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deepblue_86
Asked: 2024-10-08 16:53:17 +0800 CST

using cat command with different outputs in a loop

  • 772

I have a directory containing different text files such as:

ajac001a00.24o
ajac001a15.24o
ajac001a30.24o
.
.
areg001a00.24o
areg001a15.24o
areg001a30.24o
.
.

I need to combine these text files separately, starting with the same four characters, such as

cat *ajac* > ajac_combined
cat *areg* > areg_combined

How can I do this using a loop? There are too many files starting with different characters that exist, therefore this cannot be done using cat command manually.

command-line

3 Answers

  1. Best Answer

    You could collect all files in an array, then cut the first 4 characters to get the list of prefixes, and then iterate over the prefixes to combine the files. Like this:

    files=(*24o)
printf '%s\n' "${files[@]}" | cut -c1-4 | sort -u | 
 while read -r prefix; do 
   cat "$prefix"* >> "$prefix"_combined
done

    The printf '%s\n' "${files[@]}" | cut -c1-4 | sort -u is doing the heavy lifting. First, the printf command prints each element of the $files array on a separate line. This gives us the list of file names, and we then select the first 4 characters with cut -c1-4. Note that this assume simple ASCII file names, no unicode, so that each character is a single byte. We then pass the list of prefixes through sort -u to remove duplicates, and then feed them to the loop.

    I used cat "$prefix"* instead of cat *"$prefix"* as you had in the question since these are all prefixes and there is nothing to match before them.

    • 2

  2. In Bash you could do:

    for f in *; do cat "${f}" >>"${f:0:4}_combined"; done

    Which means: for every file in the current working directory, sorted by name according to Bash's globbing rules (i.e. numbers first, sorted numerically, followed by characters, sorted alphabetically), append the contents of the file to a file named as the first 4 characters of the file's name, followed by _combined.

    For reference, here's how those filenames would sort:

    % printf '%s\n' *
ajac001a00.24o
ajac001a15.24o
ajac001a30.24o
areg001a00.24o
areg001a15.24o
areg001a30.24o

    Which means that files named ajac*, sorted alphanumerically, will be joined into ajac_combined, that files named areg*, sorted alphanumerically, will be joined into areg_combined and so on an so forth.

    If you need to restrict this to filenames ending in .24o:

    for f in *.24o; do cat "${f}" >>"${f:0:4}_combined"; done

    % for f in *; do printf '%s\n' "${f}"; cat "${f}"; done
ajac001a00.24o
DB755B8D04AF76C9AB7F62FEFD5A5DF6
ajac001a15.24o
4FC1CE5229CCF7659F4C2BAA9157F237
ajac001a30.24o
9765C5B689BE4D8F692E52FC83CFD308
areg001a00.24o
BF56C237311136BDBB9EF6EA56D1BF56
areg001a15.24o
36D9928DD5537172523C26F9C8765627
areg001a30.24o
E482937912B11F56362E358E8F92295A
% for f in *; do cat "${f}" >>"${f:0:4}_combined"; done
% for f in *_combined; do printf '%s\n' "${f}"; cat "${f}"; done
ajac_combined
DB755B8D04AF76C9AB7F62FEFD5A5DF6
4FC1CE5229CCF7659F4C2BAA9157F237
9765C5B689BE4D8F692E52FC83CFD308
areg_combined
BF56C237311136BDBB9EF6EA56D1BF56
36D9928DD5537172523C26F9C8765627
E482937912B11F56362E358E8F92295A
    • 2

  3. You can do it in a single line command:

    ls -w 1 | awk '{ system("cat "$1" >> "substr($1,1,4)"_combined")}'

    "ls -w 1" will list all files in a single column and only the file name no other details.

    The awk command will take each line, and run the system command cat the file ($1 = filename) to the file starting with the same 4 letters and ending with _combined.

    the >> in the middle means append, so it will add to the file, if there is no file it will create it.

    Note the single quotes for the whole awk command, and the double quotes for the static text in the cmd.

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