```
sort a b | uniq # union - items in either a or b
sort a b | uniq -d # intersection - items both in a and b
sort a a b | uniq -u # difference - items in b not in a
sort a b | uniq -u # symmetric difference - items in only one file
cat a b | sort -u # a union b
cat a b | sort | uniq -d # a intersect b
cat a b b | sort | uniq -u # difference a – b
cat a b | sort | uniq -u # symmetric difference
join -t'\0' -a1 -a2 a b # union of sorted files
join -t'\0' a b # intersection of sorted files
join -t'\0' -v2 a b # difference of sorted files
join -t'\0' -v1 -v2 a b # symmetric difference of sorted files
```

```
date --date='25 Dec' +%A # what day does xmas fall on, this year
date --date='@2147483647' # convert seconds since the epoch to date
```

```
while date; do time node x.js; inotifywait -e close_write x.js; printf '\n\n'; done
```

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Since 2009 to 2015