How to add formatting to, and batch rename filenames? - bash

Question

I have around 7,000 .txt files that have been spat out by a program where the naming convention clearly broke. The only saving grace is that they follow the following structure: id, date, time.
m031060209104704.txt --> id:m031 date:060209 time:104704.txt
Sample of other filenames (again same thing):
115-060202105710.txt --> id:115- date:060202 time: 105710.txt
x138051203125338.txt etc...
9756060201194530.txt etc..
I want to rename all 7,000 files in this directory to look like the following:
m031060209104704.txt --> 090206_104704_m031.txt
i.e date_time_id (each separated by underscores or hyphens, I don't mind). I need the date format to be switched from yymmdd to ddmmyy as shown directly above though!
I'm not clear on whats overkill here, full program script or bash command (MAC OS). Again, I don't mind, any and all help is appreciated.

Answer 1

Try something like:
#!/bin/bash
# directory to store renamed files
newdir="./renamed"
mkdir -p $newdir
for file in *.txt; do
if [[ $file =~ ^(....)([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{6})\.txt$ ]]; then
# extract parameters
id=${BASH_REMATCH[1]}
yy=${BASH_REMATCH[2]}
mm=${BASH_REMATCH[3]}
dd=${BASH_REMATCH[4]}
time=${BASH_REMATCH[5]}
# then rearrange them to new name
newname=${dd}${mm}${yy}_${time}_${id}.txt
# move to new directory
mv "$file" "$newdir/$newname"
fi
done

Answer 2

Bash string indexes makes it very easy and efficient to rework the filenames as you intend. You should also validate you are only operating on input filenames of 20 characters. That can be accomplished as follows:
#!/bin/bash
for i in *.txt; do
## validate a 20 character filename
(( ${#i} == 20 )) || { printf "invalid length '%s'\n" "$i"; continue; }
echo "mv $i ${i:8:2}${i:6:2}${i:4:2}_${i:10:6}_${i:0:4}.txt" ## output rename
mv "$i" "${i:8:2}${i:6:2}${i:4:2}_${i:10:6}_${i:0:4}.txt" ## actual rename
done
Example Directory
$ ls -l
total 0
-rw-r--r-- 1 david david 0 Dec 21 19:16 115-060202105710.txt
-rw-r--r-- 1 david david 0 Dec 21 19:16 9756060201194530.txt
-rw-r--r-- 1 david david 0 Dec 21 19:15 m031060209104704.txt
-rw-r--r-- 1 david david 0 Dec 21 19:16 x138051203125338.txt
Example Use/Output
$ cd thedir
$ bash ../script.sh
mv 115-060202105710.txt 020206_105710_115-.txt
mv 9756060201194530.txt 010206_194530_9756.txt
mv m031060209104704.txt 090206_104704_m031.txt
mv x138051203125338.txt 031205_125338_x138.txt
$ ls -l
total 0
-rw-r--r-- 1 david david 0 Dec 21 19:42 010206_194530_9756.txt
-rw-r--r-- 1 david david 0 Dec 21 19:42 020206_105710_115-.txt
-rw-r--r-- 1 david david 0 Dec 21 19:42 031205_125338_x138.txt
-rw-r--r-- 1 david david 0 Dec 21 19:42 090206_104704_m031.txt
Look things over and let me know if you have any further questions.