Intel XENIX 286 Programmers Guide (86) - Tenox.tc

csh: C Shell XENIX Programming
The form
command > &! file
is used when noclobber is set and file already exists.
Finally, use the form
command > > file
to append output to the end of an existing file. If noclobber is set, then an error results
if file does not exist, otherwise the C shell creates file. The form
command > > ! file
appends to a file even if it does not exist and noclobber is set.
Creating Background and Foreground Jobs
When one or more commands are typed together as a pipeline or as a sequence of
commands separated by semicolons, a single job is created by the C shell consisting of
these commands together as a unit. Single commands without pipes or semicolons
create the simplest jobs. Usually, every line typed to the C shell creates a job. Each of
the following lines creates a job:
sort < data
Is -s I sort -n I head -5
mail harold
If the ampersand metacharacter (&) is typed at the end of a command pipeline or
sequence, then the job is started as a background job. This means that the C shell does
not wait for the job to finish, but immediately prompts for another command. The job
runs in the background at the same time that normal jobs, called foreground jobs,
continue to be read and executed by the C shell. Thus
du > usage &
runs the du program, which reports on the disk usage of your working directory, directs
the output into the file usage, and returns immediately with a prompt for the next
command without waiting for du to finish. The du program continues executing in the
background until it finishes, even though you can type and execute more commands in
the mean time. Background jobs are unaffected by any signals from the keyboard such
as the INTERRUPT or QUIT signals.
The kill command terminates a background job immediately. Normally, this is done by
specifying the process number of the job you want killed. Process numbers can be found
with the ps command.
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