regex - Command line to match lines with matching first field (sed, awk, etc.)

Question

What is fast and succinct way to match lines from a text file with a matching first field.

Sample input:

a|lorem
b|ipsum
b|dolor
c|sit
d|amet
d|consectetur
e|adipisicing
e|elit

Desired output:

b|ipsum
b|dolor
d|amet
d|consectetur
e|adipisicing
e|elit

Desired output, alternative:

b|ipsum|dolor
d|amet|consectetur
e|adipisicing|elit

I can imagine many ways to write this, but I suspect there's a smart way to do it, e.g., with sed, awk, etc. My source file is approx 0.5 GB.

There are some related questions here, e.g., "awk | merge line on the basis of field matching", but that other question loads too much content into memory. I need a streaming method.

Answer 1

For fixed width fields you can used uniq:

$ uniq -Dw 1 file
b|ipsum
b|dolor
d|amet
d|consectetur
e|adipisicing
e|elit

---

If you don't have fixed width fields here are two awk solution:

awk -F'|' '{a[$1]++;b[$1]=(b[$1])?b[$1]RS$0:$0}END{for(k in a)if(a[k]>1)print b[k]}' file
b|ipsum
b|dolor
d|amet
d|consectetur
e|adipisicing
e|elit

awk -F'|' '{a[$1]++;b[$1]=b[$1]FS$2}END{for(k in a)if(a[k]>1)print k b[k]}' file
b|ipsum|dolor
d|amet|consectetur
e|adipisicing|elit