27 linux snippets

gracefully closing node.js applications via signal handling

To make your node.js application gracefully respond to shutdown signals, use process.on(SIGNAL,HANDLER).

For example, to respond to SIGINT (typically Ctrl-c), you can use:

process.on( "SIGINT", function() {
  console.log('CLOSING [SIGINT]');
} );

Note that without the process.exit(), the program will not be shutdown. (This is you chance to override or "trap" the signal.)

Some common examples (in CoffeeScript):

process.on 'SIGHUP',  ()->console.log('CLOSING [SIGHUP]');  process.exit()
process.on 'SIGINT',  ()->console.log('CLOSING [SIGINT]');  process.exit()
process.on 'SIGQUIT', ()->console.log('CLOSING [SIGQUIT]'); process.exit()
process.on 'SIGABRT', ()->console.log('CLOSING [SIGABRT]'); process.exit()
process.on 'SIGTERM', ()->console.log('CLOSING [SIGTERM]'); process.exit()

PS: On Linux (and similar) you can enter kill -l on the command line to see a list of possible signals, and kill -N PID to send signal N to the process with process ID PID.

Published 8 Jan 2013


Command-line tool for spidering sites and extracting XML/HTML content

Xidel is a robust tool for spidering, extracting and transforming XML/HTML content from the command line.

It's like wget or curl with a CSS and XPath/XQuery engine (among other features), attached.

xidel doesn't seem to be in the package management repositories I normally use, but you can download it here.

The following example will (1) download a web page, (2) extract a list of links (specified via CSS selector) from it, (3) download the page corresponding to each of those links and finally (4) extract specific pieces of content (specified by CSS selectors) from each page:

  --follow "css('[CSS-SELECTOR-FOR-LINKS]')" \
  --extract "inner-html(css('[CSS-SELECTOR-FOR-SOME-HTML]'))"

As a concrete example, the command:

$ xidel http://reddit.com -f  "css('a')" --css title

will download every page linked from the reddit.com homepage and print the content of its title tag.

There are several more examples on the Xidel site.

Published 11 Feb 2014
Tagged linux, tool, xml, css, html, xpath, one-liner and ops.


Shell script for service-like CoffeeScript/Node.js apps using forever

This is an example of a (bash) shell script that uses the forever module to start and stop a CoffeeScript application as if it were a service.

(Also at rodw/coffee-as-a-service-via-forever.sh.)

Published 11 Feb 2014
Tagged nodejs, linux, service and ops.


Backup or mirror a website using wget

To create a local mirror or backup of a website with wget, run:

wget -r -l 5 -k -w 1 --random-wait <URL>


  • -r (or --recursive) will cause wget to recursively download files
  • -l N (or --level=N) will limit recursion to at most N levels below the root document (defaults to 5, use inf for infinite recursion)
  • -k (or --convert-links) will cause wget to convert links in the downloaded documents so that the files can be viewed locally
  • -w (or --wait=N) will cause wget to wait N seconds between requests
  • --random-wait will cause wget to randomly vary the wait time to 0.5x to 1.5x the value specified by --wait

Some additional notes:

  • --mirror (or -m) can be used as a shortcut for -r -N -l inf --no-remove-listing which enables infinite recursion and preserves both the server timestamps and FTP directory listings.
  • -np (--no-parent) can be used to limit wget to files below a specific "directory" (path).
Published 10 Feb 2014


Pre-generate pages or load a web cache using wget

Many web frameworks and template engines will defer the generation the HTML version of a document the first time it is accessed. This can make the first hit on a given page significantly slower than subsequent hits.

You can use wget to pre-cache web pages using a command such as:

wget -r -l 3 -nd --delete-after <URL>


  • -r (or --recursive) will cause wget to recursively download files
  • -l N (or --level=N) will limit recursion to at most N levels below the root document (defaults to 5, use inf for infinite recursion)
  • -nd (or --no-directories) will prevent wget from creating local directories to match the server-side paths
  • --delete-after will cause wget to delete each file as soon as it is downloaded (so the command leaves no traces behind.)
Published 10 Feb 2014


Mapping port 80 to port 3000 using iptables

Port numbers less that 1024 are considered "privileged" ports, and you generally must be root to bind a listener to them.

Rather than running a network application as root, map the privileged port to a non-privileged one:

sudo iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3000

Now requests to port 80 will be forwarded on to port 3000.

Published 8 Feb 2014


Making CAPS-LOCK into a control key in X

Using xmodmap:

$ cat ~/.xmodmap
remove Lock = Caps_Lock
keycode 0x42 = Control_L
add Control = Control_L

$ xmodmap ~/.xmodmap
Published 8 Feb 2014
Tagged linux, debian and tool.


Quickly render a 'dot' (Graphviz) graph

On Linux and OSX the command:

dot -Txlib mygraph.gv

will quickly launch a lightweight window containing a dot rendering of the graph in mygraph.gv.

The rendering should automatically refresh when mygraph.gv is updated. (I've occasionally run into small glitches with this that force me to re-launch the window, but they are rare and obvious.)

The same -Txlib parameter works for the other Graphviz rendering engines, including neato, twopi, fdp, sfdp, circo, and patchwork.

Published 1 Jan 2014


Accessing Google Calendar from Thunderbird/Lightning (Icedove/Iceowl)

Background Note: On Debian, Mozilla's email client Thunderbird is known as Icedove, and Mozilla's calendar client Lightning is known as Iceowl. This rebranding has something to do with the licensing requirements surrounding the "Mozilla" name. As I understand it, Firefox/Iceweasel, Thunderbird/Icedove and Sunbird/Lightning/Iceowl are identical save for the icons and naming.

Here's how to access Google Calendar using Lightning/Iceowl, whether as a standalone application or as a Thunderbird/Icedove add-on.

1. Install the application:

sudo aptitude install iceowl

Or the add-on (assuming you've already installed Thunderbird/Icedove):

sudo aptitude install iceowl-extension

You can install both, but note that they don't share a configuration so you'll need to set up your calendars in both Lightning/Iceowl and Thunderbird/Icedove.

2. Get the "private URL" for your calendar.

Go to the web-based Google Calendar.

On the left-side navigation bar, notice the My Calendars section (expand that if needed). Hover over one of your calendars and click on the little caret/down-arrow menu that appears. Select Calendar settings from that menu.

On the Calendar settings page, note the section labeled Private Addresses. Copy the URL that is linked to by the orange button labeled XML.

3. Create a new calendar in Lightning/Iceowl.

In the standalone app, that's File > New Calendar. Within Thunderbird/Icedove, that's File > New > Calendar.

Select On the Network (and click Next).

Select Google Calendar and paste the URL you copied in step 2 into the Location field. (Then click Next.)

Enter your Google username/password and tweak the settings as desired.

4. Repeat steps 2 and 3 for each calendar you want to integrate.

Published 26 Feb 2014
Tagged gui, debian and linux.


Toggle line-wrapping in the terminal with 'tput rmam' and 'tput sram'

The command:

tput rmam

will disable line wrapping so that long lines are truncated to width of the terminal ($COLUMNS).

The command:

tput smam

will re-enable it.

This seems to known as "automatic margin" mode, hence smam is enter_am_mode and rmam is exit_am_mode.

Some terminals may not support this functionality.

Tagged bash and linux.


Use 'less -S' for horizontal scrolling

The flag -S (or --chop-long-lines) will cause less to truncate lines at the screen (terminal) boundary, rather than wrapping as it does by default. You can then scroll horizontally (with the arrow keys, for example) to view the full lines when needed.

cat some_file_with_very_long_lines | less -S
Tagged bash, linux, one-liner and cli.


Cheat Sheet for Linux Run Levels

"Standard" Linux uses the following run levels:

  • Run Level 0 is halt (shutdown).
  • Run Level 1 is single-user mode.
  • Run Level 2 is multi-user mode (without networking)
  • Run Level 3 is multi-user mode (with networking). This is the normal "terminal" mode. (I.e., before the display manager is run).
  • Run Level 4 is undefined.
  • Run Level 5 is multi-usermode with a GUI display manager (X11).
  • Run Level 6 is reboot.

In Debian and its derivatives run levels 2 thru 5 are the same: multi-user mode with networking, and with a display manager if available.

  • Run Level 0 is halt (shutdown).
  • Run Level 1 is single-user mode.
  • Run Level 2-5 is multi-user mode with networking and a GUI display manager when available.
  • Run Level 6 is reboot.

Debian also adds Run Level S, which is executed when the system first boots.

Also see Wikipedia's article on run levels.


How to disable services in Debian/Linux

The easy way is to install sysv-rc-conf:

aptitude install sysv-rc-conf

Manually, use update-rc.d and specify the run levels, like so:

update-rc.d SERVICE_NAME stop 0 1 6 3 . start 2 4 5 .
Tagged linux, debian, service and ops.


How to right-align text in your bash prompt

Right aligning text by padding with spaces

To have text in your bash prompt ($PS1) hug the right side of the terminal:

PS1="`printf "%${COLUMNS}s\n" "${TEXT}"`$PS1"

(This assumes you want the right-aligned text to appear before the rest of your prompt, if any. Move the $PS1 bit to the left side of the string to have the right-aligned text appear after the rest of your prompt.)

The ${COLUMNS} variable contains the number of columns in the current terminal (it should change if you resize the terminal). The ${TEXT} variable is a placeholder for the text you want to right-align.

The trick here is to use printf to left-pad the string to given width. printf "%ns" "text" will left-pad the given string (here, text) with spaces until the entire string is n characters wide.

Right aligning text by padding with something other than space.

Say you want to pad with - instead of space. Try:

PS1="`printf -vch "%${COLUMNS}s" "${TEXT}"; printf "%s" "${ch// /-}"`$PS1"

This will left-pad the ${TEXT} with spaces, as above, and then replace any spaces with -.

If you have any spaces in ${TEXT} you want to preserve, one hacky work-around is to mark spaces in $TEXT with some other character, say _, and then replace _ with a space () after the other substitution:

$ PS1="$PS1`printf -vch "%${COLUMNS}s" "${TEXT}"; printf -vch "%s" "${ch// /-}"; printf "%s\n" "${ch//_/ }"`"

Drawing a line to the end of the line

I recently added a line containing the date and time to my bash prompt (so I can tell when a given command completed) and wanted to draw a line across the rest of the screen to make it visually easier to tell where a new prompt is displayed. Something like this:

-- Tue 02-Oct-2012 05:19 PM --------------------------------

(Assuming the terminal is 60 characters wide.)

Here's how I did it.

Within my $PROMPT_COMMAND I execute the following:

line="`printf -vch "%${COLUMNS}s" ""; printf "%s" "${ch// /-}"`"
dts="`date +"-- %a %d-%b-%Y %I:%M %p "`"

The first line creates a variable ($line) with ${COLUMNS} dashes (-). This line would span the length of the terminal.

The second line creates a variable ($dts) with my date and time format of choice (prefixed with -- just for kicks).

The ${dts}${line:${#dts}} bit in the third line displays my date and time string ($dts) and then a substring of $line, starting at the length of my date and time string (${#dts}). (In this particular case ${dts} is always exactly 28 characters long, so that value could be hard-coded but this way it works in the general case too.)

If you are curious, the \e[1m\e[32m bit makes the text bold (\e[1m) and green (\e[32m).

Tagged bash, linux and cli.


Find duplicate files on Linux (or OSX).

Find files that have the same size and MD5 hash (and hence are likely to be exact duplicates):

find -not -empty -type f -printf "%s\n" | \         # line 1
  sort -rn | \                                      # line 2
  uniq -d | \                                       # line 3
  xargs -I{} -n1 find -type f -size {}c -print0 | \ # line 4
  xargs -0 md5sum | \                               # line 5
  sort | \                                          # line 6
  uniq -w32 --all-repeated=separate | \             # line 7
  cut -d" " -f3-                                    # line 8

You probably want to pipe that to a file as it runs slowly.

  1. Line 1 enumerates the real files non-empty by size.
  2. Line 2 sorts the sizes (as numbers of descending size).
  3. Line 3 strips out the lines (sizes) that only appear once.
  4. For each remaining size, line 4 finds all the files of that size.
  5. Line 5 computes the MD5 hash for all the files found in line 4, outputting the MD5 hash and file name. (This is repeated for each set of files of a given size.)
  6. Line 6 sorts that list for easy comparison.
  7. Line 7 compares the first 32 characters of each line (the MD5 hash) to find duplicates.
  8. Line 8 spits out the file name and path part of the matching lines.

Some alternative approaches can be found at the original source.

Tagged linux, one-liner and ops.


Generate a random list of words with shuf

shuf is (in my experience) a little known GNU utility that selects random lines (or bytes) from a file.

For instance, the command:

shuf -n 3 /usr/share/dict/words

selects three words at random from the words dictionary.

Tagged linux, one-liner and tool.


Restore Ctrl+Alt+Backspace as a way to kill X on Linux.

Modern Debian and Ubuntu distributions have this disabled by default.

setxkbmap -option terminate:ctrl_alt_backspace
Tagged linux and debian.


Backup an SD card on Linux using 'dd'

if [ -b "/dev/$1" ]
  outfile="sdcard-backup-`date +"%s"`.dd"
  echo "cloning /dev/$1 to $outfile"
  dd if=/dev/$1 of=$outfile
  echo "tgz-ing $outfile"
  tar zcvf $outfile.tgz $outfile
  echo "done."
  echo "Usage: $0 /dev/<device>"
echo "to restore, unmount(?), then use:"
echo "tar Ozxf <file> | dd of=<device>"
Tagged linux, backup and tool.


Find large files on Linux.

UPDATE: Reader Luc Pionchon points out that sort often supports a -h parameter that sorts by "human" numbers, hence:

$ du -h * | sort -h | tail

is probably a better alternative than any of the following (for the systems that support it).

du -h * | grep "^[0-9.]*M" | sort -n

This finds files at least 1 MB in size and then sorts them by size. Change M to G for files at least 1 GB in size.

(Caveat: files 1 GB or larger will be missed by the MB version. You can use:

du -h * | egrep "^[0-9.]*(M|G)"

to get both, but then the sort -n doesn't work quite the way we'd like.)

Of course, you could use du without the -h to get file sizes by the default block size rather than the human-readable 12.4M or 16K, etc.

Tagged linux, one-liner and tool.


Set monitor resolution with xrandr

$ cvt -r -v 1920 1080
# 1920x1080 59.93 Hz (CVT 2.07M9-R) hsync: 66.59 kHz; pclk: 138.50 MHz
Modeline "1920x1080R"  138.50  1920 1968 2000 2080  1080 1083 1088 1111 +hsync -vsync

$ xrandr --newmode "1920x1080R"  138.50  1920 1968 2000 2080  1080 1083 1088 1111 +hsync -vsync

$ xrandr --addmode VGA1 "1920x1080R"

$ xrandr --output VGA1 --mode "1920x1080R"

Also handy:

$ xrandr --output LVDS1 --off --output VGA1 --auto
Tagged linux, debian and tool.


Strip characters from a field in 'awk'

E.g., the following command strips alpha characters from the second (tab delimited) field.

awk -F"\t" '{gsub(/[A-Za-z]/,"",$2); print $2 }'
Tagged linux, awk, tool and one-liner.


Strip characters from a string or file with 'sed'

$ echo "A1B2C3" | sed 's/[A-Z]//g'
Tagged linux, sed, tool and one-liner.


Some 'awk' basics

Extract tab delimited fields from a file:

$ awk -F"\t" '{print "field one=" $1 "; field two=" $2 }' file
Tagged linux, awk, tool and cheatsheet.


Skip the first N lines in file

using tail

To skip the first line of a file (and start piping data at the second line):

tail -n +2 <FILENAME>

More generally:

tail -n +M <FILENAME>

where M is the number of the first line you want to see (i.e., the number of lines to skip plus one).

using sed

To skip the first line of a file (and start piping data at the second line):

sed 1d <FILENAME>

More generally:


when you want to exclude lines A through B from the output.

Tagged linux, sed and tool.


List Available Fonts

To view a list of available fonts, use fc-list.

Tagged linux, debian and tool.


Append to ~/.bash_history "immediately"

Bash normally waits until a session (terminal) is closed before it writes commands to the history.

You can add a call to history -a to PROMPT_COMMAND to make bash to append your history to ~/.bash_history every time it displays your prompt.


The environment variable PROMPT_COMMAND is executed when bash is about to display your prompt.

The command history -a appends the current history to ~/.bash_history.

Tagged linux and bash.


Create a simple histogram or bar-chart in bash

The following script accepts (via stdin) input like this:

foo 10
bar 21
xyzzy 12

and generates (to stdout) a bar-chart like this:

|   foo | 10 | ##########
|   bar | 21 | #####################
| xyzzy | 12 | ############

where the width of the headings (category and count) is automatically determined by the input values and the width of the bar chart (the number of # characters) is automatically scaled to fit within the current terminal width.

It also demonstrates a few intermediate level awk features such as passing variables (via the -v parameter) and variable-width printf statements (via %*s).

Published 28 Feb 2018
Tagged linux, bash, data, text and awk.


This page was generated at 4:16 PM on 26 Feb 2018.
Copyright © 1999 - 2018 Rodney Waldhoff.