How to batch change the Title metadata in a bunch of PDFs with a shell script

Recently I downloaded a large number of PDFs of an old magazine that I wanted to read. I’m particular about my file names, so I used A Better Finder Rename to fix them so they all looked like this:

• 1934 0104 Old Magazine.pdf
• 1934 0111 Old Magazine.pdf
• 1934 0118 Old Magazine.pdf
• 1934 0125 Old Magazine.pdf

Things looked great, so I imported the PDFs into Calibre, free & open source software that is basically an iTunes for ebooks. Unfortunately, the Titles listed in Calibre looked like this:

• Old Magazine, Jan 04, 1934
• Old Magazine, January 11, 1934
• Old Magazine January 18, 1934
• 1934 0125 Old Magazine

Out of the 48 or so PDFs I imported, one about five imported with the correct Titles. I assumed that the file names I gave the PDFs would be the same as their Titles in Calibre. But clearly they weren’t. What was going on?

I started by opening a PDF in Preview & selecting Tools > Show Inspector > General Info. I immediately noticed the Title field, which said Old Magazine November 10, 1934. Aha! I needed to change the Title metadata in the PDF¹. But not in just one PDF—in about 200 PDFs, and programmatically, from the command line. Why? Because I sure as heck wasn’t gonna do it one at a time².

After looking at several different methods, most of which made you change the Title one file at a time, which was useless. Finally, I discovered PDFtk, short for “The PDF Toolkit”. This is a cross-platform command line app that describes itself as able to do all of the following:

• Merge PDF Documents or Collate PDF Page Scans
• Split PDF Pages into a New Document
• Rotate PDF Documents or Pages
• Decrypt Input as Necessary (Password Required)
• Encrypt Output as Desired
• Fill PDF Forms with X/FDF Data and/or Flatten Forms
• Generate FDF Data Stencils from PDF Forms
• Apply a Background Watermark or a Foreground Stamp
• Report PDF Metrics, Bookmarks and Metadata
• Add/Update PDF Bookmarks or Metadata
• Attach Files to PDF Pages or the PDF Document
• Unpack PDF Attachments
• Burst a PDF Document into Single Pages
• Uncompress and Re-Compress Page Streams
• Repair Corrupted PDF (Where Possible)

Perfect!

Start by downloading PDFtk for your Mac from it’s website. It’s a PKG file, so just click on it, answer Continue, Continue, Install, & you now have PDFtk installed at /opt/pdflabs/pdftk.

Now let’s get that metadata out of an existing PDF so we can see it.

$ cd TNY
$ pdftk 1934\ 0104\ Old\ Magazine.pdf dump_data output report.txt
$ cat report.txt
InfoBegin
InfoKey: Creator
InfoValue: PScript5.dll Version 5.2.2
InfoBegin
InfoKey: Title
InfoValue: Old Magazine, Jan 04, 1934
InfoBegin
InfoKey: Producer
InfoValue: Acrobat Distiller 8.1.0 (Windows)
InfoBegin
InfoKey: Author
InfoValue: kao
InfoBegin
InfoKey: ModDate
InfoValue: D:20091228164142-05'00'
InfoBegin
InfoKey: CreationDate
InfoValue: D:20091228164142-05'00'
PdfID0: 4ffa99d7de15c0856ed0d9d23c8bab80
PdfID1: 717e840b8d76c044a86a7ce38d9c0658
NumberOfPages: 84
PageLabelBegin
PageLabelNewIndex: 1
PageLabelStart: 1
PageLabelNumStyle: DecimalArabicNumerals

Notice these two lines:

InfoKey: Title
InfoValue: Old Magazine, Jan 04, 1934

That’s what I need to change. Here’s the rub: PDFtk expects you to do this for each PDF whose metadata you want to change:

$ pdftk old-file-name.pdf update_info instructions.txt output new-file-name.pdf

Let me break down the components of this command:

• pdftk
  The command. Duh.
• old-file-name.pdf
  The PDF with the incorrect metadata.
• update_info
  The task I want PDFtk to perform: updating, or changing, metadata for the specified PDF.
• instructions.txt
  The file that contains the changes, with one line specifying the metadata key to change, & one line for the metadata value for that key.
• output
  I’m letting PDFtk know that I want to write these changes to a new file.
• new-file-name.pdf
  Our brand spanking new PDf to create, with corrected metadata.

The problem was that you have to specify each title in instructions.txt every time. That’s a pain, but I wrote a little shell script that makes the whole process very easy. I hope all of the comments make it clear what everything is doing³.

#!/bin/bash

# Set IFS since input has spaces, but first save old IFS
# See http://www.cyberciti.biz/tips/handling-filenames-with-spaces-in-bash.html
SAVEIFS=$IFS
IFS=$(echo -en "\n\b")

for i in $(ls *.pdf)
do
  # Rename the file so I can output to the original file name
  mv "$i" _"$i"
  # Save the file name
  filename=$(echo $i)
  # Write an instructions file for pdftk
  echo "InfoKey: Title" > /tmp/pdftemp.txt
  echo "InfoValue: $filename" >> /tmp/pdftemp.txt
  # Change the PDF’s metadata, outputting to the original file name
  pdftk _"$i" update_info /tmp/pdftemp.txt output $i
  # Get rid of the renamed file
  rm _"$i"
done

# Restore IFS
IFS=$SAVEIFS

I saved the script in ~/bin as pdfname2title.sh, & then made it executable:

$ chmod 755 pdfname2title.sh

Using it is pretty simple. Just cd to the directory with the PDFs, & then run the script:

$ pdfname2title.sh

A few moments later & the script is finished. When I imported the PDFs this time, the Titles matched the file names. I’m glad I discovered PDFtk, & I’m also glad I wrote up that shell script. I have the feeling it will come in handy more than I might think.