In non-error trapping mode, errors are ignored (but the rest of that line is not used) and the next line from the shell file is executed. Echoing mode If echoing mode is enabled (by placing a + early in the file), then each line from the shell file is printed out with the character string ‘---->’ in front of it before it is executed. Use this for testing how a shell file works, or for informing a user (via display on the screen) that things are happening. If echo mode is off, then the only output which occurs is that produced by the commands which the shell file invokes. Multiple or nested shell programs The execution of shell files is fully nestable:
Shell files may be called from within shell files. Parameters may be passed from one shell file to the next with the $1, $2 mechanism. The depth of nesting of shell files is limited by the maximum number of disk files which can be open simultaneously, generally sixteen. When a shell file program is invoked by another shell file, the trapping and echoing status of the calling shell program are not normally transferred to the called one. If you desire trapping and echoing in the called shell program you will have to put ‘+’ and ‘trap’ commands in it.
If a shell program which was invoked by another shell program is in error trapping mode, and an error is detected, the error flag will be returned to the calling shell program. The calling shell program can then abort, if it is in trapping mode. When the shell file interpreter encounters a line such as command args ... stuff> eofmarker it asynchronously executes the first command with its standard input attached to a pipe. The data up to the line eofmarker is fed down the command’s normal input, after which the normal interpretation of the shell file continues. The opening eofmarker must be all UPPER CASE characters in the range A to Z only.
There must be no trailing white space after the eofmarker , and no space between the eofmarker . The closing marker must be identical to the opening marker, and must be on a line of its own. If it has a & character added to it, the shell interpreter will not wait Page 42 User Tutorial Manual Starting Programs