is first mismatched element in lists a and b
a <= b every a string is less than or equal to its b counterpart
a > b a[i] string is greater than b[i] string, where i is first mismatched element
a >= b every a string is greater than or equal to its b counterpart
a in b true if all elements of a can be found in b, or if a has no elements
! cond condition not true
cond && cond conjunction
cond || cond disjunction
( cond ) precedence grouping

include file ;
Causes jam to read the named file. The file is bound like a regular target (see Binding above) but unlike a regular target the include file cannot be built.
The include file is inserted into the input stream during the parsing phase. The primary input file and all the included file(s) are treated as a single file; that is, jam infers no scope boundaries from included files.
local vars [ = values ] ;
Creates new vars inside to the enclosing {} block, obscuring any previous values they might have. The previous values for vars are restored when the current block ends. Any rule called or file included will see the local and not the previous value (this is sometimes called Dynamic Scoping). The local statement may appear anywhere, even outside of a block (in which case the previous value is restored when the input ends). The vars are initialized to values if present, or left uninitialized otherwise.
return values ;
Within a rule body, the return statement sets the return value for an invocation of the rule. It does not cause the rule to return; a rule's value is actually the value of the last statement executed, so a return should be the last statement executed before the rule "naturally" returns.
switch value { case pattern1 : statements ; case pattern2 : statements ; ... }
The switch statement executes zero or one of the enclosed statements, depending on which, if any, is the first case whose pattern matches value. The pattern values are not variable-expanded. The pattern values may include the following wildcards:

? match any single character
* match zero or more characters
[chars] match any single character in chars
[^chars] match any single character not in chars
\x match x (escapes the other wildcards)

while cond { statements }
Repeatedly execute statements while cond remains true upon entry. (See the description of cond expression syntax under if, above).

Variables

Jam variables are lists of zero or more elements, with each element being a string value. An undefined variable is indistinguishable from a variable with an empty list, however, a defined variable may have one more elements which are null strings. All variables are referenced as $(variable).

Variables are either global or target-specific. In the latter case, the variable takes on the given value only during the updating of the specific target.

A variable is defined with:

variable = elements ;
variable += elements ;
variable on targets = elements ;
variable on targets += elements ;
variable default = elements ;
variable ?= elements ;

The first two forms set variable globally. The third and forth forms set a target-specific variable. The = operator replaces any previous elements of variable with elements; the += operation adds elements to variable's list of elements. The final two forms are synonymous: they set variable globally, but only if it was previously unset.

Variables referenced in updating commands will be replaced with their values; target-specific values take precedence over global values. Variables passed as arguments ($(1) and $(2)) to actions are replaced with their bound values; the "bind" modifier can be used on actions to cause other variables to be replaced with bound values. See Action Modifiers above.

Jam variables are not re-exported to the environment of the shell that executes the updating actions, but the updating actions can reference jam variables with $(variable).

Variable Expansion

During parsing, jam performs variable expansion on each token that is not a keyword or rule name. Such tokens with embedded variable references are replaced with zero or more tokens. Variable references are of the form $(v) or $(vm), where v is the variable name, and m are optional modifiers.

Variable expansion in a rule's actions is similar to variable expansion in statements, except that the action string is tokenized at whitespace regardless of quoting.

The result of a token after variable expansion is the product of the components of the token, where each component is a literal substring or a list substituting a variable reference. For example:

$(X) -> a b c t$(X) -> ta tb tc $(X)z -> az bz cz $(X)-$(X) -> a-a a-b a-c b-a b-b b-c c-a c-b c-c

The variable name and modifiers can themselves contain a variable reference, and this partakes of the product as well:

$(X) -> a b c $(Y) -> 1 2 $(Z) -> X Y $($(Z)) -> a b c 1 2

Because of this product expansion, if any variable reference in a token is undefined, the result of the expansion is an empty list. If any variable element is a null string, the result propagates the non-null elements:

$(X) -> a "" $(Y) -> "" 1 $(Z) -> *$(X)$(Y)* -> *a* *a1* ** *1* *$(X)$(Z)* ->

A variable element's string value can be parsed into grist and filename-related components. Modifiers to a variable are used to select elements, select components, and replace components. The modifiers are:

[n] Select element number n (starting at 1). If the variable contains fewer than n elements, the result is a zero-element list.
[n-m] Select elements number n through m.
[n-] Select elements number n through the last.
:B Select filename base.
:S Select (last) filename suffix.
:M Select archive member name.
:D Select directory path.
:P Select parent directory.
:G Select grist.
:U Replace lowercase characters with uppercase.
:L Replace uppercase characters with lowercase.
:chars Select the components listed in chars.
:G=grist Replace grist with grist.
:D=path Replace directory with path.
:B=base Replace the base part of file name with base.
:S=suf Replace the suffix of file name with suf.
:M=mem Replace the archive member name with mem.
:R=root Prepend root to the whole file name, if not already rooted.
:E=value Assign value to the variable if it is unset.
:J=joinval Concatentate list elements into single element, separated by joinval.

On VMS, $(var:P) is the parent directory of $(var:D); on Unix and NT, $(var:P) and $(var:D) are the same.

Built-in Variables

This section discusses variables that have special meaning to jam.

SEARCH and LOCATE Variables

These two variables control the binding of file target names to locations in the file system. Generally, $(SEARCH) is used to find existing sources while $(LOCATE) is used to fix the location for built targets.

Rooted (absolute path) file targets are bound as is. Unrooted file target names are also normally bound as is, and thus relative to the current directory, but the settings of $(LOCATE) and $(SEARCH) alter this:

If $(LOCATE) is set then the target is bound relative to the first directory in $(LOCATE). Only the first element is used for binding.
If $(SEARCH) is set then the target is bound to the first directory in $(SEARCH) where the target file already exists.
If the $(SEARCH) search fails, the target is bound relative to the current directory anyhow.

Both $(SEARCH) and $(LOCATE) should be set target-specific and not globally. If they were set globally, jam would use the same paths for all file binding, which is not likely to produce sane results. When writing your own rules, especially ones not built upon those in Jambase, you may need to set $(SEARCH) or $(LOCATE) directly. Almost all of the rules defined in Jambase set $(SEARCH) and $(LOCATE) to sensible values for sources they are looking for and targets they create, respectively.

HDRSCAN and HDRRULE Variables

These two variable control header file scanning. $(HDRSCAN) is an egrep(1) pattern, with ()'s surrounding the file name, used to find file inclusion statements in source files. Jambase uses $(HDRPATTERN) as the pattern for $(HDRSCAN). $(HDRRULE) is the name of a rule to invoke with the results of the scan: the scanned file is the target, the found files are the sources. This is the only place where jam invokes a rule through a variable setting.

Both $(HDRSCAN) and $(HDRRULE) must be set for header file scanning to take place, and they should be set target-specific and not globally. If they were set globally, all files, including executables and libraries, would be scanned for header file include statements.

The scanning for header file inclusions is not exact, but it is at least dynamic, so there is no need to run something like makedepend(GNU) to create a static dependency file. The scanning mechanism errs on the side of inclusion (i.e., it is more likely to return filenames that are not actually used by the compiler than to miss include files) because it can't tell if #include lines are inside #ifdefs or other conditional logic. In Jambase, HdrRule applies the NOCARE rule to each header file found during scanning so that if the file isn't present yet doesn't cause the compilation to fail, jam won't care.

Also, scanning for regular expressions only works where the included file name is literally in the source file. It can't handle languages that allow including files using variable names (as the Jam language itself does).

Platform Identifier Variables

A number of Jam built-in variables can be used to identify runtime platform:

OS OS identifier string
OSPLAT Underlying architecture, when applicable
MAC true on MAC platform
NT true on NT platform
OS2 true on OS2 platform
UNIX true on Unix platforms
VMS true on VMS platform

Jam Version Variables

JAMDATE Time and date at jam start-up.
JAMUNAME Ouput of uname(1) command (Unix only)
JAMVERSION jam version, currently "2.3"

JAMSHELL Variable

When jam executes a rule's action block, it forks and execs a shell, passing the action block as an argument to the shell. The invocation of the shell can be controlled by $(JAMSHELL). The default on Unix is, for example:

JAMSHELL = /bin/sh -c % ;

The % is replaced with the text of the action block.

Jam does not directly support building in parallel across multiple hosts, since that is heavily dependent on the local environment. To build in parallel across multiple hosts, you need to write your own shell that provides access to the multiple hosts. You then reset $(JAMSHELL) to reference it.

Just as jam expands a % to be the text of the rule's action block, it expands a ! to be the multi-process slot number. The slot number varies between 1 and the number of concurrent jobs permitted by the -j flag given on the command line. Armed with this, it is possible to write a multiple host shell. For example:

#!/bin/sh # This sample JAMSHELL uses the SunOS on(1) command to execute a # command string with an identical environment on another host. # Set JAMSHELL = jamshell ! % # # where jamshell is the name of this shell file. # # This version handles up to -j6; after that they get executed # locally. case $1 in 1|4) on winken sh -c "$2";; 2|5) on blinken sh -c "$2";; 3|6) on nod sh -c "$2";; *) eval "$2";; esac

DIAGNOSTICS

In addition to generic error messages, jam may emit one of the following:

warning: unknown rule X
A rule was invoked that has not been defined with an "actions" or "rule" statement.
using N temp target(s)
Targets marked as being temporary (but nonetheless present) have been found.
updating N target(s)
Targets are out-of-date and will be updated.
can't find N target(s)
Source files can't be found and there are no actions to create them.
can't make N target(s)
Due to sources not being found, other targets cannot be made.
warning: X depends on itself
A target depends on itself either directly or through its sources.
don't know how to make X
A target is not present and no actions have been defined to create it.
X skipped for lack of Y
A source failed to build, and thus a target cannot be built.
warning: using independent target X
A target that is not a dependency of any other target is being referenced with $(<) or $(>).
X removed
Jam removed a partially built target after being interrupted.

BUGS, LIMITATIONS

The -j flag can cause jam to get confused when single actions update more than one target at a time. jam may proceed as if the targets were built even though they are still under construction.

For parallel building to be successful, the dependencies among files must be properly spelled out, as targets tend to get built in a quickest-first ordering. Also, beware of un-parallelizable commands that drop fixed-named files into the current directory, like yacc(1) does.

With the -j flag, errors from failed commands can get staggeringly mixed up.

A poorly set $(JAMSHELL) is likely to result in silent failure.

SEE ALSO

Jam documentation and source are available from the Perforce Public Depot.

AUTHOR

Jam's author is Christopher Seiwald (seiwald@perforce.com). Documentation is provided by Perforce Software, Inc.

Copyright 1993-2002 Christopher Seiwald and Perforce Software, Inc.
Comments to info@perforce.com
Last updated: December 31, 2000
$Id: //guest/richard_geiger/intjam/src/Jam.html#14 $

`-a`	Build all targets anyway, even if they are up-to-date.
`-d n`	Enable cummulative debugging levels from 1 to n. Interesting values are: 1 Show actions (the default) 2 Show "quiet" actions and display all action text 3 Show dependency analysis, and target/source timestamps/paths 4 Show shell arguments 5 Show rule invocations and variable expansions 6 Show directory/header file/archive scans 7 Show variable settings 8 Show variable fetches 9 Show variable manipulation, scanner tokens
`-d +n`	Enable debugging level n.
`-d 0`	Turn off all debugging levels. Only errors are not suppressed.
`-f jambase`	Read jambase instead of using the built-in Jambase. Only one -f flag is permitted, but the jambase may explicitly include other files.
`-j n`	Run up to n shell commands concurrently (UNIX and NT only). The default is 1.
`-n`	Don't actually execute the updating actions, but do everything else. This changes the debug level default to -d2.
`-o file`	Write the updating actions to the specified file instead of running them (or outputting them, as on the Mac).
`-s var=value`	Set the variable var to value, overriding both internal variables and variables imported from the environment.
`-t target`	Rebuild target and everything that depends on it, even if it is up-to-date.
`-v`	Print the version of jam and exit.

`[n]`	Select element number n (starting at 1). If the variable contains fewer than n elements, the result is a zero-element list.
`[n-m]`	Select elements number n through m.
`[n-]`	Select elements number n through the last.
`:B`	Select filename base.
`:S`	Select (last) filename suffix.
`:M`	Select archive member name.
`:D`	Select directory path.
`:P`	Select parent directory.
`:G`	Select grist.
`:U`	Replace lowercase characters with uppercase.
`:L`	Replace uppercase characters with lowercase.
`:chars`	Select the components listed in chars.
`:G=grist`	Replace grist with grist.
`:D=path`	Replace directory with path.
`:B=base`	Replace the base part of file name with base.
`:S=suf`	Replace the suffix of file name with suf.
`:M=mem`	Replace the archive member name with mem.
`:R=root`	Prepend root to the whole file name, if not already rooted.
`:E=value`	Assign value to the variable if it is unset.
`:J=joinval`	Concatentate list elements into single element, separated by joinval.

OS	OS identifier string
OSPLAT	Underlying architecture, when applicable
MAC	true on MAC platform
NT	true on NT platform
OS2	true on OS2 platform
UNIX	true on Unix platforms
VMS	true on VMS platform

JAMDATE	Time and date at jam start-up.
JAMUNAME	Ouput of uname(1) command (Unix only)
JAMVERSION	jam version, currently "2.3"