Jam Language Features

Lexical Features

Jam has a interpreted, procedural language with a few select features to effect program construction. Statements in jam are rule (procedure) definitions, rule invocations, updating action definitions, flow-of-control structures, variable assignments, and sundry language support.

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 updating actions 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 (as 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.


Jam's only data type is a one-dimensional list of arbitrary strings. They arise as literal (whitespace-separated) tokens in the Jambase or included files, as the result of variable expansion of those tokens, or as the return value from a rule invocation.


The basic jam language entity is called a rule. A rule is simply a procedure definition, with a body of jam statements to be run when the rule is invoked. The syntax of rule invocation make it possible to write Jamfiles that look a bit like Makefiles.

Rules take up to 9 arguments ($(1) through $(9), each a list) and can have a return value (a single list). A rule's return value can be expanded in a list by enclosing the rule invocation with [ and ].

Updating Actions

A rule may have updating actions associated with it, in which case arguments $(1) and $(2) are treated as built targets and sources, respectively. Updating actions are the OS shell commands to execute when updating the built targets of the rule.

When an rule with updating actions is invoked, those 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, the actions are passed to the OS command shell, with $(1) and $(2) replaced by bound versions of the target names. See Binding above.


Jam's langauge has the following statements:

rulename field1 : field2 : ... : fieldN ;

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

rulename undergoes variable expansion. If the resulting list is more than one value, each rule is invoked with the same arguments, and the result of the invocation is the concatenation of all the results.

actions [ modifiers ] rulename { commands }

Define a rule's updating actions, replacing any previous definition. The first two arguments may be referenced in the action's commands as $(1) and $(2) or $(<) and $(>).

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.


Breaks out of the closest enclosing for or while loop.


Jumps to the end of the closest enclosing for or while loop.

for var in list { statements }

Executes statements for each element in list, setting the variable var to the element value.

if cond { statements } [ else statement ]

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. Marking an include file target with the NOCARE rule makes it optional: if it is missing, it causes no error.

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.

on target statement ;

Run statement under the influence of target's target-specific variables. These variables become local copies during statement's run, but they may be updated as target-specific variables using the usual "variable on targets =" syntax.

return values ;

Within a rule body, the return statement sets the return value for an invocation of the rule and terminates the rule's execution.

rule rulename [ : vars ] { statements }

Define a rule's procedure, replacing any previous definition. If vars are provided, they are assigned the values of the parameters ($(1) to $(9)) when statements are executed, as with the local statement.

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).


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 target's binding, header file scanning, and updating; and during the "on target statement" statement.

A variable is defined with:

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

The first three forms set variable globally. The last three 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 ?= operator sets variable only if it was previously unset. The last form "variable on targets ?= elements" checks to see if the target-specific, not the global, variable is set. (The ?= operator also has an old form "default =".)

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 Use value instead if the variable 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 Rules

Jam has twelve 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 remaining rules are utility rules.

ECHO args ;
Echo args ;
echo args ;
Blurts out the message args to stdout.

EXIT args ;
Exit args ;
exit args ;
Blurts out the message args to stdout and then exits with a failure status.

GLOB directories : patterns ;
Scans directories for files matching patterns, returning the list of matching files (with directory prepended). patterns uses the same syntax as in the switch statement. Only useful within the [ ] construct, to change the result into a list.

MATCH regexps : list ;
Matches the egrep(1) style regular expressions regexps against the strings in list. The result is the concatenation of matching () subexpressions for each string in list, and for each regular expression in regexps. Only useful within the [ ] construct, to change the result into a list.

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.


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. $(HDRRULE) is run under the influence of the scanned file's target-specific variables.

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:

OSOS identifier string
OSPLATUnderlying architecture, when applicable
MACtrue on MAC platform
NTtrue on NT platform
OS2true on OS2 platform
UNIXtrue on Unix platforms
VMStrue on VMS platform

Jam Version Variables

JAMDATETime and date at jam start-up.
JAMUNAMEOuput of uname(1) command (Unix only)
JAMVERSIONjam version, as reported by jam -v.


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:


# 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";;