#Text Parsing with awk, sed and paste

• Authors: Kathi Unglert
• Research field: Volcano Geophysics
• Lesson Topic: awk, sed and paste in the bash shell.

awk

Check whether you have awk installed by using the which command:

$: which awk

You should get something like this:

/usr/bin/awk

If you don't have awk installed you can get it here.

volcano_observations.dat contains the following lines (try viewing it with cat volcano_observations.dat):

month level observation
april 1 steam
may 2 ash
june 4 plinian
july 1 steam
august 2 earthquakes
september 3 subplinian
october 2 earthquakes
november 1 steam

We can achieve the same effect as cat with:

awk '{print $0}' volcano_observations.dat

• Challenge Question: How to get output to print to file?
• Solution: awk '{print $0}' volcano_observations.dat > observations2.dat

We can select individual columns to print like this:

awk '{print $3}' volcano_observations.dat

But, maybe we don't want the header line observation in our output. We can get rid of it via

awk '{if (NR > 1) print $3}' volcano_observations.dat

We can pick all entries that contain "earth" in their observation column:

awk '{if (index($3, "earth") != 0) print $3}' volcano_observations.dat

Or, we can combine if statements to pick all entries that have an "e" in their observation column, not including the header:

awk '{if (NR > 1 && index($3, "e") != 0) print $3}' volcano_observations.dat

Or, the same thing, but print out the whole entry:

awk '{if (NR > 1 && index($3, "e") != 0) print $0}' volcano_observations.dat

• Challenge Question: How to print all months with alert level of 2 or higher?
• Solution: awk '{if (NR > 1 && $2 >= 2) print $1}' volcano_observations.dat

earthquakes.dat contains the following (view with cat):

date time magnitude lat lon depth
2011/03/01 20:21:11.11 3.1 49.12 -123.10 45
2011/03/01 21:45:51.04 3.8 49.21 -123.08 42
2011/03/01 21:53:42.33 2.5 49.01 -122.89  5
2011/03/01 21:58:32.17 3.4 48.99 -122.89  2
2011/03/01 22:03:56.10 4.3 49.03 -123.12 35
2011/03/01 23:22:45.89 3.1 49.01 -122.91  1
2011/03/02 04:17:03.77 4.3 49.02 -123.01 0.5
2011/03/02 12:01:34.32 3.7 49.17 -123.20 43
2011/03/02 15:34:56.51 2.8 49.14 -123.00 46
2011/03/03 05:21:23.09 3.4 49.09 -123.11 41
2011/03/03 08:54:56.67 3.3 49.09 -123.10 43
2011/03/03 16:32:45.52 4.1 49.10 -123.12 42

In this data, shallow earthquakes may indicate volcanic activity, while deep earthquakes may originate from a subduction zone.

• Challenge Question: How to print all earthquake entries from volcano?
• Solution: awk '{ if (NR > 1 && $6 <= 10) print $0}' earthquakes.dat

Or, print out only the magnitudes of those:

awk '{ if (NR > 1 && $6 <= 10) print $3}' earthquakes.dat

Or, the magnitudes and depths:

awk '{ if (NR > 1 && $6 <= 10) print $3,$6}' earthquakes.dat

We can also add some text to what we return:

awk '{ if (NR > 1 && $6 <= 10) print "magnitude",$3,"depth",$6}' earthquakes.dat

We'd like to be able to extract which days had volcanic earthquakes. But, the year, month and day are seaparated by / instead of spaces, so they appear as one column. We can define a new 'field separator' with the -F flag; we can get the month like this:

awk -F/ '{ if (NR > 1) print $2}' earthquakes.dat

But if we ask for the next column in order to get the days, we get the whole rest of the line, too - awk is no longer breaking on spaces:

awk -F/ '{ if (NR > 1) print $3}' earthquakes.dat

We can define additional field separators:

awk '-F[/ :]+' '{if (NR > 1) print $1}' earthquakes.dat

Which lets us find the days with volcanic earthquakes:

awk '-F[/ :]+' '{if (NR > 1 && $10 <= 10) print $3}' earthquakes.dat

and remove duplicates in the list using awk:

awk '-F[/ :]+' '{if (NR > 1 && $10 <= 10) print $3}' earthquakes.dat | awk '!_[$0]++'

compare this to:

awk '-F[/ :]+' '{if (NR > 1 && $10 <= 10) print $3}' earthquakes.dat | uniq

sed

sed is a powerful text manipulation tool, most usually used for doing find-and-replace operations.

We can substitute all instances of 2011 for 2010 in earthquakes.dat (/g = globally):

sed 's/2011/2010/g' earthquakes.dat > earthquakes2.dat

If we want to substitute the month, we have to be a bit more careful; replacing every instance of 03 might lead to some unintended replacements. We can be sure that we're looking at a month if the text follows the pattern /03/, but now sed will get confused as to which backslashes are part of the command, and which backslashes are characters to be replaced. We have to 'escape' the slashes we want to count as text, using the character \:

sed 's/\/03\//\/04\//g' earthquakes.dat > earthquakes2.dat

Since this is a bit hard to read, many implementations of sed accept other delimiting characters; for example, try:

sed 's|/03/|/04/|g' earthquakes.dat > earthquakes2.dat

We can also substitute inside the original file:

sed -i 's/2011/2010/g' earthquakes2.dat

or on Mac:

sed -i.tmp 's/2011/2010/g' earthquakes2.dat

or:

sed -i '' 's/2011/2010/g' earthquakes2.dat

We can also delete lines:

sed '/2011\/03\/03/d' earthquakes.dat > earthquakes2.dat

paste

We can combine the columns of two files with the paste command:

paste -d" " earthquakes.dat earthquakes2.dat > alleqs.dat