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Explanation

GNU dd (disk dump) copies any block device to another block device or file.

• Piping the input of dd into the output of another instance seems to always improve copying speed.
• /dev/sda refers to your input device, which may vary. Check yours with fdisk -l.
• image.dd refers to the copy stored in the current working directory. You can also use another block device, such as /dev/sdb.
• The entire hdparm subshell sets dd's input block size to the buffer size of the source medium. This also usually improves copy speed, but may need adjustment (see limitations below).
• conv=noerror tells dd to ignore read errors.

Check dd's progress with: kill -USR1 $(pidof dd)

Explanation

• -d -m start screen in "detached" mode. This creates a new session but doesn't attach to it.
• -S some_name gives the screen session a label, so you can easily re-attach to it with screen -R some_name
• The shell prompt returns immediately, and of course you can logout too, the screen session will continue to run happily.
• When the specified program exits (in this example ping), its screen window will also exit, and if there are no other windows in the screen session, then the screen session itself will also exit.

Explanation

This shows a very nice hexdump of the specified file. You can edit the output and convert it back to binary with xxd -r. But the best part is that you can configure vim to do all the converting back and forth for you, effectively turning vim into a binary editor, by adding this to your .vimrc:

augroup Binary
    au!
    au BufReadPre  *.bin let &bin=1
    au BufReadPost *.bin if &bin | %!xxd
    au BufReadPost *.bin set ft=xxd | endif
    au BufWritePre *.bin if &bin | %!xxd -r
    au BufWritePre *.bin endif
    au BufWritePost *.bin if &bin | %!xxd
    au BufWritePost *.bin set nomod | endif
augroup END

This will work for editing .bin files. To use it for other file extensions too, duplicate the lines within augroup and replace *.bin with *.exe for example.

This tip is from vim's :help xxd.

Explanation

This is just for the sake of an example of finding $foo that occurs after $bar. Substitute ifconfig and the arguments of grep appropriately for your use case.

• In the output of ifconfig there are several lines with inet. We want to get to the first one that comes after a line starting with en1
• grep ^en1 -A5 will print the line starting with en1 and the next 5 lines that follow it
• grep inet will print only the lines matching inet
• head -n 1 will print only the first line

The value 5 in -A5 is really just a guess that the line we're interested in will be within the next 5 lines, the appropriate number depends on your use case.

Kind of a dumb technique, but it's easy to remember.

Explanation

Should have realized this can be done with sed, too - it's even shorter!

-n suppresses the normal printing of lines.

\@interface Ethernet3/1@,/!/ specifies the beginning and end of the stanza we want to work on, and addresses all lines in between. Because the string we want to match at the beginning contains a slash, we need to designate a different character as the delimiter for the regular expression (@ in this case - note the backslash before the first one). We are OK using slashes for the ending regex, which matches an exclamation mark.

While in the stanza of interest, we slightly abuse the s function with s/ip address/&/p. This matches the text "ip address" and replaces it with... exactly the same thing (& just represents the matched text). The reason we bother doing this is to take advantage of the p flag, which will print any line where a replacement has been made.

The result is that only lines which match "ip address" will be printed, and only when we are in the stanza of interest.

Explanation

This can easily be done by setting a variable once the lead line of the stanza is found. Take the following file as input:

interface Ethernet3/1
 ip address 172.30.10.2 255.255.0.0
 ip router isis area1
 no ip route-cache
 isis metric 10
!
interface Ethernet3/2
 ip address 192.168.10.10 255.255.255.0
 ip router isis area1
 no ip route-cache
 isis metric 10

The first awk pattern-action pair in the one-liner /interface Ethernet3\/1/ {instanza=1} sets the instanza variable to 1 (true) when it encounters the "interface Ethernet 3/1" line. The second pattern-action /!/ {instanza=0} resets instanza back to 0 (false) once an exclamation point is encountered, ending the desired stanza. The third pattern-action instanza && /ip address/ {print} prints any line containing "ip address," but only if we are still in the desired stanza (instanza is true): in this case, ip address 172.30.10.2 255.255.0.0.

If you only wanted to see the first matching line of the first matching stanza, you could change the third action to be {print; exit} and awk would quit after printing the first match.

Explanation

sort supports -h for human readable number sorting.

Explanation

• All the double quoting is necessary to handle filenames with spaces.
• Calling sh with -exec is necessary to evaluate readlink for each symlink

Explanation

This will effectively force the user to change his/her password at next login.

Explanation

The Android video player is somewhat fussy about the formats it can play. This ffmpeg script will take any movie file in 16:9 widescreen format and convert it into a form that can be played on one of the current leading QHD phones such as HTC Sensation, Samsung Galaxy S2 or Motorola Atrix. Files that are merely marked as widescreen (e.g. DVD VOBs) will have to be processed into a true widescreen format such as .m2t first.

Parameters:

$1 the input file

$2 the output file

Output file suffix should be .mp4

Requires:

ffmpeg and codecs

Explanation

versorge asks:

I have a bunch of usb volumes lying around and I would like to get a quick summary of what is on the drives. How much space is taken up by pdf, image, text or executable files. This could be output as a text summary, or a pie chart.

This one-liner produces a list like this:

  5804731229 FLAC audio bitstream data
   687302212 MPEG sequence
    99487460 data
    60734903 PDF document
    55905813 Zip archive data
    38430192 ASCII text
    32892213 gzip compressed data
    24847604 PNG image data
    16618355 XML 1.0 document text
    13876248 JPEG image data

The find command locates all regular files (-type f) below the given directory, which could be a mounted USB stick or any other directory. For each one, it runs the file -b command with the filename to print the file type; if this succeeds, it also prints the file size (-printf '%s\n'). This results in a list containing a file type on one line, followed by the file size on the next.

The awk script takes this as input. The GNU file command often produces very specific descriptions such as GIF image data, version 87a, 640 x 480 - to generalize these, we set the field separator to be a comma with the -F option. Referencing $1 then only uses what's to the left of the first comma, giving us a more generic description like GIF image data.

In the awk script, the first pattern-action pair NR%2 {i=$1} applies to each odd-numbered line, setting the variable i to be the file type description. The even-numbered lines are handled by NR%2==0 {a[i]+=$1}, adding the value of the line (which is the file size) to the array variable a[i]. This results in an array indexed by file type, with each array member holding the cumulative sum of bytes for that type. The END { ... } pattern-action pair finally prints out a formatted list of the total size for each file type.

At the end of the line, the sort command sorts the list, putting the file types with the largest numbers at the top.

Explanation

• The reason to use a for loop is to sort results with G or M or K values separately, otherwise sort -n would just sort everything by the numbers regardless of G M K suffix.
• grep "[0-9]$i\b" matches lines containing a digit followed by G or M or K followed by a "word boundary"

Explanation

• echo -e prints G for Gigabytes, M for Megabytes and K for Kilobytes in a line each.
• 2>/dev/null send stderr to /dev/null
• sort -rn sorts in reverse numerical order. Largest first

Explanation

• <<< A variant of here documents

Explanation

• bc is an arbitrary precision calculator language.
• obase defines the conversion base for output numbers, in this example 2 (binary)
• ; is a statement separator in bc
• 1234 is the decimal number to convert
• By piping the command to bc we get 1234 in binary format

Explanation

FFmpeg is a complete, cross-platform solution to record, convert and stream audio and video. It includes libavcodec - the leading audio/video codec library.

Explanation

column is a utility for formatting text. With the -t flag it detects the number of columns in the input so it can format the text into a table-like format.

For more details see man column.

Explanation

Something I do a lot is extract columns from some input where cut is not suitable because the columns are separated by not a single character but multiple spaces or tabs. So I often do things like:

... | awk '{print $7, $8}'

... which is a lot of typing, additionally slowed down when typing symbols like '{}$ ... Using the simple one-line function above makes it easier and faster:

... | col 7 8

How it works:

• The one-liner defines a new function with name col
• The function will execute awk, and it expects standard input (coming from a pipe or input redirection)
• The function arguments are processed with sed to use them with awk: replace all spaces with ,$ so that for example 1 2 3 becomes 1,$2,$3, which is inserted into the awk command to become the well formatted shell command: awk '{print $1,$2,$3}'

Explanation

• mogrify is part of ImageMagick, an image manipulation software suite
• mogrify manipulates the specified images. If you prefer to keep the original image untouched and write the manipulated image to a different file, simply replace mogrify with convert, the syntax is the same, but the last command line argument will be the target image to write to.
• The -geometry flag is to resize the image, it requires a dimension parameter in the format WIDTHxHEIGHT
• The dimension in this example has no width, which means the image will be resized to height=31 pixels, and the width will be proportional.

Explanation

As explained superbly in man bash:

   (list) list is executed in a subshell environment (see  COMMAND  EXECU-
          TION  ENVIRONMENT below).  Variable assignments and builtin com-
          mands that affect the  shell's  environment  do  not  remain  in
          effect  after  the  command completes.  The return status is the
          exit status of list.

In other words, this is a handy way to do something somewhere else without having to go there and coming back.

Explanation

The recode utility installed on many systems converts between character sets. This command is shorthand for recode IBM-PC..latin1 file1 file2 file3 which converts the given files from CRLF to LF line endings.

Explanation

Sure, you could figure out the link target from the output of ls -l a_symbolic_link_to_somewhere too, but the output of readlink is simply the target of the symbolic link itself, so it is cleaner and easier to read.

Explanation

• The arguments starting with + are commands in vi that will be executed
• set ff=unix is a shortcut for set fileformat=unix and means to use "unix" file format, i.e. without the carriage return \r character.
• %s/^M$// is a pattern substitution for all lines in the entire buffer, the pattern is "carriage return at end of the line", where ^M is not two literal characters but actually one, to enter it on the command line press Ctrl followed by Enter/Return
• bufdo means to run command in all buffers (each file is opened in a separate buffer)
• q is to quit vi

Note: the set ff=unix is necessary, otherwise the pattern substitution will not do anything if all the lines end with \r = the file is in dos format, because in that case the line ending character will not be considered as part of the line.

Note: if a shell-script has "accidentally" some carriage returns in it, then when you try to execute you may get an error: bad interpreter: No such file or directory. This one-liner fixes that problem. If you know that all the lines in the file have the carriage return, and there is only one file to fix, then a simplified version of the one-liner is enough:

vi +'set ff=unix' +wq file1

Explanation

Knowing the octal, hexadecimal or decimal code of the ASCII character set can be handy at times. In the past, too often I did things like:

perl -e 'for my $n (1 .. 255) { print $n, chr($n), $n, "\n"; }'

... when a simple man ascii would have done the trick...

On a related note, these all print the letter "A":

echo -e '\0101'
printf '\101'
printf '\x41'
perl -e 'print "\x41"'

Explanation

We open a file with vi and run two vi commands (specified with +):

1. %!sort | uniq
   • % = range definition, it means all the lines in the current buffer.
   • ! = run filter for the range specified. Filter is an external program, in this example sort | uniq
2. wq = write buffer contents to file and exit.