Updated: 2015-01-04 12:53 EST
cut
vs. awk
to extract fields from linesBecause of the power of Unix pipes and the rich set of command-line tools available, Unix/Linux system administrators are often asked to extract or “mine” data from various text files, or to convert files from one format to another format.
The “mining” operation can take many forms; but, a common form is to process a stream of text and extract certain fields from certain lines. One set of commands selects the lines to extract; the other set of commands picks off the desired fields from those lines (or vice-versa). Often these two operations are repeated, narrowing down the selection until just the desired information is displayed.
Data mining is easy, if you build up the Unix pipeline slowly, adding one command at a time and watching the output each time.
Some Unix commands select lines from a text stream, others select fields, and some can do both:
Select lines from text streams:
grep, awk, sed, head, tail, look, uniq, comm, diff
Select fields in lines or parts of lines:
awk, sed, cut
Transform text (change characters or words in lines):
awk, sed, tr
The sort
command is also useful for putting lines of text in order, often for counting using uniq -c
.
Become familiar with the data mining capabilities of the above commands.
Problem: “Print the fifth directory from your PATH
environment variable.”
This problem can be generalized to print the fifth field or fifth line of any input stream. We will do an iterative solution built up slowly using simple commands. There are several solutions.
tr
, head
, tail
IndexFirst, we echo the PATH
variable onto our screen and note that fields are separated by colon (:
) characters:
$ echo "$PATH"
/usr/bin:/usr/sbin:/sbin:/bin:/usr/games:/usr/lib/xscreensaver
Next, we convert the colons separating directories into newlines, so that each directory is on a separate line. We do this so that we can later use “line selection” commands to select the fifth directory as the fifth line instead:
$ echo "$PATH" | tr ':' '\n'
/usr/bin
/usr/sbin
/sbin
/bin
/usr/games
/usr/lib/xscreensaver
Now that the fields are on separate lines, we can use a “line selection” command to select the first five lines:
$ echo "$PATH" | tr ':' '\n' | head -5
/usr/bin
/usr/sbin
/sbin
/bin
/usr/games
Now, we use a “line selection” command to select the last line (of the first 5):
$ echo "$PATH" | tr ':' '\n' | head -5 | tail -1
/usr/games
This is the answer – /usr/games
is the fifth directory (the last line of the first five lines).
awk
IndexWe can also do the same operation using the “field selection” commands to extract the fifth field.
awk
command '{print $1}'
prints the first field on each line.awk
command '{print $2}'
prints the second field on each line.awk
command '{print $NF}'
prints the last field on each line.By default, awk
separates fields by blanks; so, we need to turn the colons in PATH
into blanks. Again, build up the command iteratively:
$ echo "$PATH"
/usr/bin:/usr/sbin:/sbin:/bin:/usr/games:/usr/lib/xscreensaver
$ echo "$PATH" | tr ':' ' '
/usr/bin /usr/sbin /sbin /bin /usr/games /usr/lib/xscreensaver
$ echo "$PATH" | tr ':' ' ' | awk '{print $5}'
/usr/games
However, awk
has a convenient option to use another separator character:
$ echo "$PATH" | awk -F: '{print $5}'
/usr/games
cut
IndexIf you happen to know that cut
can split a line using any delimiter character, you could also use:
$ echo "$PATH" | cut -d: -f5
/usr/games
sed
IndexEven sed
lets us pick off the fifth field separated by colons using a “regular expression” pattern (though this is very messy!):
$ echo "$PATH" | sed -e 's/^[^:]*:[^:]*:[^:]*:[^:]*:\([^:]*\):.*/\1/'
/usr/games
Problem: “Print the second-to-last directory from your PATH
environment variable.”
This can be generalized to print the second-to-last line of any input.
Use the same basic line-oriented form as the previous example, only select the fields from the end of the list instead of the beginning. Build up the command one-by-one:
$ echo "$PATH"
$ echo "$PATH" | tr ':' '\n'
$ echo "$PATH" | tr ':' '\n' | tail -2
$ echo "$PATH" | tr ':' '\n' | tail -2 | head -1
This is the answer – it is the second-to-last directory (the first line of the last two lines). We can also do the same operation using the “field selection” commands to extract the fifth field. Build up the pipe line iteratively:
$ echo "$PATH"
$ echo "$PATH" | tr ':' ' '
$ echo "$PATH" | tr ':' ' ' | awk '{print $(NF-1)}'
-OR-
$ echo "$PATH" | awk -F: '{print $(NF-1)}'
-OR-
$ echo "$PATH" | sed -e 's/^.*:\([^:]*\):[^:]*$/\1/'
Note the use of single quotes to protect the dollar signs in the command-line script fragments from expansion the shell. We want the commands themselves to see the dollar symbols; we don’t want the shell to expand them.
Problem: “Sort the elements in the PATH
variable in ascending order.”
This can be generalized to sort any field-delimited list.
Since the sort
command only works on lines, not fields, we must first transform the PATH
into a list of directories, one per line:
$ echo "$PATH"
/usr/bin:/usr/sbin:/sbin:/bin:/usr/games:/usr/lib/xscreensaver
$ echo "$PATH" | tr ':' '\n'
/usr/bin
/usr/sbin
/sbin
/bin
/usr/games
/usr/lib/xscreensaver
Now, we can add the sort command:
$ echo "$PATH" | tr ':' '\n' | sort
/bin
/sbin
/usr/bin
/usr/games
/usr/lib/xscreensaver
/usr/sbin
Now, we can put the line back together by changing all the newlines back into colons:
$ echo "$PATH" | tr ':' '\n' | sort | tr '\n' ':'
/bin:/sbin:/usr/bin:/usr/games:/usr/lib/xscreensaver:/usr/sbin:
The above line adds an extra colon (:
) on the end of the PATH
, which isn’t correct. To get rid of the final colon, a final edit with sed
:
$ echo "$PATH" | tr ':' '\n' | sort | tr '\n' ':' | sed -e 's/:$//'
/bin:/sbin:/usr/bin:/usr/games:/usr/lib/xscreensaver:/usr/sbin
Problem: “Keep only the first five elements of the PATH
.”
This can be generalized to keep the first N fields of any list.
We will again transform the fields of PATH
into directories on separate lines, select the first five lines, then put the directories back together again. Build up the pipeline iteratively, checking each step:
$ echo "$PATH"
$ echo "$PATH" | tr ':' '\n'
$ echo "$PATH" | tr ':' '\n' | head -5
$ echo "$PATH" | tr ':' '\n' | head -5 | tr '\n' ':'
$ echo "$PATH" | tr ':' '\n' | head -5 | tr '\n' ':' | sed -e 's/:$//'
Make sure to get rid of the trailing colon added by the final newline.
Problem: “How many unique shells are in the /etc/passwd
file?”
This can be generalized to count the unique field values in any data.
Build up the solution iteratively, starting with simple commands.
The shell field is the seventh colon-delimited field in the passwd
file. The commands awk
, sed
, or cut
can pick out a field from a file. We will use cut
to pick out the seventh field delimited by a colon.
Once we have only the seventh field being output, we can use sort
and uniq
to reduce the repeated lines to only unique lines, and then count them.
Because the /etc/passwd
file some machines is huge (and the output on our screen would be huge), we will start making our pipeline with only the first 10 lines of the passwd
file until we know we have the correct command line, then we will use the solution on the whole passwd
file.
First, get 10 lines from the top of the passwd
file:
$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
Cut out only the seventh field in each line, delimited by a colon:
$ head /etc/passwd | cut -d : -f 7
/bin/bash
/bin/sh
/bin/sh
/bin/sh
/bin/sync
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
Sort the fields so that identical field values are adjacent:
$ head /etc/passwd | cut -d : -f 7 | sort
/bin/bash
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sync
Reduce the output to unique lines by removing adjacent duplicate lines with uniq
:
$ head /etc/passwd | cut -d : -f 7 | sort | uniq
/bin/bash
/bin/sh
/bin/sync
Count the unique lines:
$ head /etc/passwd | cut -d : -f 7 | sort | uniq | wc -l
3
We have the correct command line. Now use the solution on the whole file, not just on the first ten lines:
$ cat /etc/passwd | cut -d : -f 7 | sort | uniq | wc -l
6
Note that the cut
command is quite capable of reading files itself - there is no need to use a superfluous and unnecessary cat
command to do it:
$ cut -d : -f 7 /etc/passwd | sort | uniq | wc -l
6
The sort command also has a option that only outputs unique lines. If we knew about it, we would write:
$ cut -d : -f 7 /etc/passwd | sort -u | wc -l
6
Does the pipeline below (the reverse of the above) give the same output?
$ sort -u /etc/passwd | cut -d : -f 7 | wc -l
48 # WRONG ANSWER!
When selecting lines and fields from a text stream, often the order in which you do the selection matters.
Problem: “Extract the temperature for Ottawa from a weather web page.”
Here is a URL that has the RSS feed for the Ottawa weather:
http://weather.gc.ca/rss/city/on-118_e.xml
We will look at the unformatted page and two formatted versions of the page, and decide which version is easiest to work with:
$ url='http://weather.gc.ca/rss/city/on-118_e.xml'
$ wget -O wget.txt "$url"
...raw XML RSS page downloads here into wget.txt file...
$ lynx -dump "$url" >lynx.txt
...formatted web page is in lynx.txt...
$ alias ee='elinks -dump -no-numbering -no-references'
$ ee "$url" >elinks.txt
...formatted web page is in elinks.txt...
The formatted elinks
page is the easiest to work with.
The first step is to extract from the page the line we want. The Temperature we want is on this line in the elinks
output:
[...]
Temperature: -8.6°C
[...]
That’s easy to extract. Send the page fetched to standard output, into a pipe into fgrep
to extract the right line:
$ ee "$url" | fgrep 'Temperature:'
Temperature: -8.6°C
If we only wanted the actual number, we could extract the last field from the line:
$ ee "$url" | fgrep 'Temperature:' | awk '{ print $NF }'
-8.6°C
In a script, we might use shell Command Substitution to place the output into a variable for later use:
$ cat temp.sh
#!/bin/sh -u
city='Ottawa'
url='http://weather.gc.ca/rss/city/on-118_e.xml'
temp=$( elinks -dump -no-numbering -no-references "$url" \
| fgrep 'Temperature:' | awk '{ print $NF }' )
echo "The temperature in $city is: $temp"
$ ./temp.sh
The temperature in Ottawa is: -8.6°C
Sometimes the pattern we want matches more than one line in the weather report, and we might get multiple output lines:
$ ee "$url" | fgrep 'Wind'
Wind Chill: -16
Wind: NW 19 km/h gust 28 km/h
2014-04-16T05:01:00Z 2014-04-16T05:01:00Z Partly cloudy. Wind northwest 20
2014-04-16T05:01:00Z 2014-04-16T05:01:00Z Sunny. Wind northwest 20 km/h
We can limit the output to just the lines we want using head
and tail
, but be careful that the order and number of lines never changes or else the output will be wrong:
$ ee "$url" | fgrep 'Wind' | head -n2 | tail -n1
Wind: NW 19 km/h gust 28 km/h
The above fails when the Wind Chill line is not included in the weather report!
Here are command pipelines that extract information from the system authorization log file /var/log/auth.log
on who is trying to attack the Course Linux Server (may require privileged read permission on the log files). The format of each line in this file is like this:
Jan 2 09:51:17 idallen-ubuntu sshd[28008]: Failed password for root from 50.46.204.2 port 33092 ssh2
Each line starts with the date, followed by some text. (Different log files may have different date formats.)
Each of these commands below is one, long single command line, but to make the long lines easier to read in this document the long lines have been split into multiple lines by using a backslash at the end of a line to mean “continue this with the next line”. You can either type the lines with the backslashes at the end, as shown below, or else simply type one long line and omit the trailing backslashes:
/var/log/auth.log
uniq -c
fgrep 'Failed password' /var/log/auth.log \
| awk '{print $9}' | sort | uniq -c | sort -nr | head
Jan
for “January”. (Different log files may have different date formats.)fgrep
to further limit the lines to ones that contain the date string Jan
followed by a blank:fgrep 'Failed password' /var/log/auth.log \
| fgrep 'Jan ' \
| awk '{print $9}' | sort | uniq -c | sort -nr | head
Feb
for “February”.Jan
to Feb
in the search string:fgrep 'Failed password' /var/log/auth.log \
| fgrep 'Feb ' \
| awk '{print $9}' | sort | uniq -c | sort -nr | head
Failed password
to refused connect
in the search string./var/log/auth.log
:Jan 2 02:18:27 idallen-ubuntu sshd[18078]: refused connect from 222.189.239.75 (222.189.239.75)
fgrep 'refused connect' /var/log/auth.log \
| fgrep 'Feb ' \
| awk '{print $10}' | sort | uniq -c | sort -nr | head
Sometimes the data you are reading isn’t nicely separated into lines on which you can use grep
. You can use the tr
(“translate”) command to split up the file into separate lines, based on some list of delimiters.
For parsing HTML pages, it’s often useful to split the long lines in HTML pages on angle brackets and/or quotes. This can put the data you want to extract on separate lines so that grep
can find it easily.
Let’s try to extract the URL that has the Ottawa weather from the master index page of weather in major Canadian cities:
$ url='http://weather.gc.ca/canada_e.html'
$ wget -q -O - "$url" | fgrep 'Ottawa'
<area href="/city/pages/on-118_metric_e.html" title="Ottawa" alt="Ottawa" shape="poly" coords="417,445,428,445,431,453,442,453,442,463,427,463,427,474,462,474,462,508,420,508,417,480,414,463,406,463,406,449" />
<td headers="header1"><a href='/city/pages/on-118_metric_e.html'>Ottawa (Kanata - Orléans)</a></td><td headers="header2">Clear</td><td headers="header3" class="align-right">-9°C</td></tr>
The initial try finds two (long) lines. Both lines contain the href=
URL for the Ottawa weather page. We decide to use the second one, so we refine the search to only select that line and not the first one:
$ wget -q -O - "$url" | fgrep '>Ottawa'
<td headers="header1"><a href='/city/pages/on-118_metric_e.html'>Ottawa (Kanata - Orléans)</a></td><td headers="header2">Clear</td><td headers="header3" class="align-right">-9°C</td></tr>
That very long line is messy for getting at the URL for Ottawa, which is inside single quotes in the middle of the line. Let’s split that one long line into separate lines by turning single quotes into newlines:
$ wget -q -O - "$url" | fgrep '>Ottawa' | tr -s "'" '\n'
<td headers="header1"><a href=
/city/pages/on-118_metric_e.html
>Ottawa (Kanata - Orléans)</a></td><td headers="header2">Clear</td><td headers="header3" class="align-right">-9°C</td></tr>
We want that /city/pages/...
URL, which is now on its own line and easy to extract with another grep
:
$ wget -q -O - "$url" | fgrep '>Ottawa' | tr -s "'" '\n' | fgrep '/city/pages/'
/city/pages/on-118_metric_e.html
In a script, we usually save the output in a variable using shell Command Substitution. Later, we output an HTML hyperlink to the Ottawa weather page:
$ cat ottawa.sh
#!/bin/sh -u
baseurl='http://weather.gc.ca'
canadaurl="$baseurl/canada_e.html"
wanturl=$( wget -q -O - "$canadaurl" \
| fgrep '>Ottawa' | tr -s "'" '\n' | fgrep '/city/pages/' )
fullurl="$baseurl$wanturl"
echo "Link to Weather in <a href='$fullurl'> Ottawa </a>"
$ ./ottawa.sh
Link to Weather in <a href='http://weather.gc.ca/city/pages/on-118_metric_e.html'> Ottawa </a>
cut
vs. awk
to extract fields from linesIndexThe cut
command treats every occurrence of the delimiter as the beginning of a new field. This makes it wrong to use in many situations.
$ echo "a b c" | awk '{print $2}' # output is: b
$ echo "a b c" | cut -d ' ' -f 2 # output is: b
$ echo "a b c" | cut -d ' ' -f 3 # output is: c
$ echo "a b c" | awk '{print $2}' # output is: b
$ echo "a b c" | cut -d ' ' -f 2 # output is: # WHY?
$ echo "a b c" | cut -d ' ' -f 3 # output is: b # WHY?
$ echo "a b c" | awk '{print $2}' # output is: b
$ echo "a b c" | cut -d ' ' -f 2 # output is: # WHY?
$ echo "a b c" | cut -d ' ' -f 3 # output is: # WHY?
$ echo "a b c" | cut -d ' ' -f 4 # output is: b # WHY?
For example, you might try to use cut
to extract the current day from the date string (though there are easier ways to get this information):
$ date=$( date )
$ echo "The date is $date"
The date is Wed Oct 16 13:51:54 EDT 2008
$ echo "$date" | cut -d ' ' -f 3
16 # just the day number
This looks like it’s working fine, until next month…
$ date=$( date )
$ echo "The date is $date"
The date is Fri Nov 1 12:15:45 EDT 2008
$ echo "$date" | cut -d ' ' -f 3
# empty! WHY?
$
Woops! The extra blank in front of the day 1
has caused cut
to come up with an empty third field. This is not what we want.
The awk
command behaves more reasonably when looking for fields separated by one or more blanks. By default, awk
splits up lines on any non-zero amount of whitespace (blanks and tabs), so awk
does not get confused by the extra blanks between fields:
$ date=$( date )
$ echo "The date is $date"
The date is Fri Nov 1 12:15:45 EDT 2002
$ echo "$date" | awk '{print $3}'
1
This works much better. awk
doesn’t care if there is one blank or many blanks; it still divides the line up into the same number of fields.
Usually awk
does what your eyes expect when extracting fields separated by blanks. Avoid cut
for extracting fields unless you know that only one delimiter exists between fields. (The password and group files are always delimited by exactly one colon between each field.)
Looking for lines containing all of a list of strings can be done by chaining grep
commands together:
$ fgrep 'string one' filenames... | fgrep 'string two'
Only lines containing both string one
and string two
will display. This can be extended to any number of text strings with more grep
.
To find lines that contain any of a list of strings, use the -e
option to grep
:
$ fgrep -e 'string one' -e 'string two' filenames...
Lines containing string one
or string two
will display. This can be extended to any number of text strings with more -e
options.
You want to extract a block of lines, starting at a line matching the first pattern and stopping at the line matching the second pattern.
... | grep -A 1000 "start pattern" | grep -B 1000 "end pattern"
# The end pattern must be within 1000 lines of the start pattern in
# the input stream.
-OR-
... | sed -n -e '/start pattern/,/end pattern/p'
# This above solution uses the "stream editor".