How to find missing files with sequential names?

I have thousands of files named 1.txt 2.txt and so on. Some of those files are missing. Which would be the easiest way to find out which files are missing?

Answers:

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Method 1

ub=1000 # Replace this with the largest existing file's number.
seq "$ub" | while read -r i; do
    [[ -f "$i.txt" ]] || echo "$i.txt is missing"
done

You can easily find the proper value for ub by doing ls | sort -n or similar. This relies on the files being in the format output by seq, notably here without leading zeroes.

Method 2

$ ls
1.txt  3.txt
$ seq 1 10 | xargs -I {} ls {}.txt >/dev/null
ls: cannot access 2.txt: No such file or directory
ls: cannot access 4.txt: No such file or directory
ls: cannot access 5.txt: No such file or directory
ls: cannot access 6.txt: No such file or directory
ls: cannot access 7.txt: No such file or directory
ls: cannot access 8.txt: No such file or directory
ls: cannot access 9.txt: No such file or directory
ls: cannot access 10.txt: No such file or directory
$

Method 3

This is the function I will be using

missing () {
  #ub gets the largest sequential number
  ub=$(ls | sort -n | tail -n 1 | xargs basename -s .txt)
  seq "$ub" | while read -r i; do
    [[ -f "$i.jpg" ]] || echo "$i.txt is missing"
  done
}

Method 4

Another (bash):

comm -23 <(printf '%d.txtn' {1..1000} | sort) <(ls *.txt |sort)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0