Executes a system command. When the os attribute is specified, then the command is only executed when Apache Ant is run on one of the specified operating systems.

Note that you cannot interact with the forked program, the only way to send input to it is via the input and inputstring attributes. Also note that since Ant 1.6, any attempt to read input in the forked program will receive an EOF (-1). This is a change from Ant 1.5, where such an attempt would block.

If you want to execute an executable using a path relative to the project's basedir, you may need to use vmlauncher=false on some operating systems—but even this may fail (Solaris 8/9 has been reported as problematic). The resolveexecutable attribute should be more reliable, as would be something like

<property name="executable-full-path"
<exec executable="${executable-full-path}" ...

Windows Users

The <exec> task delegates to Runtime.exec which in turn apparently calls ::CreateProcess. It is the latter Win32 function that defines the exact semantics of the call. In particular, if you do not put a file extension on the executable, only .EXE files are looked for, not .COM, .CMD or other file types listed in the environment variable PATHEXT. That is only used by the shell.

Note that .bat files cannot in general by executed directly. One normally needs to execute the command shell executable cmd using the /c switch.

<target name="help">
  <exec executable="cmd">
    <arg value="/c"/>
    <arg value="ant.bat"/>
    <arg value="-p"/>

A common problem is not having the executable on the PATH. In case you get an error message Cannot run program "...":CreateProcess error=2. The system cannot find the path specified. have a look at your PATH variable. Just type the command directly on the command line and if Windows finds it, Ant should do it too. (Otherwise ask on the user mailinglist for help.) If Windows can not execute the program, add the directory of the program to the PATH (set PATH=%PATH%;dirOfProgram) or specify the absolute path in the executable attribute in your buildfile.

Cygwin Users

The <exec> task will not understand paths such as /bin/sh for the executable parameter. This is because JVM in which Ant is running is a standard Windows executable and is not aware of the Cygwin environment (i.e., doesn't load cygwin1.dll). The only work-around for this is to compile a JVM under Cygwin (at your own risk). See for instance OpenJDK build instructions for cygwin.

OpenVMS Users

The command specified using executable and <arg> elements is executed exactly as specified inside a temporary DCL script. This has some implications:

Please note that JVM provided by HP doesn't follow OpenVMS' conventions of exit codes. If you run a JVM with this task, the task may falsely claim that an error occurred (or silently ignore an error). Don't use this task to run JAVA.EXE, use a <java> task with the fork attribute set to true instead as this task will follow the JVM's interpretation of exit codes.

RedHat S/390 Users

It has been reported on linux-390 that shell scripts invoked via the Ant Exec task must have their interpreter specified, i.e., the scripts must start with something like:


or the task will fail as follows:

[exec] Warning: UNIXProcess.forkAndExec native error: Exec format error
[exec] Result: 255

Running Ant as a background process on Unix(-like) systems

If you run Ant as a background process (like ant &) and use the <exec> task with spawn set to false, you must provide explicit input to the forked process or Ant will be suspended because it tries to read from the standard input.


Attribute Description Required
command the command to execute with all command line arguments. Deprecated, use executable and nested <arg> elements instead. Exactly one of the two
executable the command to execute without any command line arguments.
dir the directory in which the command should be executed. No; if vmlauncher is true, defaults to the current working directory, otherwise the project's basedir
os list of Operating Systems on which the command may be executed. If the current OS's name is contained in this list, the command will be executed. The OS's name is determined by JVM and is set in the os.name system property. No
osfamily OS family as used in the <os> condition. since Ant 1.7 No
spawn whether or not you want the command to be spawned
If you spawn a command, its output will not be logged by Ant.
The input, output, error, and result property settings are not active when spawning a process.
since Ant 1.6
No; default is false
output Name of a file to which to write the output. If the error stream is not also redirected to a file or property, it will appear in this output. No
error The file to which the standard error of the command should be redirected. since Ant 1.6 No
logError This attribute is used when you wish to see error output in Ant's log and you are redirecting output to a file/property. The error output will not be included in the output file/property. If you redirect error with the error or errorProperty attributes, this will have no effect. since Ant 1.6 No
append Whether output and error files should be appended to or overwritten. No; defaults to false
outputproperty The name of a property in which the output of the command should be stored. Unless the error stream is redirected to a separate file or stream, this property will include the error output. No
errorproperty The name of a property in which the standard error of the command should be stored. since Ant 1.6 No
input A file from which the executed command's standard input is taken. This attribute is mutually exclusive with the inputstring attribute. since Ant 1.6 No
inputstring A string which serves as the input stream for the executed command. This attribute is mutually exclusive with the input attribute. since Ant 1.6 No
resultproperty the name of a property in which the return code of the command should be stored. Only of interest if failonerror=false. No
timeout Stop the command if it doesn't finish within the specified time (given in milliseconds). No
failonerror Stop the build process if the command exits with a return code signaling failure. No; defaults to false
failifexecutionfails Stop the build if we can't start the program. No; defaults to true
newenvironment Do not propagate old environment when new environment variables are specified. No; default is false
vmlauncher Run command using the JVM's execution facilities where available. If set to false the underlying OS's shell, either directly or through the antRun scripts, will be used. Under some operating systems, this gives access to facilities not normally available through JVM including, under Windows, being able to execute scripts, rather than their associated interpreter. If you want to specify the name of the executable as a relative path to the directory given by the dir attribute, it may become necessary to set vmlauncher to false as well. No; default is true
resolveexecutable When this attribute is true, the name of the executable is resolved firstly against the project basedir and if that does not exist, against the execution directory if specified. On Unix systems, if you only want to allow execution of commands in the user's path, set this to false. since Ant 1.6 No; default is false
searchpath When this attribute is true, then system path environment variables will be searched when resolving the location of the executable. since Ant 1.6.3 No; default is false
discardOutput Whether output should completely be discarded. This setting is incompatible with any setting that redirects output to files or properties.
If you set this to true error output will be discared as well unless you redirect error output to files, properties or enable logError. Since Ant 1.10.10
No; defaults to false
discardError Whether error output should completely be discarded. This setting is incompatible with any setting that redirects error output to files or properties as well as logError. Since Ant 1.10.10 No; defaults to false


<exec dir="${src}" executable="cmd.exe" os="Windows 2000" output="dir.txt">
  <arg line="/c dir"/>

Parameters specified as nested elements


Command line arguments should be specified as nested <arg> elements. See Command line arguments.


It is possible to specify environment variables to pass to the system command via nested <env> elements.

Attribute Description Required
key The name of the environment variable.
Note: since Ant 1.7, for Windows, the name is case-insensitive.
value The literal value for the environment variable. Exactly one of these
path The value for a PATH-like environment variable. You can use ; or : as path separators and Ant will convert it to the platform's local conventions.
file The value for the environment variable. Will be replaced by the absolute filename of the file by Ant.


Since Ant 1.6.2

A nested I/O Redirector can be specified. In general, the attributes of the redirector behave as the corresponding attributes available at the task level. The most notable peculiarity stems from the retention of the <exec> attributes for backwards compatibility. Any file mapping is done using a null sourcefile; therefore not allMapper types will return results. When no results are returned, redirection specifications will fall back to the task level attributes. In practice this means that defaults can be specified for input, output, and error output files.

Errors and return codes

By default the return code of a <exec> is ignored; when you set failonerror to true then any return code signaling failure (OS specific) causes the build to fail. Alternatively, you can set resultproperty to the name of a property and have it assigned to the result code (barring immutability, of course).

If the attempt to start the program fails with an OS dependent error code, then <exec> halts the build unless failifexecutionfails is set to false. You can use that to run a program if it exists, but otherwise do nothing.

What do those error codes mean? Well, they are OS dependent. On Windows boxes you have to look at the documentation; error=2 means 'no such program', which usually means it is not on the path. Any time you see such an error from any Ant task, it is usually not an Ant bug, but some configuration problem on your machine.


Start emacs on display 1 of the X Window System.

<exec executable="emacs">
  <env key="DISPLAY" value=":1.0"/>

Add ${basedir}/bin to the PATH of the system command.

<property environment="env"/>
<exec ... >
  <env key="PATH" path="${env.PATH}:${basedir}/bin"/>

Start the ${browser} with the specified ${file} and end the Ant process. The browser will remain.

<property name="browser" location="C:/Program Files/Internet Explorer/iexplore.exe"/>
<property name="file" location="ant/docs/manual/index.html"/>

<exec executable="${browser}" spawn="true">
    <arg value="${file}"/>

Send the string blah before blah to the cat executable, using an <inputfilterchain> to replace before with after on the way in. Output is sent to the file redirector.out and stored in a property of the same name. Similarly, error output is sent to a file and a property, both named redirector.err.

<exec executable="cat">
    <redirector outputproperty="redirector.out"
                inputstring="blah before blah">
            <replacestring from="before" to="after"/>
        <outputmapper type="merge" to="redirector.out"/>
        <errormapper type="merge" to="redirector.err"/>

Note: do not try to specify arguments using a simple arg-element and separate them by spaces. This results in only a single argument containing the entire string.

Timeouts: If a timeout is specified, when it is reached the sub process is killed and a message printed to the log. The return value of the execution will be -1, which will halt the build if failonerror=true, but be ignored otherwise.