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Software and software installers in Ubuntu need to be marked as [[SecurityTeam/Policies#ExecutePermissionBitRequired|"executable"]] in order to be treated as runnable programs. This indicates to Ubuntu that the file is actually software and is trusted.  Normally and preferably software should be installed using the [[https://wiki.ubuntu.com/SoftwareCenter|Ubuntu Software Center]] which is the primary source of software in Ubuntu.

You should only mark a file as executable via the file [[https://help.ubuntu.com/community/FilePermissions|permissions]] if you know what you are doing.

Files from outside the supported [[https://help.ubuntu.com/community/Repositories/Ubuntu|software repository]] are not marked as "executable" since they did not get installed via a trusted source.  Because of this, attempting to open/run downloaded files that are software will fail.  These files are blocked for security reasons to help unsuspecting users avoid [[http://en.wikipedia.org/wiki/Malware|malware]] (i.e. malicious software like trojan horses, worms, and viruses).

Ubuntu Policy requires that [[https://wiki.ubuntu.com/SecurityTeam/Policies#Execute-Permission%20Bit%20Required|software not marked as executable not be runnable]].  One of the most common ways you will see this is by having a package ship a {{{.desktop}}} file for a Mime``Type which executes the target file (.EXE, .JAR, etc).  This is not allowed unless the target file is already executable (or installed by a trusted software repository).

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