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package - Man Page

Facilities for package loading and version control


package forget ?package package ...?
package ifneeded package version ?script?
package names
package present package ?requirement...?
package present -exact package version
package provide package ?version?
package require package ?requirement...?
package require -exact package version
package unknown ?command?
package vcompare version1 version2
package versions package
package vsatisfies version requirement...
package prefer ?latest|stable?


This command keeps a simple database of the packages available for use by the current interpreter and how to load them into the interpreter. It supports multiple versions of each package and arranges for the correct version of a package to be loaded based on what is needed by the application. This command also detects and reports version clashes. Typically, only the package require and package provide commands are invoked in normal Tcl scripts;  the other commands are used primarily by system scripts that maintain the package database.

The behavior of the package command is determined by its first argument. The following forms are permitted:

package forget ?package package ...?

Removes all information about each specified package from this interpreter, including information provided by both package ifneeded and package provide.

package ifneeded package version ?script?

This command typically appears only in system configuration scripts to set up the package database. It indicates that a particular version of a particular package is available if needed, and that the package can be added to the interpreter by executing script. The script is saved in a database for use by subsequent package require commands;  typically, script sets up auto-loading for the commands in the package (or calls load and/or source directly), then invokes package provide to indicate that the package is present. There may be information in the database for several different versions of a single package. If the database already contains information for package and version, the new script replaces the existing one. If the script argument is omitted, the current script for version version of package package is returned, or an empty string if no package ifneeded command has been invoked for this package and version.

package names

Returns a list of the names of all packages in the interpreter for which a version has been provided (via package provide) or for which a package ifneeded script is available. The order of elements in the list is arbitrary.

package present ?-exact? package ?requirement...?

This command is equivalent to package require except that it does not try and load the package if it is not already loaded.

package provide package ?version?

This command is invoked to indicate that version version of package package is now present in the interpreter. It is typically invoked once as part of an ifneeded script, and again by the package itself when it is finally loaded. An error occurs if a different version of package has been provided by a previous package provide command. If the version argument is omitted, then the command returns the version number that is currently provided, or an empty string if no package provide command has been invoked for package in this interpreter.

package require package ?requirement...?

This command is typically invoked by Tcl code that wishes to use a particular version of a particular package.  The arguments indicate which package is wanted, and the command ensures that a suitable version of the package is loaded into the interpreter. If the command succeeds, it returns the version number that is loaded;  otherwise it generates an error.

A suitable version of the package is any version which satisfies at least one of the requirements, per the rules of package vsatisfies. If multiple versions are suitable the implementation with the highest version is chosen. This last part is additionally influenced by the selection mode set with package prefer.

In the “stable” selection mode the command will select the highest stable version satisfying the requirements, if any. If no stable version satisfies the requirements, the highest unstable version satisfying the requirements will be selected.  In the “latest” selection mode the command will accept the highest version satisfying all the requirements, regardless of its stableness.

If a version of package has already been provided (by invoking the package provide command), then its version number must satisfy the requirements and the command returns immediately. Otherwise, the command searches the database of information provided by previous package ifneeded commands to see if an acceptable version of the package is available. If so, the script for the highest acceptable version number is evaluated in the global namespace; it must do whatever is necessary to load the package, including calling package provide for the package. If the package ifneeded database does not contain an acceptable version of the package and a package unknown command has been specified for the interpreter then that command is evaluated in the global namespace;  when it completes, Tcl checks again to see if the package is now provided or if there is a package ifneeded script for it. If all of these steps fail to provide an acceptable version of the package, then the command returns an error.

package require -exact package version

This form of the command is used when only the given version of package is acceptable to the caller.  This command is equivalent to package require package version-version.

package unknown ?command?

This command supplies a “last resort” command to invoke during package require if no suitable version of a package can be found in the package ifneeded database. If the command argument is supplied, it contains the first part of a command;  when the command is invoked during a package require command, Tcl appends one or more additional arguments giving the desired package name and requirements. For example, if command is foo bar and later the command package require test 2.4 is invoked, then Tcl will execute the command foo bar test 2.4 to load the package. If no requirements are supplied to the package require command, then only the name will be added to invoked command. If the package unknown command is invoked without a command argument, then the current package unknown script is returned, or an empty string if there is none. If command is specified as an empty string, then the current package unknown script is removed, if there is one.

package vcompare version1 version2

Compares the two version numbers given by version1 and version2. Returns -1 if version1 is an earlier version than version2, 0 if they are equal, and 1 if version1 is later than version2.

package versions package

Returns a list of all the version numbers of package for which information has been provided by package ifneeded commands.

package vsatisfies version requirement...

Returns 1 if the version satisfies at least one of the given requirements, and 0 otherwise. Each requirement is allowed to have any of the forms:


This form is called “min-bounded”.


This form is called “min-unbound”.


This form is called “bounded”.

where “min” and “max” are valid version numbers. The legacy syntax is a special case of the extended syntax, keeping backward compatibility. Regarding satisfaction the rules are:


The version has to pass at least one of the listed requirements to be satisfactory.


A version satisfies a “bounded” requirement when


For min equal to the max if, and only if the version is equal to the min.


Otherwise if, and only if the version is greater than or equal to the min, and less than the max, where both min and max have been padded internally with “a0”. Note that while the comparison to min is inclusive, the comparison to max is exclusive.


A “min-bounded” requirement is a “bounded” requirement in disguise, with the max part implicitly specified as the next higher major version number of the min part. A version satisfies it per the rules above.


A version satisfies a “min-unbound” requirement if, and only if it is greater than or equal to the min, where the min has been padded internally with “a0”. There is no constraint to a maximum.

package prefer ?latest|stable?

With no arguments, the commands returns either “latest” or “stable”, whichever describes the current mode of selection logic used by package require.

When passed the argument “latest”, it sets the selection logic mode to “latest”.

When passed the argument “stable”, if the mode is already “stable”, that value is kept.  If the mode is already “latest”, then the attempt to set it back to “stable” is ineffective and the mode value remains “latest”.

When passed any other value as an argument, raise an invalid argument error.

When an interpreter is created, its initial selection mode value is set to “stable” unless the environment variable TCL_PKG_PREFER_LATEST is set.  If that environment variable is defined (with any value) then the initial (and permanent) selection mode value is set to “latest”.

Version Numbers

Version numbers consist of one or more decimal numbers separated by dots, such as 2 or 1.162 or The first number is called the major version number. Larger numbers correspond to later versions of a package, with leftmost numbers having greater significance. For example, version 2.1 is later than 1.3 and version 3.4.6 is later than 3.3.5. Missing fields are equivalent to zeroes:  version 1.3 is the same as version 1.3.0 and, so it is earlier than 1.3.1 or In addition, the letters “a” (alpha) and/or “b” (beta) may appear exactly once to replace a dot for separation. These letters semantically add a negative specifier into the version, where “a” is -2, and “b” is -1. Each may be specified only once, and “a” or “b” are mutually exclusive in a specifier. Thus 1.3a1 becomes (semantically) 1.3.-2.1, 1.3b1 is 1.3.-1.1. Negative numbers are not directly allowed in version specifiers. A version number not containing the letters “a” or “b” as specified above is called a stable version, whereas presence of the letters causes the version to be called is unstable. A later version number is assumed to be upwards compatible with an earlier version number as long as both versions have the same major version number. For example, Tcl scripts written for version 2.3 of a package should work unchanged under versions 2.3.2, 2.4, and 2.5.1. Changes in the major version number signify incompatible changes: if code is written to use version 2.1 of a package, it is not guaranteed to work unmodified with either version 1.7.3 or version 3.1.

Package Indices

The recommended way to use packages in Tcl is to invoke package require and package provide commands in scripts, and use the procedure pkg_mkIndex to create package index files. Once you have done this, packages will be loaded automatically in response to package require commands. See the documentation for pkg_mkIndex for details.


To state that a Tcl script requires the Tk and http packages, put this at the top of the script:

package require Tk
package require http

To test to see if the Snack package is available and load if it is (often useful for optional enhancements to programs where the loss of the functionality is not critical) do this:

if {[catch {package require Snack}]} {
    # Error thrown - package not found.
    # Set up a dummy interface to work around the absence
} else {
    # We have the package, configure the app to use it

See Also

msgcat(n), packagens(n), pkgMkIndex(n)


package, version

Referenced By

geometry(n), load(n), msgcat(n), pkg_create(n), pkg_mkIndex(n), safe(n), safe_loadTk(n), Tcl_PkgRequire(3), Tcl_StaticPackage(3), tm(n).

7.5 Tcl Built-In Commands