bash for loop with numerated names - macos

Question

I'm currently working on a maths project and just run into a bit of a brick wall with programming in bash.
Currently I have a directory containing 800 texts files, and what I want to do is run a loop to cat the first 80 files (_01 through to _80) into a new file and save elsewhere, then the next 80 (_81 to _160) files etc.
all the files in the directory are listed like so: ath_01, ath_02, ath_03 etc.
Can anyone help?
So far I have:
#!/bin/bash
for file in /dir/*
do
echo ${file}
done
Which just simple lists my file. I know I need to use cat file1 file2 > newfile.txt somehow but it's confusing my with the numerated extension of _01, _02 etc.
Would it help if I changed the name of the file to use something other than an underscore? like ath.01 etc?
Cheers,

Answer 1

Since you know ahead of time how many files you have and how they are numbered, it may be easier to "unroll the loop", so to speak, and use copy-and-paste and a little hand-tweaking to write a script that uses brace expansion.
#!/bin/bash
cat ath_{001..080} > file1.txt
cat ath_{081..160} > file2.txt
cat ath_{161..240} > file3.txt
cat ath_{241..320} > file4.txt
cat ath_{321..400} > file5.txt
cat ath_{401..480} > file6.txt
cat ath_{481..560} > file7.txt
cat ath_{561..640} > file8.txt
cat ath_{641..720} > file9.txt
cat ath_{721..800} > file10.txt
Or else, use nested for-loops and the seq command
N=800
B=80
for n in $( seq 1 $B $N ); do
for i in $( seq $n $((n+B - 1)) ); do
cat ath_$i
done > file$((n/B + 1)).txt
done
The outer loop will iterate n through 1, 81, 161, etc. The inner loop will iterate i over 1 through 80, then 81 through 160, etc. The body of the inner loops just dumps the contents if the ith file to standard output, but the aggregated output of the loop is stored in file 1, then 2, etc.

Answer 2

You could try something like this:
cat "$file" >> "concat_$(( ${file#/dir/ath_} / 80 ))"
with ${file#/dir/ath_} you remove the prefix /dir/ath_ from the filename
$(( / 80 )) you get the suffix divided by 80 (integer division)
Also change the loop to
for file in /dir/ath_*
So you only get the files you need

Answer 3

If you want groups of 80 files, you'd do best to ensure that the names are sortable; that's why leading zeroes were often used. Assuming that you only have one underscore in the file names, and no newlines in the names, then:
SOURCE="/path/to/dir"
TARGET="/path/to/other/directory"
(
cd $SOURCE || exit 1
ls |
sort -t _ -k2,2n |
awk -v target="$TARGET" \
'{ file[n++] = $1
if (n >= 80)
{
printf "cat"
for (i = 0; i < 80; i++)
printf(" %s", file[i]
printf(" >%s/%s.%.2d\n", target, "newfile", ++number)
n = 0
}
END {
if (n > 0)
{
printf "cat"
for (i = 0; i < n; i++)
printf(" %s", file[i]
printf(" >%s/%s.%.2d\n", target, "newfile", ++number)
}
}' |
sh -x
)
The two directories are specified (where the files are and where the summaries should go); the command changes directory to the source directory (where the 800 files are). It lists the names (you could specify a glob pattern if you needed to) and sorts them numerically. The output is fed into awk which generates a shell script on the fly. It collects 80 names at a time, then generates a cat command that will copy those files to a single destination file such as "newfile.01"; tweak the printf() command to suit your own naming/numbering conventions. The shell commands are then passed to a shell for execution.
While testing, replace the sh -x with nothing, or sh -vn or something similar. Only add an active shell when you're sure it will do what you want. Remember, the shell script is in the source directory as it is running.
Superficially, the xargs command would be nice to use; the difficulty is coordinating the output file number. There might be a way to do that with the -n 80 option to group 80 files at a time and some fancy way to generate the invocation number, but I'm not aware of it.
Another option is to use xargs -n to execute a shell script that can deduce the correct output file number by listing what's already in the target directory. This would be cleaner in many ways:
SOURCE="/path/to/dir"
TARGET="/path/to/other/directory"
(
cd $SOURCE || exit 1
ls |
sort -t _ -k2,2n |
xargs -n 80 cpfiles "$TARGET"
)
Where cpfiles looks like:
TARGET="$1"
shift
if [ $# -gt 0 ]
then
old=$(ls -r newfile.?? | sed -n -e 's/newfile\.//p; 1q')
new=$(printf "%.2d" $((old + 1)))
cat "$#" > "$TARGET/newfile. $new
fi
The test for zero arguments avoids trouble with xargs executing the command once with zero arguments. On the whole, I prefer this solution to the one using awk.

Answer 4

Here's a macro for #chepner's first solution, using GNU Make as the templating language:
SHELL := /bin/bash
N = 800
B = 80
fileNums = $(shell seq 1 $$((${N}/${B})) )
files = ${fileNums:%=file%.txt}
all: ${files}
file%.txt : start = $(shell echo $$(( ($*-1)*${B}+1 )) )
file%.txt : end = $(shell echo $$(( $* * ${B} )) )
file%.txt:
cat ath_{${start}..${end}} > $#
To use:
$ make -n all
cat ath_{1..80} > file1.txt
cat ath_{81..160} > file2.txt
cat ath_{161..240} > file3.txt
cat ath_{241..320} > file4.txt
cat ath_{321..400} > file5.txt
cat ath_{401..480} > file6.txt
cat ath_{481..560} > file7.txt
cat ath_{561..640} > file8.txt
cat ath_{641..720} > file9.txt
cat ath_{721..800} > file10.txt