What does grep stand for?

[John Abreau]

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"Scott Prive" <scottprive at earthlink.net> writes:

> > g/[re]/p
> 
> Obviously re is regular expression... but what's g and p (or g/ /p)?

My first exposure to Unix was Version 7 on a PDP-11/34a, back in 1983; 
the grep command already existed by then, and as I recall, its man page 
explained that in prior versions of Unix, users would use "ed" to do 
pattern searches using the "g/[re]/p" syntax. Presumably the first 
implementation of grep was a simple shell script that invoked ed, 
something similar to the following:

    #! /bin/sh
    RE="$1"
    shift
    for filename do
    ed - $filename << "EOF"
    g/"$RE"/p
    EOF
    done

line or range of lines specified as a prefix to the command. If you 
specified a line, the default command was to set the current line 
to that line silently. The "p" command would print the current line. 

The "/[re]/" command would search for the next occurrence of a line 
matching the pattern "[re]" and set the current line to that line, 
and if it was followed by a command, it would then run that command. 
So, for instance, "/^$/d" would delete the next blank line. 

The "/[re]/" command also had "g" and "v" prefixes: "g/[re]/" would 
match every line that matched the pattern, and "v/[re]/" would match 
every line that didn't match the pattern. 

"g/[re]/p" would therefore find all lines that matched the pattern 
and print them, and "v/[re]/p" would find all lines that didn't 
match the pattern and print them. That's also why the grep command
uses "-v" to negate the search pattern; that also comes from ed. 


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John Abreau / Executive Director, Boston Linux & Unix
Email jabr at blu.org / WWW http://www.abreau.net / PGP-Key-ID 0xD5C7B5D9
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- --
John Abreau / Executive Director, Boston Linux & Unix
Email jabr at blu.org / WWW http://www.abreau.net / PGP-Key-ID 0xD5C7B5D9
PGP-Key-Fingerprint 72 FB 39 4F 3C 3B D6 5B E0 C8 5A 6E F1 2C BE 99


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