[TUHS] Shell control through external commands
dds at aueb.gr
Fri Jul 15 00:38:38 AEST 2016
On 14/07/2016 17:11, schoedel at kw.igs.net wrote:
> On Thu, 14 Jul 2016 16:18:43 +0300, Diomidis Spinellis wrote
>> I remember hearing that originally the Unix shell had control structures
>> (e.g. if, while, case) implemented through external commands. However,
>> I can't see this reflected in the source code. The 7th Edition
>> Bourne shell has these commands built-in (usr/src/cmd/sh/cmd.c), while
>> the 6th Edition (usr/source/s2/sh.c) seems to lack them completely.
Thank you! So:
- if(1) gets as arguments an expression to evaluate and the command to
execute if the expression is true, and
- goto(1) changes the seek offset of the shared standard input file
descriptor to match the label's position in the file.
This is a much simpler implementation that what I thought would be the
case, but, true to the spirit of Unix, remarkably effective.
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