Concatenating text files in bash

Question

I have many text files with only one line float value in one folder and I would like to concatenate them in bash in order for example: file_1.txt, file_2.txt ...file_N.txt. I would like to have them in one txt file in the order from 1 to N. Could someone please help me ? Here is the code I have but it just concatenates them in random manner. Thank you

for file in *.txt
do 
  cat ${file} >>  output.txt  
done 

Answer 1

As much as I recommend against parsing the output of ls, here we go.

ls has a "version sort" option that will sort numbered files like you want. See below for a demo.

To concatenate, you want:

ls -v file*.txt | xargs cat > output

$ touch file{1..20}.txt
$ ls
file1.txt   file12.txt  file15.txt  file18.txt  file20.txt  file5.txt  file8.txt
file10.txt  file13.txt  file16.txt  file19.txt  file3.txt   file6.txt  file9.txt
file11.txt  file14.txt  file17.txt  file2.txt   file4.txt   file7.txt
$ ls -1
file1.txt
file10.txt
file11.txt
file12.txt
file13.txt
file14.txt
file15.txt
file16.txt
file17.txt
file18.txt
file19.txt
file2.txt
file20.txt
file3.txt
file4.txt
file5.txt
file6.txt
file7.txt
file8.txt
file9.txt
$ ls -1v
file1.txt
file2.txt
file3.txt
file4.txt
file5.txt
file6.txt
file7.txt
file8.txt
file9.txt
file10.txt
file11.txt
file12.txt
file13.txt
file14.txt
file15.txt
file16.txt
file17.txt
file18.txt
file19.txt
file20.txt

Answer 2

for file in *.txt
do 
  cat ${file} >>  output.txt  
done 

This works for me as well as :

for file in *.txt
do 
  cat $file >>  output.txt  
done

You don't need {}

But the simpler is still :

cat file*.txt > output.txt

So if you have more than 9 files as suggested in the comment, you can do one of the following :

files=$(ls file*txt | sort -t"_" -k2g)
files=$(find . -name "file*txt" | sort -t "_" -k2g)
files=$(printf "%s\n" file_*.txt | sort -k1.6n) # Thanks to glenn jackman

and then:

cat $files

or

cat $(find . -name "file*txt" | sort -t "_" -k2g)

Best is still to number your files correctly, so file_01.txt if you have less than 100 files, et file_001.txt if less than 1000, an so on.

---

example :

ls file*txt
file_1.txt  file_2.txt  file_3.txt  file_4.txt  file_5.txt  file_10.txt

They contain only their corresponding number.

$ cat $files
1
2
3
4
5
10