PostgreSQL or pl/psSQL equivalent to MS SQL Server's xp_cmdshell?

Does anyone know of the PostgreSQL or pl/psSQL equivalent to MS SQL Server's
xp_cmdshell? This is the command that allows you issue command-line
statements from within SQL, e.g., you would do xp_cmdshell 'dir c:\' if you
wanted to see the contents of the c: drive.

TM
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