如何使用 sed 或 awk 执行“大于”以删除特定行？

Question

as tittle, how sed perform "greater than" to delete specific line?
I got a file like this:

bash-4.2$ cat testfile.txt
string1 1 1
AAA 2 2
string2 2 2
BBB 3 3
string3 3 3
string4 4 4
string5 5 5
string6 6 6
CCC 6 6
string7 7 7
string8 8 8
string9 9 9
string10 10 10
DDD 11 11
string11 11 11
string12 12 12
string13 13 13

I wanna delete some lines which contains "string[[:digit:]]" but string1 to string"$num" is needed, num is defined by a variable. For example, I wanna keep those lines which contains string1-5 and delete string6-string99, I'd tried:

#!/bin/bash
read -p "Please Assign the Number of String Line that You Wanna Keep: " num
cat testfile.txt | sed -e "/string[`expr $num + 1`-9]/d" > new_testfile.txt

but it's only working in range 0-8, is there any way to perform it with sed or awk?

Answer 1

This 'awk` should do:

awk '/^string/ {n=substr($1,7)+0;if (n>5 && n<100) next}1' file
string1 1 1
AAA 2 2
string2 2 2
BBB 3 3
string3 3 3
string4 4 4
string5 5 5
CCC 6 6
DDD 11 11

It just skips any line with string"x" where x is larger then 5 and less then 100

---

If high/low comes from variables, this should do:

high=99
low=6
awk '/^string/ {n=substr($1,7)+0;if (n>=l && n<=h) next}1' h="$high" l="$low" file
string1 1 1
AAA 2 2
string2 2 2
BBB 3 3
string3 3 3
string4 4 4
string5 5 5
CCC 6 6
DDD 11 11

Answer 2

Here is one way with awk :

$ read -p "Please Assign the Number of String Line that You Wanna Keep: " num
Please Assign the Number of String Line that You Wanna Keep: 5
$ awk -v max="$num" '/string/{line=$0;sub(/string/,"",$1);if($1+0<=max){print line};next}1' file
string1 1 1
AAA 2 2
string2 2 2
BBB 3 3
string3 3 3
string4 4 4
string5 5 5
CCC 6 6
DDD 11 11

Answer 3

This is an old question, but if you are coming from a duplicate, perhaps the important thing to understand is that sed does not have any facilities for arithmetic, which is why all the old answers here use Awk.

If you can articulate a regex which reimplements your mathematical constraint as a textual constraint, these things are possible;

sed 's/string\([1-9][0-9]\+\|[6-9]\)//' testfile.txt

To briefly spell this out, this finds and replaces string if it is followed by two or more digits, or a single digit which is 6-9 , which implements the requirement by matching the digits as string sequences.

GNU sed also has a limited facility for executing external commands on the matched text with the non-standard flag /e , but my advice would be to switch to Awk at that point anyway, which allows you to reason about mathematical properties of numbers with a more readable and beginner-friendly syntax as well as vastly better efficiency by way of avoiding to spawn an external process for each expression you want to evaluate.