1. Linux Tips¶

A collection dating back to 2001.08.23.

The following tips are ones that I’ve become tired of looking up in the man and info pages, (or searching the Internet for) then whittling down to their barest essences.

NOTE: «text within French quotes» indicates variable text – often a filename.

1.1. Redirecting Standard Error to Standard Out¶

The proper way to redirect stderr is to first decide where stdout is going, and THEN redirect the stderr to stdout. So, for example:

$ «yada-yada» | «pager» 2>&1

takes the output of «yada-yada» and pipes it into «pager», then tells the system to send stderr to wherever stdout is going. (A pager is a program that allows a user to scroll through long documents. I use one called most, but older, commonly used ones include more and less.)

1.2. Making a patch file¶

Assume we have a Red Hat source RPM:

$ rpm -Uvh «package».src.rpm
$ cd /usr/src/redhat/SPECS
$ rpm -bp «package».spec
$ cd /usr/src/redhat/BUILD
$ mv «package» «package».orig
$ cd /usr/src/redhat/SPECS
$ rpm -bp «package».spec
$ cd /usr/src/redhat/BUILD
  (edit to your heart's content)
$ diff -Naur «package».orig «package» > ../SOURCES/«package».patch
$ rm -rf «package».orig
$ cd /usr/src/redhat/SPECS
$ emacs «package».spec
  ...
  Source: ...
  Patch: «package».patch
  ...
  %prep
  %setup
  %patch -p 1
  ^X^S^X^C
$ rpm -ba «package».spec

The idea is to create two directories with identical contents, then modify one of them. Create a diff file of the changes and save it. I THINK I got all the basic steps in there… However, it may be necessary to create the tarball too, in which case you need something like:

$ tar czvf «archive».tar.gz «directory_to_archive»

1.3. Copying directory trees¶

Often, it becomes necessary to copy entire directory trees from one directory to another. The method I saw somewhere was:

$ cd «olddir» ; tar cf - . | (cd «newdir» ; tar xpf -)

This creates a tarball that never actually becomes a file. The tarball is piped directly to a little script subroutine which changes to the new directory, and untars the tarball on the fly.

According to JP Abgrall, there’s an optimized way to do this:

$ tar cf - -C «olddir» . | tar xpf - -C «newdir»

1.4. Getting landscape output¶

It looks like mpage will do the trick:

$ mpage -1lvH «filename» | lpr

The man page suggests that there’s a way to pass pr switches to mpage (using -p instead of -H), but I’ve been unable to pass the line-numbering switch -n together with pr into mpage. (What I want is the headings a la -H together with line numbering.)

A fancier way, requiring a bit more study, is to use enscript. In fact, enscript is neat for LOTS of stuff – customizable layout, “Page X of Y”, line numbers, etc.

1.5. Verifying Red Hat packages¶

You can verify each package with the following command:

$ rpm --checksig

If you only wish to verify that each package has not been corrupted or tampered with, examine only the md5sum with the following command:

$ rpm --checksig --nopgp

1.6. Security: Watching the watchers…¶

The netstat command is a handy tool for seeing who’s poking around at any given moment:

$ netstat -v | most

1.7. Stripping comments:¶

Assuming you have a script that uses the number sign (a.k.a. pound symbol, hash mark) as a comment character, and you wish to examine only those lines containing “active” commands and options, the following will produce such a listing:

$ grep -v "^\#" «scriptfile» | grep -v "^[[:space:]]*$"

What’s happening: The file is first stripped of lines beginning with #. Then, that result is stripped of any lines which have 0 or more whitespace characters (and nothing else) between the start and end of the line.

1.8. Verifying all RPM’s¶

Here’s a small script that constructs a list of all package names sans version numbers, then feeds the list to rpm with the --verify option. It also echoes the package name:

$ for i in $(rpm -qa --queryformat "%{NAME}\n" | sort)
$ do
$   echo $i":" >>verify.log 2>&1
$   rpm --verify $i >>verify.log 2>&1
$ done

The stuff enclosed in $(...) gets run as a script within a script, and the output of that is fed to the outer script. (See man eval and other stuff about evaluating.)

1.9. Viewing post-installation RPM scripts¶

Occasionally, after the files are dropped onto the system, hither and yon, RPM will run a script embedded in the package file. It’s nice to see what the squirrels are doing under the hood:

$ rpm -q --scripts «package-name»

1.10. What are we listening to?¶

lsof shows which processes are listening on a given port. (Actually it stands for “list open files”, which shows what files are currently open.) lsof -i will list which ports are open on the machine:

$ lsof -i

1.11. Finding duplicate files with identical contents¶

There’s probably a better way, but this worked for me:

$ diff -qrs «directory1» «directory2» 2>&1 | \
  grep "identical$" > «unedited-shell-script.sh»

Then edit the file «unedited-shell-script.sh» to your heart’s content.

1.12. Listing DNS stuff¶

Lots of different ways to do this, but I like:

$ nslookup
> ls -d gallaudet.edu

1.13. Who’s been sleeping in MY bed?¶

Here’s a more informative way to use the last command:

$ last -adf «wtmp_file»

1.14. Clip-clip. Taking care of really LONG lines¶

Lots of times, we only need to see the beginning of lines in a file to determine something. (For example, a subroutine that takes a single string argument may have a really long string literal.) To see just the first 100 characters on a line use the cut command. Like so:

$ cut -b -100 «FY2000.sql» | land

(land is an alias I’ve set up to print a file in landscape orientation using enscript command – a very nice printing program.)

1.15. Find and delete¶

Sometimes it’s nice to do something (like delete) a bunch of files based on a searchable criteria, e.g. portion of the filename, size, date, etc. Here’s how:

$ find . -empty -exec rm -v {} \;

This is a specific example that searches for empty files and directories from the current working directory down, and then it deletes them. The important parts are the {} which gets replaced with whatever has been found, and the \;, an escaped semicolon indicating the end of the command to be “exec’ed”. (And the -v is the ubiquitous “verbose” option, to tell you what’s happening.)

A better approach, I’m told, is:

$ find . -empty | xargs rm -v

1.16. Formatting and using a floppy¶

Without mounting anything, just pop a floppy in the drive and:

$ fdformat /dev/fd0H1440
$ mke2fs /dev/fd0H1440
$ mount /dev/fd0 /mnt/floppy ...
$ umount /mnt/floppy

1.17. Mounting an NSF device¶

To mount an NSF disk:

$ mount «server»:«remote_directory» /«local_mount_point» -t nfs -o ro

1.18. Searching for files and manipulating them¶

To find files in or beneath the current directory, of type “file”, of size 800 KB or greater, and then pipe the results through the ls command:

$ find . -type f -size +800k -exec ls -l {} \;

As mentioned in an earlier tip, it’s better with xargs. The command above could be improved as:

$ find . -type f -size +800k | xargs ls -l

To find all files that match the pattern *.o, print the full filespec (%p), the last-access time (%a) and the last-modified time (%t), and prompt for deletion:

$ find . -name "*.o" -printf "%p\nA: %a\nM: %t\n" -exec rm -i {} \;

To find files whose data has changed since midnight:

$ find . -daystart -mtime 0

(The -mtime can be replaced with the -ctime to show files whose status has changed.)

1.19. Starting a remote X windows program on a local screen¶

I must have had to do this at some point:

$ xon «remote_host»           \
      -access                 \
      -user «remote_username» \
      «remote_program_path»

1.20. Making a boot floppy¶

What the heck is a “floppy”? Well, if you have one:

$ dd bs=8192 if=/vmlinuz of=/dev/fd0

That allowed me to recover from a machine that someone infected with a boot sector virus. I still had to rebuild the kernel.

1.21. Switching parallel from printer to ZIP¶

Speaking of dead hardware… ZIP drives that connect to the parallel port:

$ modprobe -r lp
$ modprobe ppa
$ mount /dev/sdc4 /mnt/ZIP

1.22. More fun with RPM’s¶

That --queryformat be some powerful voodoo. Convert the datetime tags to human readable form via the system command:

$ convdate -c `rpm -q --queryformat "%{INSTALLTIME}\n" «package name» `

1.23. Burning CD’s¶

Unfortunately, with no CD-burner on any of the Linux boxes, you have to resort to M$ to do the actual burn. But just copying the files and trying to burn things didn’t seem to work. So, on a Linux box, create a disk image, then move the image to a M$ machine. Like so:

$ mkisofs -vrTJV "«Volume Label»" -o «image filename».iso «root directory»/
$ mount -t iso9660 -o ro,loop=/dev/loop0 «image filename».iso /mnt/cdrom

The first line makes an ISO file system and write it to a file. The .iso just makes it easier for the Windoze software to recognize it as a CD image. The command line options used are:

-v verbose,
-r Rockridge extensions,
-T make TRANS.TBL files,
-J Joliett extensions,
-V Volume label,
-o output file.

The second line tests the image, by mounting it as though it were a real device.

Fancying the image creation up a bit, the following puts an abstract on the CD and hides the TRANS.TBL from systems that can handle long file names:

$ mkisofs -vrTJV "«Volume Label»"         \
          -abstract "«Short description»" \
          -hide-joliet-trans-tbl          \
          -o «image filename».iso «root directory»/

And a variation with some Macintosh options thrown in:

$ mkisofs -vrJV "«Volume Label»" \
          -hfs                   \
          -magic «magic file»    \
          -probe                 \
          -o «image filename».iso «root directory»/

The magic file helps mkisofs determine which CREATOR and TYPE to use so that a Macintosh knows how to open the files. It appears the magic file is only needed if the system cannot already determine what the file is by examining the first few bytes. (I used /dev/null for the magic file.)

If you DO have a burner on your box, you can add the command:

$ cdrecord -v -speed=«##» dev=«#,#,#» -data «image filename»

In my case the -speed is 10 and the dev is 2,1,0. This burns the image file created by the mkisofs command onto your CD. If you don’t know the dev, you can find it with one of the following two lines:

$ cdrecord -scanbus
$ cdrecord dev=ATA -scanbus

depending on your kernel version and your CD burner controller. The second version picks up an ATA CD-burner under kernel 2.6.

1.24. Turning off NetQUE broadcasts¶

NetQue boxes attached to dumb printers keep sending RWHO packets (UDP/513) all over campus. This is annoying. To turn it off:

telnet into the NetQue
Type SU at the prompt.
It will display a Password> prompt. Type a Control-H (ASCII backspace) and then SYSTEM and hit enter. (SYSTEM is the default password.)
If you get to the prompt, type DEFINE SERVER ANNOUNCEMENT DISABLE
Type SYNC
Type LO
Power-cycle the printer server for it to take affect.

1.25. Checking out from CVS¶

I don’t yet understand what I’ve done, but apparently, I got it right. The following pulled the latest version of Boa Constructor:

$ cvs -z3 \
      -d:pserver:anonymous@cvs.Boa-Constructor.sourceforge.net:/cvsroot/boa-constuctor \
      co boa

1.26. Streaming with Icecast and Darwin¶

Icecast streams MP3 and Ogg Vorbis, Darwin is Apple’s QuickTime streamer. Again, I’m not certain of all the details, but I’ve got them both going.:

$ icecast -b
$ liveice -F ~/liveice.test 2> /dev/null
$ /usr/local/sbin/streamingadminserver.pl

The first line starts Icecast listening. The second sends a stream to Icecast for rebroadcast. The third starts the Darwin server. Be sure to check the configuration files in /etc/icecast and ~/liveice.test.

1.27. Pretty-printing code as web pages¶

My favorite program lister enscript, can generate color-coded web pages, as well as color-coded Postscript. Separate colors are used for comments, keyowrds, functions, and quoted strings. To generate a page, complete with a table of contents, the magic is:

$ enscript -E -C -G -j -Whtml --color --toc -p«output».html \
           «program1» [«program2» «program3» ...]

1.28. md5sum¶

MD5 checksums are frequently distributed with files to be downloaded, in an effort to insure data integrity. The program md5sum for Linux is fairly easy to find, and may already be on your system.

To check a file, download the corresponding MD5SUM file (possibly named «filename».md5) to the same directory where the files to be checked live. Then issue the command:

$ md5sum -c «filename».md5

To create an MD5 checksum file for others to use against your files, issue the command:

$ md5sum «filenames» > «filename».md5

I donwnloaded a Microsoft DOS/Windows version of md5sum from https://etree.org/md5com.html. The web page suggests the directory in which to save the program. It needs to be run from the DOS command prompt.

1.29. Slow Hand¶

The problem: During a rescue operation, I needed to copy a HUGE file. Unfortunately, while booted up in Red Hat’s rescue mode, memory management seems to have some problems. Every attempt to copy this would go for a while then run out of memory and force a reboot.

Solution: I hypothesized that if I could slow the machine down, I might give it time to recover its memory. (I know I forget things when I think too fast.)

So, how?

Break the file into chunks and copy a chunk at a time, with delays between chunks.
The file (a bzipped tarball) was 689459745 bytes long.
dd (a file copying program) writes nulls when it hasn’t got any data. So, I couldn’t write more data than was actually in the original file. Otherwise I’d end up with nulls at the end.
That means, the blocksize times the number of blocks had to exactly match the file size.
I found a web page with a factoring calculator, and learned that the prime factors of 689459745 are 3, 5, 13, 19, 379 and 491.
Armed with that info, I opted for 379 blocks of 1819155 bytes each.
Finally, I wrote a little script

#!/bin/sh
# SLOW DOWN! Copy slooooowly, and provide a running
# progress report comparing the file sizes of the
# two files in question. When done, compute the MD5
# checksum for each file, for comparison.
#
# NOTE: A 1-second delay wasn't enough. A 10-second
# delay was, but it was also probably overkill.

cd /mnt/sysimage/usr/share
rm /mnt/jaz/home.tbz
touch /mnt/jaz/home.tbz
ls -al home.tar.bz2
ls -al /mnt/jaz/home.tbz
sleep 1

for ((blk = 0; blk < 379; blk++))
do
  dd if=home.tar.bz2 of=/mnt/jaz/home.tbz \
     bs=1819155 count=1 \
     seek=$blk  skip=$blk
  ls -al home.tar.bz2
  ls -al /mnt/jaz/home.tbz
  sleep 10
done
md5sum home.tar.bz2 /mnt/jaz/home.tbz

UPDATE: Apparently, I misread or misunderstood the dd command. It doesn’t pad its output with NULL’s unless you explicitly ask it to by using the conv=sync option. So any reasonable block size should have worked above…

Bob Solomon <wogsol (at) bestweb (dot) net> wrote to me about a different way to split up a file: Given a list of files that you want to tar, but make into several “chunks”:

$ tar -czf - «file1 file2 file3 ...» | split -b «###»m - «filename».tgz.

(where ### is a block size in megabytes.) This creates files filename.tgz.aa, filename.tgz.ab, filename.tgz.ac… and so on, with each file being ### MB long.

(Bob also uses an environment variable $DATE which he sets to the current date in the form yy-mm-dd, in the base filename of the split command.)

1.30. Synchronizing with rsync¶

To copy big directory trees across the entire universe, and maintain protections, user and group ID’s etc, use rsync. It appears to have a few kinks–like it’s slow as molassas on my machine, I think I caused a kernel panic the first time I used it, and now that it’s finished a copy it appears to be hanging, but it gets the job done.

Important tip: It seems to do better at pushing files out to the remote machine, rather than pulling from the remote:

$ rsync -avz --rsh=ssh «local_directory_tree» \
                       «remote_machine»:«remote_destination»

1.31. Exploring binary RPM’s without installing them¶

Sometimes it’s nice to see what’s in an RPM file without actually installing it. If you have the source RPM (.src) then it’s easy. Just make the binary. But if you don’t have it and don’t want to bother getting it, you can extract the CPIO “heart” of the RPM and explore that. (cpio = copy in and out.):

$ rpm2cpio «package».rpm > «package».cpio
$ cpio -it --verbose < «package».cpio | most
$ cpio -id --verbose < «package».cpio

The first line pulls out the CPIO from the RPM file. The second gives a verbose listing of the contents of the file, and the third actually does the extraction, forcing the creation of directories that aren’t already present.

1.32. Renaming all files in a directory¶

Sometimes you want to rename all the files in a directory, and the new names will in some way be based on the old names. Here’s one way to tackle the problem (not necessarily the best way):

$ ls | grep -v "[on]names" > onames
$ ls | grep -v "[on]names" > nnames
# (edit nnames and change the old filenames to their new filenames.)
# (edit onames and insert "mv " at the start of each line.)
$ paste onames nnames > script.sh
$ bash script.sh
$ rm onames nnames script.sh

The first two lines make identical files containing all the filenames in the directory, sans the two files being created. The paste command in step 5 puts them together, line by line, so you end up with several lines of mv old-name new-name, which you just push through your shell.

If all you want to do is lowercase the names, this will do the trick:

$ for i in *; do mv $i $(echo $i | tr [A-Z] [a-z]); done

1.33. “rpm -qil” in Debian¶

To query a Debian package and obtain both a package description and a list of files within the package, use the command:

$ (dpkg -p «package» ; dpkg -L «package» ) | «pager»

To just get a list of ALL packages (installed and uninstalled, use:

$ dpkg -l '*'

1.34. Handling NULL’s in PostgreSQL¶

The COALESCE function is your friend. It allows you to substitute a string for a NULL value. The example below shows how to combine several fields together into a single string:

SELECT  COALESCE(name,'')    || '-' ||
        COALESCE(version,'') || '-' ||
        COALESCE(release,'') || '\n\t' ||
        COALESCE(summary,'') || '\n\t' ||
        COALESCE(url,'')
FROM     gri
WHERE    name ~* '.*devel.*' and release ~* '.*ximian.*'
ORDER BY name;

When run on my database of installed RPMs, that produces output like this:

...
sane-backends-devel-1.0.8-1.ximian.1
       The SANE (a universal scanner interface) development toolkit.
       https://www.sane-project.org/
SDL-devel-1.2.4-1.ximian.3
       Files needed to develop Simple DirectMedia Layer applications.
       https://www.libsdl.org/
xmms-devel-1.2.7-1.ximian.4
       XMMS - Static libraries and header files.
       https://www.xmms.org/
(28 rows)

1.35. Using BitTorrent with RedHat¶

According to https://www.redhat.com/en, RPMS for Red Hat Linux 7.3 through 9 of BitTorrent are available from:

https://torrent.dulug.duke.edu/btrpms/

Usage is simple:

$ btdownloadcurses.py --url «https://URL.torrent»

Allow incoming TCP 6881 - 6889 to join the torrent swarm.

1.36. Decoding base-64 encoded text¶

If you end up with a file that is base-64 encoded, fetch a copy of uudecode and add a line to the top of the base-64 encoded file (if it isn’t there already) that looks like:

begin-base64 664 «ASCII-file»

Then issue the command:

$ uudecode -m «b64-file»

This should read in b64-file and create ASCII-file.

1.37. Optimizing SQL field lengths¶

Obvious, when one thinks about it, but… To obtain the lengths of all entries for a particular field, use:

SELECT LENGTH(«field_name») AS «column_name»
FROM «table_name»
GROUP BY «column_name»;

To list only the length of the longest entry in a field, use:

SELECT MAX(LENGTH(«field_name»)) AS «column_name»
FROM «table_name»;

1.38. A light at the end of the tunnel¶

I don’t yet consider myself an expert by any means, but I’m making progress understanding tunneling. Here’s an example:

$ ssh -L 7000:134.231.8.45:80 -l kevin.cole gallaudet.edu
http://localhost:7000/

The first line establishes a tunnel on the localhost, going out port 7000 to 134.231.8.45, port 80, via ssh logged in as kjcole on gri.gallaudet.edu.

The second line is the URL to make use of the above tunnel, effectively connecting to https://134.231.8.45/.

1.39. Paper size¶

To switch to 8.5 * 11 paper size:

$ cd /usr/share/libgnomeprint/.../printers/

Edit the files GENERIC.xml and PDF-WRITER.xml. Change the PhysicalSize from A4 to USLetter (no spaces).

1.40. GNU Privacy Guard (GPG) tip¶

Apparently, the GPG honor-http-proxy keyserver-option is buggy. (Either that, or privoxy is.) So, in order to use commands like:

$ gpg -v --keyserver x-hkp://pgp.mit.edu --refresh-keys

or:

$ gpg -v --keyserver x-hkp://pgp.mit.edu --send-keys

remember to issue the shell command unset http_proxy first.

1.41. Searching for strings using grep the right way¶

I was constantly annoyed by grep hanging indefinitely when searching recursively. One possible reason for the trouble was that grep would encounter a “file” which was in fact a pipe or other weird beastie that doesn’t really have a beginning or end. As a result, grep would search such a file indefinitely. So, instead, use find to guarantee that grep only searches actual normal files:

$ find «path» -type f | xargs \
  grep -H "«And I still haven't found what I'm looking for»"

And, an often related nusance: When find encounters a filename with spaces, what it pipes to grep ends up as several arguments. When it finds a file with a filename like “Scholarly Work.txt”, grep interprets as a file named Scholarly and a second file named Work.txt. Not at all what I had intended. So, in the simple case, a solution is:

$ find «path» -type f -exec \
  grep -Hil "«And I still haven't found what I'm looking for»" {} \;

grep receives each filename supplied by find whole and intact.

1.42. Handling files with spaces in the name¶

Those nasty Mac OS X people, and later those nasty Windows folks have made life messy for us saints of Free / Libre Open Source Software. But, there is hope:

#!/bin/bash
#
# This script iterates through several file types and makes substitutions
# within them. It's done this way to get around funky directory and file
# names containing spaces. The IFS indicates an inter-file separator,
# in this case NUL. The "set -f" turns off pathname expansion, allowing
# the "htm*" and "php*" to be passesd as-is to the find command.  And
# finally, the ${1:-.} says to use a dot (current directory) if no
# directory path is supplied on the command line.

IFS="$(echo -ne '\000')"
set -f

for filetype in "htm*" "php*" "py" "cgi" "c" "pl" "txt" \
                "js" "asp" "shtm*" "pm" "java" "lore" "kid"
do
  find ${1:-.}/ -iname "*.$filetype" -print0 | while read -d "$IFS" file
  do
    echo "\"$file\""
    perl -p -i -e "s|\<i\>|\<em\>|g;"       "$file"
    perl -p -i -e "s|\<I\>|\<em\>|g;"       "$file"
    perl -p -i -e "s|\</i\>|\</em\>|g;"     "$file"
    perl -p -i -e "s|\</I\>|\</em\>|g;"     "$file"
    perl -p -i -e "s|\<b\>|\<strong\>|g;"   "$file"
    perl -p -i -e "s|\<B\>|\<strong\>|g;"   "$file"
    perl -p -i -e "s|\</b\>|\</strong\>|g;" "$file"
    perl -p -i -e "s|\</B\>|\</strong\>|g;" "$file"
  done
done

1.43. Fancier apache protection¶

First, enable some apache modules: auth_digest, dav, and ssl. The auth_digest module enables the use of better-encrypted usernames and passwords. The dav module enables WebDAV, which allows those with the appropriate permissions to look at the files in a directory using a file browser / manager, with drag-n-drop, and the ability to add and delete files to the directory. And, finally, the ssl module gets down and dirty with data transfer encryption. SSL’s a bitch, and therefore not covered here:

$ a2enmod dav
$ a2enmod auth_digest
$ a2enmod ssl

Now, edit the file containing directives for your web directories (in the case of Ubuntu, one of the files in /etc/apache2/sites-available/ e.g. default). Add in a stanza for the URL you want to protect:

<Location «relative URL»>
    Order Allow,Deny
    Allow from all
    Dav On
    AuthType Digest
    AuthName "«realm»"
    AuthDigestDomain «relative URL»
    AuthDigestProvider file
    AuthUserFile «/path/to/password.file»
    Require valid-user
</Location>

Finally, create the password file:

$ cd «/path/to/password.file»
$ htdigest -c «password.file» "«realm»" «username»

The realm is a short description of the area to be protected. The realm in the htdigest command should match the realm specified with the AuthName directive in the apache configuration file. Ditto for the path to the password file. The htdigest command will create (-c) the password file password.file add username to it, prompting for a new password.

It appears that it’s also a good idea to match up the argument in the <Location> directive with that in the AuthDigestDomain directive. This should be “relative” to the DocumentRoot. In other words, it’s what appears after the https://host.domain.tld/

Addendum: Oh the perversity that is Micro$oft Winblows. Every variation of URL, username, etc. failed to create a mapped network drive. What finally worked… sort of? From inside Micro$oft Weird, opening a Network Place. But, not exactly. You see, it cannot create a new file directly. It needs to create a folder. So, it creates a new FOLDER inside the already shared WebDAV folder. That means all the files we expected to find were one level above the Network Place we created and had to be moved into the newly created directory.

1.44. Restoring files with cp¶

To preserve dates, permissions, links, etc. when copying files:

$ cp -rvP --preserve=all «source directory» «destination»

1.45. QR Codes as SVG¶

qrencode creates QR Code images as PNG files. If that’s all you need, then the first line below will suffice. However, if you want to convert the PNG to an SVG, go through an intermediate step that makes a BMP:

$ qrencode -s 20 -o «filename».png "«blablabla.bla»"
$ convert «filename».png «filename».bmp
$ potrace -b svg -o «filename».svg «filename».bmp
$ rm «filename».bmp «filename».png

qrencode produces the PNG (Portable Network Graphic), convert converts it to a BMP (bitmap), and potrace converts the BMP to an SVG. (The rm removes the first two files.) Use the same filename extensions as shown above. “blablabla.bla” is just any text you want to encode. You can also pipe a file to the encoder. -s 20 sets the dot size to 20 pixels. The default size is 3 pixels. -o indicates the name of the output file. -b svg specifies the “backend” which tells potrace the output format. (qrencode is part of the qrencode package, potrace is part of the potrace package, and convert is part of the imagemagick package.)

1.46. Inserting lines at the start of a file¶

sed, the stream editor, is our friend here. To insert a single line:

sed -i '1s/^/«line to be inserted»\n/' «filename»

But a more clever approach that handles more than one line, methinks:

printf '%s\n%s\n' "«text to be inserted»" "$(cat «filename»)" > «filename»

1.47. Floating image in reStructuredText¶

To have an image with text appearing beside it in reStructuredText:

In the .rst file:

.. container:: twocol

   .. container:: leftside

      .. figure:: /_static/illustrations/structure.svg

   .. container:: rightside

      Bla-bla-blah, and yada-yada.

In the custom CSS (I used a copy of sphinxdoc.css which I put in ./source//_static/):

div.leftside {
    width: 414px;
    padding: 0px 3px 0px 0px;
    float: left;
}

div.rightside {
    margin-left: 425px;
}

Each ..container:: becomes a <div>. In my case, I wanted a fixed width for the image and a variable width for the remainder. And, with a wee bit o’ tweaking of the LaTeX produced by Sphinx, it also did a decent job of producing two-column output for that section.

1.48. Learning Assembler !!!¶

(We’ll see how long this exercise in terror / futility lasts.)

Write some C code in a file “scratchpad.c” (or whatever name you’d like). Then:

$ gcc -S -fverbose-asm scratchpad.c

or:

$ gcc -g -c scratchpad.c
$ objdump -drwC -Mintel scratchpad.o

(The later is actually a dis-assembly from the object binary. The first form is MUCH more useful to me: It interleaves the original C code with the assembly language it generates, making it easy to see “Oh. That ‘if’ statement compiles into these four assembly language instructions.)

I have a feeling there’s much room for improvement on the process (e.g. adding -O3 to optimize the hell out of the compilation, adding -g to the first form to include more debugging info) but this is quite nice. There are more command-line options for the GCC compiler than I’ve seen for any other Unix-y / Linux-y thing ever. So, fine-tuning the output for learnability is for a future date.

Source: StackOverflow, naturally: Using GCC to produce readable assembly?

Continuing on the adventure, the Executable and Linkable Format, better known as ELF:

1.49. Rectangular blocks in emacs¶

I’m sure there was a time when I did this regularly, but it’s been ages! In any case, the magic:

Move the cursor to the starting location.
Mark the current location with Ctrl-x SPACE.
Move the cursor to the end location.
Kill the region Ctrl-x r k
Move to the new location
Yank the region Ctrl-x r k

1.50. git “mirroring”¶

With a very small tweak, you can make your local git repositories push out to as many hosting sites as you like. Once per each hosting site that you would like to replicate to (though you have to have created a repository there first, even it’s empty):

git remote set-url --add --push origin «clone URL»

After that, any time you push, it will go to all of the sites that you’ve used that line for. So now, many of my repositories are pushed out to Codeberg, GitBox, and GitLab with every “git push” command. (”git pull” will still pull from the one you initially cloned from, or did your initial setup with.)

NOTE: The remote repository has to exist before adding it to the list of push destinations.

1.51. Copying between two remote machines¶

It turns out that it is as simple as 1, 2, -3:

$ scp -3 «remote1:/path/to/sources» «remote2:/path/to/destination»

1.52. Sledgehammer “git pull”¶

Probably not the smartest thing I’ve ever done, but at least not as disasterous as doing pip update on everything in sight, thus trashing packages installed by apt, yum or pacman…

This little ditty goes through as many git repositories as it can find, and tries to issue a git pull for each of them:

$ for i in $(locate /.git/ | sed -e "s/\/\.git.*//;" | sort | uniq)
$ do
$   cd $i
$   git pull
$ done

(It presumes the locate package is installed and that updatedb has been run recently.)

In some cases, this leads to conflicts, merge problems, etc.

1.53. Sucking down a web site directory tree¶

(There’s probably a more clever way with curl these days…)

When a URL reveals a directory of stuff you want to get, while avoiding the stuff you don’t want:

$ wget -r -np -R "index.html*" -e robots=off «URL»

Option	Meaning
`-r`	recursive
`-np`	no parent (don’t go to the root of the URL)
`-R "index.html*"`	don’t include the index.html files
`-e robots=off`	ignore what robots.txt is telling you to do

1.54. Stop annoying animated GIFs¶

In Firefox go to the url about:config and change image.animation_mode from normal to none.

1.55. Platform-provided Python packages¶

It turns out, that while in the virtual environment, I don’t have access to some of the Python packages installed via apt. In particular, the PyQt5 stuff. But, there’s an answer:

$ cd dirname
$ rm -rf ~/.local/share/virtualenvs/dirname*
$ rm Pipfile.lock
$ pipenv --three --site-packages
$ pipenv shell
$ pipenv update

Passing --site-packages during the initial pipenv setup adds magic to ~/.local/share/virtualenvs/dirname... or so it would appear. As near as I can determine, it adds an include-system-site-packages line to ~/.local/share/virtualenvs/dirname-.../pyvenv.cfg. Like so:

home = /usr
implementation = CPython
version_info = 3.8.5.final.0
virtualenv = 20.0.23
include-system-site-packages = true
base-prefix = /usr
base-exec-prefix = /usr
base-executable = /usr/bin/python3.8
prompt = (dirname)

And that’s where it gets the command line prompt as well.

1.56. Pretty-print XML¶

Sometimes one wants to read that billion-character single-line XML file in order to make sense of it. First, set the indentation.

For tab indentation:

export XMLLINT_INDENT=`echo -e '\t'`

For four space indentation:

export XMLLINT_INDENT=\ \ \ \

Then:

xmllint -format -recover nonformatted.xml > formated.xml

1.57. Reformatting XML a la Emacs¶

Actually, I suspect this works for a lot of different source material, provided Emacs recognizes the file type. In emacs parlance:

C-x h C-M-\

In more readable form:

Ctrl-X h    Ctrl-Alt-\

Explanation:

Ctrl-X h      runs the command    "mark-whole-buffer"
Ctrl-Alt-\    runs the command    "indent-region"

1.58. Adding an upstream repository to a forked repository¶

So, after forking a repository, it would be nice to be able to keep it synchronized. First, set up an upstream branch:

$ git clone git@github.com:kjcole/obs-midi.git
Cloning into 'obs-midi'...
remote: Enumerating objects: 106, done.
remote: Counting objects: 100% (106/106), done.
remote: Compressing objects: 100% (69/69), done.
remote: Total 5924 (delta 64), reused 69 (delta 37), pack-reused 5818
Receiving objects: 100% (5924/5924), 2.21 MiB | 6.04 MiB/s, done.
Resolving deltas: 100% (3778/3778), done.
$ cd obs-midi/

$ git remote -v
origin  git@github.com:kjcole/obs-midi.git (fetch)
origin  git@github.com:kjcole/obs-midi.git (push)

$ git remote add upstream git@github.com:cpyarger/obs-midi.git

$ git remote -v
origin    git@github.com:kjcole/obs-midi.git (fetch)
origin    git@github.com:kjcole/obs-midi.git (push)
upstream  git@github.com:cpyarger/obs-midi.git (fetch)
upstream  git@github.com:cpyarger/obs-midi.git (push)

Then at a later date, periodically lather, rinse, repeat:

$ git fetch upstream
$ git checkout main
$ git merge upstream/main

1.59. Searching for Unicode characters¶

My unprintables and expletives aliases (shown below) come in handy for finding everything that is not pure, printable, easy to type ASCII. But sometimes, just sometimes, I need to search for a specific Unicode character. For example, a long dash. most (or hexdump if you prefer) can reveal that at least one variant of a long dash is hexadecimal E2B8BA. To search for it with grep use:

$ grep $'\xE2\xB8\xBA' *

The aforementioned and very handy aliases are:

$ alias unprintable='grep --color="auto" -P -n "[\x00-\x1E]"'
$ alias expletives='grep  --color="auto" -P -n "[^\x00-\x7E]" '
$ alias decomment='egrep -v "^[[:space:]]*((#|;|//).*)?$" '

unprintables searches for any lines containing control characters (ASCII characters in the range 0 to 31). expletives searches for any characters beyond ASCII, i.e. NOT in the range 0 to 127. (And, because it is probably my most used alias decomment shows the contents of files sans any comment lines using #, ; or // as the comment delimiter.)

1.60. Running multiple commands in Bash¶

There are three different ways to combine commands in the terminal:

;     Command 1 ;  Command 2    Run command 1 first and then command 2
&&    Command 1 && Command 2    Run command 2 only if command 1 ends sucessfully
||    Command 1 || Command 2    Run command 2 only if command 1 fails

1.61. Bash: for i in range()¶

Two different techniques:

Method 1:

for i in {0..10..1}
do
  printf "%02d\n" $i   # Print as a 2-digit number with leading zeros
done

Method 2:

export END=10
for i in $(seq 0 $END)
do
  printf "%02d\n" $i   # Print as a 2-digit number with leading zeros
done

1.62. Bash substrings¶

This little ditty goes through a directory tree looking for filenames ending in “~” and comparing them to the same filename NOT ending in “~”. In other words it quickly compares backup version of each file with the current version of the file. Like so:

for i in $(find . -name "*~")
do
    diff -u $i ${i:0:-1} | most
done

The new tidbit for me was the substring syntax:

If $i is a variable, ${i:offset:length} is the substring (e.g. ${i:1:2} will give the 2nd and 3rd character), but using negative values for the length parameter indicates how many letters should be cut from the end. (Using negative values for the offset appears to have no effect and behaves the same as zero.) Either offset or length can be omitted:

$ x="This string"
$ echo ${x::5}
This
$ echo ${x:5}
string

Marco the Marvelous suggests an improvement: Just remove last character:

${i%?}

This one is just saying output $i except remove the matching pattern after %, which is a single any character. A more specific version of the previous one, since all your files end with a tilde:

"${i%\~}"

This one is just saying output $i except for the matching pattern \~. (When escaped is just the ending tilde.)

For more fancy variable stuff, see GNU Bash - Shell Parameter Expansion

1.63. Compare two files ignoring whitespace AND newlines¶

Suppose you have two files:

$ cat file-A
The quick brown fox jumped over the lazy dog's back. Now    is     the    time...

$ cat file-B
The quick brown fox jumped
over the lazy dog's back.
Now is the time.

The text is identical, but the spacing is different. How can you determine that? A search of the package repositories turns up:

dwdiff    - diff program that operates word by word
docdiff   - Compares two files word by word / char by char
icdiff    - terminal side-by-side colorized word diff
numdiff   - Compare similar files with numeric fields
rfcdiff   - compares two internet draft files and outputs the difference
wdiff     - Compares two files word by word
wdiff-doc - Documentation for GNU wdiff

all of which may be worth exploring. However, StackExchange provides, once more:

$ diff <( tr -d " \n" <file-A ) \
       <( tr -d " \n" <file-B )

The command uses tr to delete all spaces and newlines from each file, and then feeds them as inputs to diff.

NOTE: The whitespace in the command is necessary.

Better still, git provides! git has a --word-diff option:

git diff --word-diff file-A file-B

1.64. Convert JPG to SVG in one swell foop¶

Easy (using ImageMagick as the middle-app):

convert -channel RGB -compress None input.jpg bmp:- | \
potrace -s - -o output.svg

1.65. Convert MIDI to MP3 in one swell foop¶

Thanks to StackOverflow - Convert midi to mp3 we have:

for i in *.midi
do
  timidity $i -Ow -o - | \
  ffmpeg -i - -acodec libmp3lame -ab 64k ${i:0:-5}.mp3
done

1.66. Expand MP4 to PNG frames¶

Sometimes, one just wants a single frame from a video:

ffmpeg -ss «timestamp»       \
       -i «source_movie».mp4 \
       -t «duration»         \
       «destination_frames»_%04d.png

will produce a series of images, «destination_frames»_0000.png through «destination_frames»_%9999.png. The -ss «timestamp» indicates the point from which the first frame should be extracted (e.g. 00:00 to start at the beginning) and the -t «duration» is the number of minutes and seconds to continue extracting ((e.g. 01:04 to get one minute and four seconds of frames).

The %04d indicates that the generated frames should have filenames that contain consecutive four-digit, zero-padded decimal number.

1.67. Running two applications in parallel¶

For this specific example, I wanted to have a GUI-based audio player play several tunes with an option for me to adjust the speed of playback, while simultaneously displayng a window with the sheet music for the corresponding tune. The magic that worked:

for i in polkas/john_ryans_polka \
         polkas/peg_ryans_polka  \
         single_jigs-slides/denis_murphys_slide-no._1
do
    evince $i.pdf &   \
    vlc    $i.mp3 &&  \
    fg
done

1.68. Create a new file with lines matching several patterns deleted¶

Suppose you have a mailing list, and you want to create a new mailing list with several addresses excluded. Easy. fgrep is your friend:

cat > remove
user1@host1.tld
user2@host2.tld
...
^D

fgrep -v -f remove source.csv > destination.csv
wc -l source.csv destination.csv remove

The fgrep uses remove as a file full of patterns to match. (And the wc is only there as a sanity check: The first number should equal the sum of the second and third numbers.)

1.69. Making terminal command-line “movies”¶

The script command will start a second Bash shell and record all terminal I/O including user input, command output and ANSI escape sequences for colorizing, cursor positioning, screen clearing, etc. The default behavior is to save the data to a file named typescript.

However, viewing such a file using standard tools is problematic: Using cat typescript shows the correct output, but it scrolls off the screen too quickly. Using a pager such as most prevents scrolling but does not know how to interpret the ANSI escape sequences, showing them as raw data, which drastically decreases the readability of the script.

Enter scriptreplay. By adding a timing file option to the script command, and then using that file with scriptreplay the typescript file will be displayed as if one was watching a ghost user enter commands into a terminal: The timing file preserves the original pauses that a user has while typing:

script -T timecodes
...
exit

produces two files: typescript with the terminal I/O and timecodes which contains the timing data for the keystrokes. By later typing:

scriptreplay -t timecodes

you can watch the “movie” in realtime.

The caveat: After issuing the scriptreplay command, the first thing you will see is the command prompt, making it appear as if the scriptreplay command has failed. This is an illusion: You are, in fact, NOT seeing the command prompt: You are seeing the recording of a command prompt: It is the first output in the typescript file. Wait. Eventually, the “ghost user” will begin typing.

The above caveat can be mitigated by creating an alias for the scriptreplay command that adds in messages indicating that the movie is about to start (or has just ended) and putting it into .bash_aliases, optionally adding an alias for script as well. The following aliases make script automatically produce a timecodes file, and produce colorized (bright / bold red on yellow) informative messages regarding the starting and ending of a script replay to the scriptreplay command:

alias script='script -T timecodes '
alias scriptreplay='clear ; echo -e "\e[1;43;31m### STARTING replay ###\e[0m\n" ; scriptreplay -t timecodes ; echo -e "\e[1;43;31m### FINISHED replay ###\e[0m" '

(That second alias is a bit long, and may eventually be changed to a Bash function in .bashrc. A wag of my acquainance, the goode Mr. Flint suggested the name scriptease. Clever. I like it.)

Ultimately, functions that use a special directory and time-stamp both the I/O data file an the timing file that pairs with it might be a good way to keep multiple recordings, rather than overwrite the files on each use. For example:

mkdir -p ~/scripts/$(date --iso)
mv typescript ~/scripts/$(date --iso)/
mv timecodes  ~/scripts/$(date --iso)/

1.70. Reversing a video¶

To save a video that plays backwards:

ffmpeg -i input.mp4 -vf reverse output.mp4

Then with a tool like pitivi you can append the output to the input and get a nice looping effect.