Opening a command pipeline

As mentioned previously, the open command can be used to open a command pipeline. A command pipeline is a mechanism that allows us to read from or write to a command. The syntax is similar to the standard open command. However, if the first character passed as an argument to the open command is a pipe character (|) the remaining characters are treated as a list of arguments describing a command pipeline that is to be invoked. The syntax is as follows:

open | command access_permissions

The arguments provided by command are similar to those used for the exec command. The open command will return a file pointer that may be used to write to the specified command's input pipe or read from its output pipe. The specific functionality (read or write) is determined by the access permissions.

If the open command or one of the commands provided as arguments should return an error, a Tcl error will be generated when the close command is invoked on the channel unless the pipeline...