Implement tail with awk - linux

I am struggling with this awk code which should emulate the tail command
num=$1;
{
vect[NR]=$0;
}
END{
for(i=NR-num;i<=NR;i++)
print vect[$i]
}
So what I'm trying to achieve here is an tail command emulated by awk/
For example consider cat somefile | awk -f tail.awk 10
should print the last 10 lines of a text file, any suggestions?

All of these answers store the entire source file. That's a horrible idea and will break on larger files.
Here's a quick way to store only the number of lines to be outputted (note that the more efficient tail will always be faster because it doesn't read the entire source file!):
awk -vt=10 '{o[NR%t]=$0}END{i=(NR<t?0:NR);do print o[++i%t];while(i%t!=NR%t)}'
more legibly (and with less code golf):
awk -v tail=10 '
{
output[NR % tail] = $0
}
END {
if(NR < tail) {
i = 0
} else {
i = NR
}
do {
i = (i + 1) % tail;
print output[i]
} while (i != NR % tail)
}'
Explanation of legible code:
This uses the modulo operator to store only the desired number of items (the tail variable). As each line is parsed, it is stored on top of older array values (so line 11 gets stored in output[1]).
The END stanza sets an increment variable i to either zero (if we've got fewer than the desired number of lines) or else the number of lines, which tells us where to start recalling the saved lines. Then we print the saved lines in order. The loop ends when we've returned to that first value (after we've printed it).
You can replace the if/else stanza (or the ternary clause in my golfed example) with just i = NR if you don't care about getting blank lines to fill the requested number (echo "foo" |awk -vt=10 … would have nine blank lines before the line with "foo").

for(i=NR-num;i<=NR;i++)
print vect[$i]
$ indicates a positional parameter. Use just plain i:
for(i=NR-num;i<=NR;i++)
print vect[i]
The full code that worked for me is:
#!/usr/bin/awk -f
BEGIN{
num=ARGV[1];
# Make that arg empty so awk doesn't interpret it as a file name.
ARGV[1] = "";
}
{
vect[NR]=$0;
}
END{
for(i=NR-num;i<=NR;i++)
print vect[i]
}
You should probably add some code to the END to handle the case when NR < num.

You need to add -v num=10 to the awk commandline to set the value of num. And start at NR-num+1 in your final loop, otherwise you'll end up with num+1 lines of output.

This might work for you:
awk '{a=a b $0;b=RS;if(NR<=v)next;a=substr(a,index(a,RS)+1)}END{print a}' v=10

Related

separate columns of a text file

Hii experts i have a big text file that contain many columns.Now i want to extract each column in separate text file serially with adding two strings on the top.
suppose i have a input file like this
2 3 4 5 6
3 4 5 6 7
2 3 4 5 6
1 2 2 2 2
then i need to extract each column in separate text file with two strings on the top
file1.txt file2.txt .... filen.txt
s=5 s=5
r=9 r=9
2 3
3 4
2 3
1 2
i tried script as below:but it doesnot work properly.need help from experts.Thanks in advance.
#!/bin/sh
for i in $(seq 1 1 5)
do
echo $i
awk '{print $i}' inp_file > file_$i
done
Could you please try following, written and tested with shown samples in GNU awk. Following doesn't have close file function used because your sample shows you have only 5 columns in Input_file. Also created 2 awk variables which will be printed before actual column values are getting printed to output file(named var1 and var2).
awk -v var1="s=5" -v var2="r=9" '
{
count++
for(i=1;i<=NF;i++){
outputFile="file"i".txt"
if(count==1){
print (var1 ORS var2) > (outputFile)
}
print $i > (outputFile)
}
}
' Input_file
In case you can have more than 5 or more columns then better close output files kin backend using close option, use this then(to avoid error too many files opened).
awk -v var1="s=5" -v var2="r=9" '
{
count++
for(i=1;i<=NF;i++){
outputFile="file"i".txt"
if(count==1){
print (var1 ORS var2) > (outputFile)
}
print $i >> (outputFile)
}
close(outputFile)
}
' Input_file
Pretty simple to do in one pass through the file with awk using its output redirection:
awk 'NR==1 { for (n = 1; n <= NF; n++) print "s=5\nr=9" > ("file_" n) }
{ for (n = 1; n <= NF; n++) print $n > ("file_" n) }' inp_file
With GNU awk to internally handle more than a dozen or so simultaneously open files:
NR == 1 {
for (i=1; i<=NF; i++) {
out[i] = "file" i ".txt"
print "s=5" ORS "r=9" > out[i]
}
}
{
for (i=1; i<=NF; i++) {
print $i > out[i]
}
}
or with any awk just close them as you go:
NR == 1 {
for (i=1; i<=NF; i++) {
out[i] = "file" i ".txt"
print "s=5" ORS "r=9" > out[i]
close(out[i])
}
}
{
for (i=1; i<=NF; i++) {
print $i >> out[i]
close(out[i])
}
}
split -nr/$(wc -w <(head -1 input) | cut -d' ' -f1) -t' ' --additional-suffix=".txt" -a4 --numeric-suffix=1 --filter "cat <(echo -e 's=5 r=9') - | tr ' ' '\n' >\$FILE" <(tr -s '\n' ' ' <input) file
This uses the nifty split command in a unique way to rearrange the columns. Hopefully it's faster than awk, although after spending a considerable amount of time coding it, testing it, and writing it up, I find that it may not be scalable enough for you since it requires a process per column, and many systems are limited in user processes (check ulimit -u). I submit it though because it may have some limited learning usefulness, to you or to a reader down the line.
Decoding:
split -- Divide a file up into subfiles. Normally this is by lines or by size but we're tweaking it to use columns.
-nr/$(...) -- Use round-robin output: Sort records (in our case, matrix cells) into the appropriate number of bins in a round-robin fashion. This is the key to making this work. The part in parens means, count (wc) the number of words (-w) in the first line (<(head -1 input)) of the input and discard the filename (cut -d' ' -f1), and insert the output into the command line.
-t' ' -- Use a single space as a record delimiter. This breaks the matrix cells into records for split to split on.
--additional-suffix=".txt" -- Append .txt to output files.
-a4 -- Use four-digit numbers; you probably won't get 1,000 files out of it but just in case ...
--numeric-suffix=1 -- Add a numeric suffix (normally it's a letter combination) and start at 1. This is pretty pedantic but it matches the example. If you have more than 100 columns, you will need to add a -a4 option or whatever length you need.
--filter ... -- Pipe each file through a shell command.
Shell command:
cat -- Concatenate the next two arguments.
<(echo -e 's=5 r=9') -- This means execute the echo command and use its output as the input to cat. We use a space instead of a newline to separate because we're converting spaces to newlines eventually and it is shorter and clearer to read.
- -- Read standard input as an argument to cat -- this is the binned data.
| tr ' ' '\n' -- Convert spaces between records to newlines, per the desired output example.
>\$FILE -- Write to the output file, which is stored in $FILE (but we have to quote it so the shell doesn't interpret it in the initial command).
Shell command over -- rest of split arguments:
<(tr -s '\n' ' ' < input) -- Use, as input to split, the example input file but convert newlines to spaces because we don't need them and we need a consistent record separator. The -s means only output one space between each record (just in case we got multiple ones on input).
file -- This is the prefix to the output filenames. The output in my example would be file0001.txt, file0002.txt, ..., file0005.txt.

changing two lines of a text file

I have a bash script which gets a text file as input and takes two parameters (Line N° one and line N° two), then changes both lines with each other in the text. Here is the code:
#!/bin/bash
awk -v var="$1" -v var1="$2" 'NR==var {
s=$0
for(i=var+1; i < var1 ; i++) {
getline; s1=s1?s1 "\n" $0:$0
}
getline; print; print s1 s
next
}1' Ham > newHam_changed.txt
It works fine for every two lines which are not consecutive. but for lines which follows after each other (for ex line 5 , 6) it works but creates a blank line between them. How can I fix that?
I think your actual script is not what you posted in the question. I think the line with all the prints contains:
print s1 "\n" s
The problem is that when the lines are consecutive, s1 will be empty (the for loop is skipped), but it will still print a newline before s, producing a blank line.
So you need to make that newline conditional.
awk -v var="4" -v var1="6" 'NR==var {
s=$0
for(i=var+1; i < var1 ; i++) {
getline; s1=s1?s1 "\n" $0:$0
}
getline; print; print (s1 ? s1 "\n" : "") s
next
}1' Ham > newHam_changed.txt
Using getline makes awk scripts always a bit complicated. It is better to prevent the use of getline and just make use of the awk pattern { action } syntax. This will make perfectly readable scripts. In any other language you would just do a loop and get the next line, but in awk I think it is best to make good use of this feature.
awk -v var="$1" -v var1="$2" '
NR==var {s=$0; collect=1; next;}
NR==var1 {collect=0; print; printf inbetween; print s}
collect {inbetween=inbetween""$0"\n"; next;}
1' Ham
Here I capture the first line in s when I found it and set the collect flag. This will trigger the collect block on the next iteration which collects all lines in between. Whenever the second line is found it sets the collect back to zero and prints first the current line, than the inbetween lines and then s. If the lines are consecutive inbetween is empty and printf will than do nothing.
Too complex for my taste, here is something quite simple that achieves the same task:
#!/bin/bash
ORIGFILE='original.txt' # original text file
PROCFILE='processed.txt' # copy of the original file to be proccesed
CHGL1=`sed "$1q;d" $ORIGFILE` # get original $1 line
CHGL2=`sed "$2q;d" $ORIGFILE` # get original $2 line
`cat $ORIGFILE > $PROCFILE`
sed -i "$2s/^.*/$CHGL1/" $PROCFILE # replace
sed -i "$1s/^.*/$CHGL2/" $PROCFILE # replace
More code doesn't mean more useful, keep it simple. This code do not use for and instead goes directly to the specific lines.
EDIT:
A simple way on one line to do this task:
printf '%s\n' 14m26 26-m14- w q | ed -s file
Found in this answer.

bash script and awk to sort a file

so I have a project for uni, and I can't get through the first exercise. Here is my problem:
I have a file, and I want to select some data inside of it and 'display' it in another file. But the data I'm looking for is a little bit scattered in the file, so I need several awk commands in my script to get them.
Query= fig|1240086.14.peg.1
Length=76
Score E
Sequences producing significant alignments: (Bits) Value
fig|198628.19.peg.2053 140 3e-42
> fig|198628.19.peg.2053
Length=553
Here on the picture, you can see that there are 2 types of 'Length=', and I only want to 'catch' the "Length=" that are just after a "Query=".
I have to use awk so I tried this :
awk '{if(/^$/ && $(NR+1)/^Length=/) {split($(NR+1), b, "="); print b[2]}}'
but it doesn't work... does anyone have an idea?
You need to understand how Awk works. It reads a line, evaluates the script, then starts over, reading one line at a time. So there is no way to say "the next line contains this". What you can do is "if this line contains, then remember this until ..."
awk '/Query=/ { q=1; next } /Length/ && q { print } /./ { q=0 }' file
This sets the flag q to 1 (true) when we see Query= and then skips to the next line. If we see Length and we recently saw Query= then q will be 1, and so we print. In other cases, set q back to "not recently seen" on any non-empty line. (I put in the non-empty condition to allow for empty lines anywhere without affecting the overall logic.)
awk solution:
awk '/^Length=/ && r~/^Query/{ sub(/^[^=]+=/,""); printf "%s ",$0 }
NF{ r=$0 }END{ print "" }' file
NF{ r=$0 } - capture the whole non-empty line
/^Length=/ && r~/^Query/ - on encountering Length line having previous line started with Query(ensured by r~/^Query/)
It sounds like this is what you want for the first part of your question:
$ awk -F'=' '!NF{next} f && ($1=="Length"){print $2} {f=($1=="Query")}' file
76
but idk what the second part is about since there's no "data" lines in your input and only 1 valid output from your sample input best I can tell.

Select lines having count greater than x and less than y using linux command

I have the following file:
1,A
2,B
3,C
10000,D
20,E
4000,F
I want to select the lines having a count greater than 10 and less than 5000. the output should be E and F. In C++ or any other language is a piece of cake. I really wanted to know how can I do it with a linux command.
I tried the following command
awk -F ',' '{$1 >= 10 && $1 < 5000} { count++ } END { print $1,$2}' test.txt
But it is only givine 4000,F.
just do:
awk -F',' '$1 >= 10 && $1 < 5000' test.txt
you put boolean check in {....}, and don't use the result at all. it doesn't make any sense. You should do either {if(...) ...} or booleanExpression{do...}
useless count++
you have only print statement in END so only last line was printed out.
Your script does actually:
print the last line of the test.txt, no matter what it is.

extracting first line from file command such that

I have a file with almost 5*(10^6) lines of integer numbers. So, my file is big enough.
The question is all about extract specific lines, filtering them by a condition.
For example, I'd like to:
Extract the N first lines without read entire file.
Extract the lines with the numbers less or equal X (or >=, <=, <, >)
Extract the lines with a condition related a number (math predicate)
Is there a cleaver way to perform these tasks? (using sed or awk or cat or head)
Thanks in advance.
To extract the first $NUMBER lines,
head -n $NUMBER filename
Assuming every line contains just a number (although it will also work if the first token is one), 2 can be solved like this:
awk '$1 >= 1234 && $1 < 5678' filename
And keeping in spirit with that, 3 is just the extension
awk 'condition' filename
It would have helped if you had specified what condition is supposed to be, though. This way, you'll have to read the awk documentation to find out how to code it. Again, the number will be represented by $1.
I don't think I can explain anything about the head call, it's really just what it says on the tin. As for the awk lines: awk, like sed, works linewise. awk fetches lines in a loop and applies your code to each line. This code takes the form
condition1 { action1 }
condition2 { action2 }
# and so forth
For every line awk fetches, the conditions are checked in the order they appear, and the associated action to each condition is performed if the condition is true. It would, for example, have been possible to extract the first $NUMBER lines of a file with awk like this:
awk -v number="$NUMBER" '1 { print } NR == number { exit }' filename
where 1 is synonymous with true (like in C) and NR is the line number. The -v command line option initializes the awk variable number to $NUMBER. If no action is specified, the default action is { print }, which prints the whole line. So
awk 'condition' filename
is shorthand for
awk 'condition { print }' filename
...which prints every line where the condition holds.

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