gs (GhostScript) cheat sheet

Cheat-sheet for versatile interpreter GhostScript. It can be used to tweak, convert, produce high quality Postscript and PDF files.

Convert postscript (ps) to pdf:

gs -q -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=fileout.pdf filein.ps

Merge ps and/or pdf:

gs -q -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=fileout.pdf filein1.ps filein2.pdf filein3.pdf

Extract page(s) from a ps or a pdf document:

gs -q -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -dFirstPage=3 -dLastPage=3 -sOutputFile=fileout.pdf filein.ps

Embed fonts in a pdf:

gs -q -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=fileout.pdf -dPDFSETTINGS=/prepress -dEmbedAllFonts=true -dSubsetFonts=true -dCompatibilityLevel=1.6 filein.pdf

Convert images to pdf:

Check this post. Or else use ImageMagic's convert.

PDF optimization options

-dPDFSETTINGS=/screen (screen-view-only quality, 72 dpi images) -dPDFSETTINGS=/ebook (low quality, 150 dpi images) -dPDFSETTINGS=/printer (high quality, 300 dpi images) -dPDFSETTINGS=/prepress (high quality, color preserving, 300 dpi imgs) -dPDFSETTINGS=/default (almost identical to /screen)

Paper size options

-sPAPERSIZE=letter -sPAPERSIZE=a4 -dDEVICEWIDTHPOINTS=w -dDEVICEHEIGHTPOINTS=h (point=1/72 of an inch) -dFIXEDMEDIA (force paper size over the PostScript defined size) -gWIDTHxHEIGHT (page size in pixels)

Output devices:

-sDEVICE=pdfwrite -sDEVICE=ps2write -sDEVICE=png16m (24-bit RGB color) -sDEVICE=pnggray (grayscale) -sDEVICE=pngmono (black-and-white) -sDEVICE=pngalpha (32-bit RGBA color) -sDEVICE=jpeg (color JPEG) -sDEVICE=jpeggray (grayscale JPEG) -sDEVICE=epswrite (encapsulated postscript) -sDEVICE=txtwrite (text output, UTF-8) Check -sOutputFile below, for separate images for each page of a multi-page document. Tiff, PNM and many more formats are supported. Check details in Ghostscript output devices.

Help & list of available devices:

gs -h

Other options

-dNOPAUSE (no pause after page) -dBATCH (exit after last file) -sOutputFile=ABC-%03d.pgm (produces 'ABC-001.pgm'..'ABC-010.pgm'..) -dEmbedAllFonts=true -dSubsetFonts=true (Embeds only the characters used in document) -dCompatibilityLevel=1.4 (Adobe's PDF specifications, >=1.4 for font embedding, =1.6 for OpenType font embedding) -dCompressPages=true (compress page content) -dFirstPage=pagenumber -dLastPage=pagenumber -dAutoRotatePages=/PageByPage (or /All or /None) -rXRESxYRES (XRES & YRES in pixels/inch) -rRES (same XRES & YRES, affects images and fonts converted to bitmaps) -sPDFPassword=password More details & defaults on ps2pdf documentation.
Adobe® PDF Creation Settings - Used by GhostScript.
GhostScript Documentation.

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Cheat Sheets

Cheat sheets in this blog are not static posts and they keep getting updates from time to time. Here is the list of cheat sheet (from this blog) for different applications:

• Useful \(\LaTeX\) tricks
• BibTex hacks
• Git - distributed version control system
• GhostScript - an interpreter for the PostScript language and PDF
• Infrequent Linux commands
• Emacs - A highly customizable and advanced text editor
• CVS - version control (Not updated anymore)

Some other useful cheat sheet resources: 

• http://www.cheat-sheets.org/
• http://www.quicklycode.com/
• http://devcheatsheet.com/
• http://www.sosmath.com/ - A somewhat detailed & categorized math cheat sheet
• http://tutorial.math.lamar.edu/cheat_table.aspx - Another math cheat sheet

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Where did my disk space go?

Have you ever noticed missing disk space on newly bought hard disks? Here are some possible reasons:

1. Unit confusion: The most common confusion in between GB/GiB or MB/MiB.
2. Reserved space in linux partitions.
3. Filesystem overheads.

Unit Confusion

Many people have questions like: Why my 1 TB hard disk shows only 930 GB? This is purely a confusion of units and bases used when words like 'kilo', 'mega' etc. are used. Following table, from AskUbuntu, explains the situation. Prefix Bytes Prefix Bytes 1 Byte = (2^10)^0 = 1 1 Byte = (10^3)^0 = 1 1 Kibibyte(KiB) = (2^10)^1 = 1024 1 Kilobyte(KB) = (10^3)^1 = 1000 1 Mebibyte(MiB) = (2^10)^2 = 1048576 1 Megabyte(MB) = (10^3)^2 = 1000000 1 Gibibyte(GiB) = (2^10)^3 = 1073741824 1 Gigabyte(GB) = (10^3)^3 = 1000000000 1 Tebibyte(TiB) = (2^10)^4 = 1099511627776 1 Terabyte(TB) = (10^3)^4 = 1000000000000 Many softwares and operating systems often calculate size in GiB and put GB when they are displaying it. This becomes even worse because most of the harddisks are labelled with actual GBs and hence when numbers are reported in GiB, it creates the confusion. Online calculators like this can help to remove the confusion. NIST states very interesting historical reason of this confusing prefix:

Once upon a time, computer professionals noticed that 2¹⁰ was very nearly equal to 1000 and started using the SI prefix "kilo" to mean 1024. That worked well enough for a decade or two because everybody who talked kilobytes knew that the term implied 1024 bytes. But, almost overnight a much more numerous "everybody" bought computers, and the trade computer professionals needed to talk to physicists and engineers and even to ordinary people, most of whom know that a kilometer is 1000 meters and a kilogram is 1000 grams.
Then data storage for gigabytes, and even terabytes, became practical, and the storage devices were not constructed on binary trees, which meant that, for many practical purposes, binary arithmetic was less convenient than decimal arithmetic. The result is that today "everybody" does not "know" what a megabyte is. When discussing computer memory, most manufacturers use megabyte to mean 2²⁰ = 1 048 576 bytes, but the manufacturers of computer storage devices usually use the term to mean 1 000 000 bytes. Some designers of local area networks have used megabit per second to mean 1 048 576 bit/s, but all telecommunications engineers use it to mean 10⁶ bit/s. And if two definitions of the megabyte are not enough, a third megabyte of 1 024 000 bytes is the megabyte used to format the familiar 90 mm (3 1/2 inch), "1.44 MB" diskette. The confusion is real, as is the potential for incompatibility in standards and in implemented systems.

Reserved space

While formatting Linux partitions like ext2/ext3/ext4, 5% of the drive's total space is reserved for the super-user (root) so that the operating system can still write to the disk even if it is full. This space is wasted if the drive is being used solely for data storage especially on large partitions. The details of the reserved space can be seen by running following command (replace sda1 by your drive name)sudo tune2fs -l /dev/sda1 The reserved space can be decreased to 1% of the drive's total space by running following command. To completely remove this reserved space replace 1 by 0. sudo tune2fs -m 1 /dev/sda1 As explained in Ubuntu Documentation, using this command does not change any existing data on the drive. You can use it on a drive which already contains data. This should be left to 5% on drives containing filesystem /, /var, /tmp in order to avoid any problems.

Filesystem overheads

Journaling file systems like ext2/ext3/ext4/NTFS used file tables for managing the filesystem. These tables take up lots of space (2-3% of drive size). Different file system allocates space to these tables differently. For example ext4 allocates all its space during the format itself, which shows up as used disk space. Where as NTFS allocates it when more files are written to disk.

The exact details are more complicated than this handy wavy explanation. Many forum posts has discussed this issue.

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Wireless power management in Ubuntu

Tested on: Ubuntu 11.04 (natty)

I have been experiencing extremely slow network connection whenever my laptop has been on battery. Speedtest results showed a 50 fold speed reduction when my laptop was on battery. I speculated that lower speed has to do with power-management, which reduces network speed in order to save battery. Ubuntu, like other operating systems, also has power management options installed by default. There is a frontend/GUI for Power Management preferences. But I could not find any thing related to wireless or Wi-Fi in the GUI. Looking around on internet, I found many pages. Here is a log of the things I found.

• Confirming the behaviour: In order to make sure that power-management is the culprit of the slow network access run following command both when on power and when on battery.iwconfig The output should look like following: wlan0 IEEE 802.11bgn ESSID:"USC Wireless" Mode:Managed Frequency:2.462 GHz Access Point: 00:00:00:00:00:00 Bit Rate=65 Mb/s Tx-Power=15 dBm Retry long limit:7 RTS thr:off Fragment thr:off Power Management:on Link Quality=68/70 Signal level=-42 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0 Tx excessive retries:27329 Invalid misc:204 Missed beacon:0 Notice line five of the output. If it says anything other than Power Management:off while on battery, then it could be power management which is slowing down then network. Check power argument in iwconfig man page for other possible options.
  
  
• Temporarily disable the power-management: While on battery, run following command (replace wlan0 with your Ethernet name) sudo iwconfig wlan0 power off This will only affect the wireless settings of power management. and should change the output of previous command to Power Management:off and most-likely revert the network speed to usual speed. However, on restarting the system the power management will be enabled.
  
  
• Permanently disable power-management for wireless: If you do not want to keep running the above command each time you restart you laptop, create a file /etc/pm/power.d/wireless with following content: #!/bin/sh /sbin/iwconfig wlan0 power off Make the file executable by running following command:sudo chmod +x /etc/pm/power.d/wireless This new file will override the default wireless script run by power management and will also make sure that wireless power management is turned off. The above trick is taken from Ubuntu Documentation (Check few lines before the example). It says:
  
  When you run pm-powersave it combines the scripts in these two directories and executes them in sorted order. If both directories contain a script with the same name, the one in /etc/pm/power.d/ has a higher precedence and only this one will be executed. Therefore to disable a script from /usr/lib/pm-utils/power.d/ simply create an empty file in /etc/pm/power.d/ with the same name and without the execute bit set.

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vsftpd behind router/firewall

Tested on: Ubuntu 11.04 (natty)

This is how I got vsftpd server work behind a router. If you are getting errors like "Server sent passive reply with unroutable address. Using server address instead." or "500 illegal port command" on list (ls) command, this post might help. Setting passive mode configuration correctly is the trick.

Please note that this may not be the perfect solution or even worse - not very secure. Read vsftpd.conf manual before applying any changes.

Make sure port 20, 21 and few more ports, for example 4242-4252, are being forwarded to the server. We will need these extra ports for passive mode - set pasv_min_port and pasv_max_port accordingly in the configuration file. Add following to /etc/vsftpd.conf . connect_from_port_20=YES pasv_enable=YES pasv_addr_resolve=YES pasv_address=myaddress.dyndns.com pasv_min_port=4242 pasv_max_port=4252 Set pasv_address to your domain name. You can use dyndns for dynamic ip-address. vsftpd will convert your domain name to ip address when vsftpd server starts. Note that if your ip address changes after vsftpd has started it will not update the ip-address. There has been some attempt to the solve it.
Here are some more configurations with comments: # Only allow FTP access to users listed in file /etc/vsftpd.userlist # Other users will not have ftp access userlist_enable=YES userlist_deny=NO userlist_file=/etc/vsftpd.userlist # Disable delete commands cmds_denied=DELE,RMD # Verbose logging log_ftp_protocol=YES # You may restrict local users to their home directories. See FAQ for # the possible risks in this before using chroot_local_user chroot_local_user=YES
For more security, you can also modify the login-shell of a dummy ftp user to something like /bin/false - which does not exist. Use following command sudo usermod -s /bin/false <username> In order to make this work you have to add /bin/false to the file /etc/shells. Or else vsftpd will generate login error when the user logs in using ftp.

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