Linux / Unix Command To Optimize and Compress PNG Files In Bulk

by on November 28, 2012 · 6 comments· LAST UPDATED November 28, 2012

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I have lots of images in png format (over 250k+ png). Properly formatting and compressing png images can save many bytes of data. How do I compress and optimize png images on my amazon cloud account so that I can save bandwidth on cloudfront cdn account? How do I use an image compressor to create lossless compression on PNG files, with no effect on image quality in bulk using Red Hat Enterprise Linux bash shell?

Tutorial details
DifficultyIntermediate (rss)
Root privilegesYes
Estimated completion timeN/A

The following image format is recommended and used by big web sites such as Google / Yahoo / Amazon / Facebook etc:

  1. PNG file format is recommended for web.
  2. GIF file format is recommended for small size images.
  3. JPG file format is recommended for high resolution photographic-style images.
  4. Do not use BMPs or TIFFs.

Say hello to optipng

You need to use a tool called optipng. It is a PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections. This tool can be installed on any server powered by Unix or Linux operating systems.


First, turn on EPEL repo and type the following yum command to install optipng:
# yum install optipng
Sample outputs:

Loaded plugins: auto-update-debuginfo, protectbase, rhnplugin
0 packages excluded due to repository protections
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package optipng.x86_64 0:0.6.4-1.el6 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
 Package           Arch             Version               Repository      Size
 optipng           x86_64           0.6.4-1.el6           epel            82 k
Transaction Summary
Install       1 Package(s)
Total download size: 82 k
Installed size: 181 k
Is this ok [y/N]: y
Downloading Packages:
optipng-0.6.4-1.el6.x86_64.rpm                          |  82 kB     00:00
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : optipng-0.6.4-1.el6.x86_64                                  1/1
  Verifying  : optipng-0.6.4-1.el6.x86_64                                  1/1
  optipng.x86_64 0:0.6.4-1.el6

How do I use optipng command?

The syntax is:

optipng file
optipng [options] file
optipng [options] input.png

My sample test.png image:

test.png - Original file

test.png - Original file

To display png image size, type, and compression info, enter:
$ pnginfo -t test.png
Sample outputs:

test.png (tiffinfo compatible labels)...
  Image Width: 1164 Image Length: 911
  Bits/Sample: 8
  Samples/Pixel: 3
  Pixel Depth: 24
  Colour Type (Photometric Interpretation): RGB
  Image filter: Single row per byte filter
  Interlacing: No interlacing
  Compression Scheme: Deflate method 8, 32k window
  Resolution: 0, 0 (unit unknown)
  FillOrder: msb-to-lsb
  Byte Order: Network (Big Endian)
  Number of text strings: 1 of 9
    Software (xTXt deflate compressed): Shutter

Use ls command to see file size:
$ ls -lh test.png
Sample outputs:

-rw-r--r-- 1 vivek vivek 279K Nov 29 00:10 test.png

Use optipng optimization program as follows:
$ cp test.{png,bak}
$ optipng test.png

Sample outputs:

OptiPNG 0.6.4: Advanced PNG optimizer.
Copyright (C) 2001-2010 Cosmin Truta.
** Processing: test.png
1164x911 pixels, 3x8 bits/pixel, RGB
Input IDAT size = 284778 bytes
Input file size = 285286 bytes
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 215274
Selecting parameters:
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 215274
Output IDAT size = 215274 bytes (69504 bytes decrease)
Output file size = 215374 bytes (69912 bytes = 24.51% decrease)

File size is reduced by 24.51%:
$ ls -lh test.png
Sample outputs:
-rw-r--r-- 1 vivek vivek 211K Nov 29 02:40 test.png
You can verify the quality of both images (click to enlarge):

Optimized test.png file ensures that it loads faster for users on slow connections. It will also save 24.51% bytes per image view in terms of both network bandwidth and storage. According to Google:

You should see a benefit for any image file that can be reduced by 25 bytes or more (less than this will not result in any appreciable performance gain).

How do I optimize files in bulk?

Use a bash for loop as follows:

cd /path/to/png/storage/2010/01/c/
mkdir optimized
## store optimized images in optimized directory ##
## Keep file system permission and make a backup of original PNG (see options below)  ##
for i in *.png; do optipng -o5 -quiet -keep -preserve -dir optimized -log optipng.log "$i"; done

You need to make changes to your code. For example, url path to optimize url will be as follows:
Alternatively, you can overwrite existing PNG images:

cd /path/to/png/storage/2010/01/c/
## Overwrite images ##
## Keep file system permission and make a backup of original PNG (see options below)  ##
for i in *.png; do optipng -o5 -quiet -keep -preserve -log optipng.log "$i"; done

Next, purge all images from your CDN account. See how to purge old images / invalidating object from cloud front for more information.


General options:
    -fix		enable error recovery
    -force		enforce writing of a new output file
    -keep		keep a backup of the modified files
    -preserve		preserve file attributes if possible
    -quiet		quiet mode
    -simulate		simulation mode
    -snip		cut one image out of multi-image or animation files
    -out <file>		write output file to <file>
    -dir <directory>	write output file(s) to <directory>
    -log <file>		log messages to <file>
    --			stop option switch parsing
Optimization options:
    -f  <filters>	PNG delta filters (0-5)			default 0,5
    -i  <type>		PNG interlace type (0-1)		default <input>
    -zc <levels>	zlib compression levels (1-9)		default 9
    -zm <levels>	zlib memory levels (1-9)		default 8
    -zs <strategies>	zlib compression strategies (0-3)	default 0-3
    -zw <window size>	zlib window size (32k,16k,8k,4k,2k,1k,512,256)
    -full		produce a full report on IDAT (might reduce speed)
    -nb			no bit depth reduction
    -nc			no color type reduction
    -np			no palette reduction
    -nx			no reductions
    -nz			no IDAT recoding
Optimization details:
    The optimization level presets
        -o0  <=>  -o1 -nx -nz
        -o1  <=>  [use the libpng heuristics]	(1 trial)
        -o2  <=>  -zc9 -zm8 -zs0-3 -f0,5	(8 trials)
        -o3  <=>  -zc9 -zm8-9 -zs0-3 -f0,5	(16 trials)
        -o4  <=>  -zc9 -zm8 -zs0-3 -f0-5	(24 trials)
        -o5  <=>  -zc9 -zm8-9 -zs0-3 -f0-5	(48 trials)
        -o6  <=>  -zc1-9 -zm8 -zs0-3 -f0-5	(120 trials)
        -o7  <=>  -zc1-9 -zm8-9 -zs0-3 -f0-5	(240 trials)
    The libpng heuristics
        -o1  <=>  -zc9 -zm8 -zs0 -f0		(if PLTE is present)
        -o1  <=>  -zc9 -zm8 -zs1 -f5		(if PLTE is not present)
    The most exhaustive search (not generally recommended)
      [no preset] -zc1-9 -zm1-9 -zs0-3 -f0-5	(1080 trials)
    optipng file.png				(default speed)
    optipng -o5 file.png			(moderately slow)
    optipng -o7 file.png			(very slow)
    optipng -i1 -o7 -v -full -sim experiment.png

Other png optimization tools for Linux / Unix

Debian / Ubuntu Linux and other operating *nix based systems can use the following tools for the same purpose:

  • pngcrush - optimizes PNG (Portable Network Graphics) files.
  • pngnq - tool for optimizing PNG (Portable Network Graphics) images. It is a tool for quantizing PNG images in RGBA format.
  • pngquant - PNG (Portable Network Graphics) image optimising utility. It is a command-line utility for converting 24/32-bit PNG images to paletted (8-bit) PNGs.
Recommended readings:
  1. A guide to PNG optimization.
  2. Visit optipng home page to grab the latest version.
  3. Analyze and optimize your website with PageSpeed tools.
  4. 6 Tools To Find Out Website Load Speed.
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{ 6 comments… read them below or add one }

1 Justin Dorfman November 29, 2012 at 1:49 am

This is great, thanks for sharing Vivek.


2 Jalal Hajigholamali November 29, 2012 at 3:46 am


Thanks for very nice article…


3 Philippe Petrinko December 3, 2012 at 9:58 am

Fine topic too, nicely done, THX Vivek. & KUTGW.


4 Linuxaria December 3, 2012 at 10:45 pm

Thanks for the useful command.
Instead of the for I’ve used a find, so it can scan all the subdir:

find /path/to/myuploads/ -name *.png -exec optipng -o5 -quiet -keep -preserve -log /tmp/optipng.log ‘{}’ \;


5 Chris Quinn October 23, 2013 at 3:58 pm

I can’t tell you how many code snippets / tutorials I already use on your site. Whenever I’m searching for something online and I see your site appear in the search results, I already know before viewing the content that whatever I need done will surely work when you’re the one writing it!

Seriously, this is going to save me so much time now….I used to run PageSpeed manually in chrome and go to the “Optimize Images” area for the results…and then manually save the file that appeared after clicking “See Optimized Content” – it was very time consuming…

One question though…I run virtualmin and have several domains being hosting…they allow follow the schema of “/home/$username/public_html” for the home directory. I almost always work exclusively with Wordpress and/or Opencart.

With that – Wordpress images are located in “wp-content/uploads/year/month” and “wp-content/themes/themename/images”

Opencart images are in “image/data”

Based on this information, would it be possible to modify the script above (the one that replaces the old images with optimized images) and have it scan these directories (even if it scanned the whole “wp-content/themes/” directory, but NOT the “wp-content” directory)….and then optimize the images once they were found?

Is replacing the images dangerous? Is there a chance an image can be optimized and then the file gets broken?

I know I could run mod_pagespeed, and I have done that in the past. The fact is, I’m kind of anti-GXXGLE right now with the whole PRI$M / spy stuff…They are getting way to integrated into everything and it’s very unsettling….

I appreciate it!


6 djay August 31, 2014 at 12:40 pm

I’m getting this error
root@cp [~]# yum install optipng
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base:
* extras:
* updates:
Setting up Install Process
No package optipng available.
Error: Nothing to do

Please help


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