sed -n '1,/Copying/p' bldlinux.log | tail -3
x=`awk '/Copying/{print NR-2","NR}' bldlinux.log `
sed -n "$x p" bldlinux.log
awk '{a[++i]=$0;}/Copying/{for(j=NR-2;j<=NR;j++)print a[j];}' bldlinux.log
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
Different ways to print the next few lines after pattern match
grep, awk or a sed command is used to print the line matching a particular pattern. However, at times, we need to print a few more lines following the lines matching the pattern. In this article, we will see the different ways in which we can get this done. The first part explains ways to print one line following the pattern along with the pattern, few examples to print only the line following the pattern and in the end we have ways to print multiple lines following the pattern.
Let us consider a file with the following contents as shown below:
$ cat file
Unix
Linux
Solaris
AIX
SCO
1. The simplest is using the grep command. In
GNU grep, there is an option -A which prints lines following the pattern.
$ grep -A1 Linux file
Linux
Solaris
In the above example, -A1 will print one line following the pattern along with the line matching the pattern. To print 2 lines after the pattern, it is -A2.
2.
sed has the N command which will read the next line into the pattern space.
$ sed -n '/Linux/{N;p}' file
Linux
Solaris
First, the line containing the pattern
/Linux/ is found. The command within the braces will run on the pattern found.
{N;p} means read the next line and print the pattern space which now contains the current and the next line. Similarly, to print 2 lines, you can simply put:
{N;N;p}. Example 7 onwards explains for printing multiple lines following the pattern.
3.
awk has the getline command which reads the next line from the file.
$ awk '/Linux/{print;getline;print;}' file
Linux
Solaris
Once the line containing the pattern
Linux is found, it is printed. getline command reads the next line into $0. Hence, the second print statement prints the next line in the file.
4. In this, the same thing is achieved using only one print statement.
$ awk '/Linux/{getline x;print $0 RS x;}' file
Linux
Solaris
getline x reads the next line into variable x. x is used in order to prevent the getline from overwriting the current line present in $0. The print statement prints the current line($0), record separator(RS) which is the newline, and the next line which is in x.
5. To
print only the line following the pattern without the line matching the pattern:
$ sed -n '/Linux/{n;p}' file
Solaris
The
n command reads the next line into the pattern space thereby overwriting the current line. On printing the pattern space using the p command, we get the next line printed.
6. Same using awk:
$ awk '/Linux/{getline;print;}' file
Solaris
Multiple lines after the pattern:
GNU grep may not available in every Unix box. Excluding grep option, the above examples are good only to print a line or two following the pattern. Say, if you have to print some 10 lines after the pattern, the command will get clumsy. Let us now see how to print
n lines following the pattern along with the pattern:
7. To
print multiple(2) lines following the pattern using awk:
$ awk '/Linux/{x=NR+2}(NR<=x){print}' file
Linux
Solaris
Aix
To print 5 lines after the pattern, simply replace the number 2 with 5. This above example is a little tricky. Once the pattern Linux is found, x is calculated which is current line number(NR) plus 2. So, we will print lines from the current line till the line number(NR) reaches x.
8. To
print 2 lines following the pattern
without the line matching the pattern:
$ awk '/Linux/{x=NR+2;next}(NR<=x){print}' file
Solaris
Aix
The
next command makes the current line, which is the pattern matched, to get skipped. In this way, we can exclude the line matching the pattern from getting printed.
9. To
print 2 lines following the pattern along with the pattern matching line in another way.
$ x=`grep -n Linux file | cut -f1 -d:`
$ awk -v ln=$x 'NR>=ln && NR<=ln+2' file
Using grep and cut command, the line number of the pattern in the file is found. By
passing the shell variable to awk, we make it print only those lines whose line number is between x and x+2.
10. One more way using sed and awk combination. First we calculate the from and to line numbers in the variables x and y. Using
sed printing range of lines, we can get the same. sed can not only deal with numbers, it can work with variables as well:
$ x=`awk '/Linux/{print NR}' file`
$ y=`awk '/Linux/{print NR+2}' file`
$ sed -n "$x,$y p" file OR
$ x=`awk '/Linux/{print NR+2}' file`
$ sed -n "/Linux/,$x p" file
- See more at: http://www.theunixschool.com/2012/05/different-ways-to-print-next-few-lines.html#sthash.9JcTMhUL.dpuf
Different ways to print the next few lines after pattern match
grep, awk or a sed command is used to print the line matching a particular pattern. However, at times, we need to print a few more lines following the lines matching the pattern. In this article, we will see the different ways in which we can get this done. The first part explains ways to print one line following the pattern along with the pattern, few examples to print only the line following the pattern and in the end we have ways to print multiple lines following the pattern.
Let us consider a file with the following contents as shown below:
$ cat file
Unix
Linux
Solaris
AIX
SCO
1. The simplest is using the grep command. In
GNU grep, there is an option -A which prints lines following the pattern.
$ grep -A1 Linux file
Linux
Solaris
In the above example, -A1 will print one line following the pattern along with the line matching the pattern. To print 2 lines after the pattern, it is -A2.
2.
sed has the N command which will read the next line into the pattern space.
$ sed -n '/Linux/{N;p}' file
Linux
Solaris
First, the line containing the pattern
/Linux/ is found. The command within the braces will run on the pattern found.
{N;p} means read the next line and print the pattern space which now contains the current and the next line. Similarly, to print 2 lines, you can simply put:
{N;N;p}. Example 7 onwards explains for printing multiple lines following the pattern.
3.
awk has the getline command which reads the next line from the file.
$ awk '/Linux/{print;getline;print;}' file
Linux
Solaris
Once the line containing the pattern
Linux is found, it is printed. getline command reads the next line into $0. Hence, the second print statement prints the next line in the file.
4. In this, the same thing is achieved using only one print statement.
$ awk '/Linux/{getline x;print $0 RS x;}' file
Linux
Solaris
getline x reads the next line into variable x. x is used in order to prevent the getline from overwriting the current line present in $0. The print statement prints the current line($0), record separator(RS) which is the newline, and the next line which is in x.
5. To
print only the line following the pattern without the line matching the pattern:
$ sed -n '/Linux/{n;p}' file
Solaris
The
n command reads the next line into the pattern space thereby overwriting the current line. On printing the pattern space using the p command, we get the next line printed.
6. Same using awk:
$ awk '/Linux/{getline;print;}' file
Solaris
Multiple lines after the pattern:
GNU grep may not available in every Unix box. Excluding grep option, the above examples are good only to print a line or two following the pattern. Say, if you have to print some 10 lines after the pattern, the command will get clumsy. Let us now see how to print
n lines following the pattern along with the pattern:
7. To
print multiple(2) lines following the pattern using awk:
$ awk '/Linux/{x=NR+2}(NR<=x){print}' file
Linux
Solaris
Aix
To print 5 lines after the pattern, simply replace the number 2 with 5. This above example is a little tricky. Once the pattern Linux is found, x is calculated which is current line number(NR) plus 2. So, we will print lines from the current line till the line number(NR) reaches x.
8. To
print 2 lines following the pattern
without the line matching the pattern:
$ awk '/Linux/{x=NR+2;next}(NR<=x){print}' file
Solaris
Aix
The
next command makes the current line, which is the pattern matched, to get skipped. In this way, we can exclude the line matching the pattern from getting printed.
9. To
print 2 lines following the pattern along with the pattern matching line in another way.
$ x=`grep -n Linux file | cut -f1 -d:`
$ awk -v ln=$x 'NR>=ln && NR<=ln+2' file
Using grep and cut command, the line number of the pattern in the file is found. By
passing the shell variable to awk, we make it print only those lines whose line number is between x and x+2.
10. One more way using sed and awk combination. First we calculate the from and to line numbers in the variables x and y. Using
sed printing range of lines, we can get the same. sed can not only deal with numbers, it can work with variables as well:
$ x=`awk '/Linux/{print NR}' file`
$ y=`awk '/Linux/{print NR+2}' file`
$ sed -n "$x,$y p" file OR
$ x=`awk '/Linux/{print NR+2}' file`
$ sed -n "/Linux/,$x p" file
- See more at: http://www.theunixschool.com/2012/05/different-ways-to-print-next-few-lines.html#sthash.9JcTMhUL.dpuf
Different ways to print the next few lines after pattern match
http://www.theunixschool.com/2012/05/different-ways-to-print-next-few-lines.html
grep, awk or a sed command is used to print the line matching a particular pattern. However, at times, we need to print a few more lines following the lines matching the pattern. In this article, we will see the different ways in which we can get this done. The first part explains ways to print one line following the pattern along with the pattern, few examples to print only the line following the pattern and in the end we have ways to print multiple lines following the pattern.
Let us consider a file with the following contents as shown below:
$ cat file
Unix
Linux
Solaris
AIX
SCO
1. The simplest is using the grep command. In GNU grep, there is an option -A which prints lines following the pattern.
$ grep -A1 Linux file
Linux
Solaris
In the above example, -A1 will print one line following the pattern along with the line matching the pattern. To print 2 lines after the pattern, it is -A2.
2. sed has the N command which will read the next line into the pattern space.
$ sed -n '/Linux/{N;p}' file
Linux
Solaris
First, the line containing the pattern /Linux/ is found. The command within the braces will run on the pattern found. {N;p} means read the next line and print the pattern space which now contains the current and the next line. Similarly, to print 2 lines, you can simply put: {N;N;p}. Example 7 onwards explains for printing multiple lines following the pattern.
3. awk has the getline command which reads the next line from the file.
$ awk '/Linux/{print;getline;print;}' file
Linux
Solaris
Once the line containing the pattern Linux is found, it is printed. getline command reads the next line into $0. Hence, the second print statement prints the next line in the file.
4. In this, the same thing is achieved using only one print statement.
$ awk '/Linux/{getline x;print $0 RS x;}' file
Linux
Solaris
getline x reads the next line into variable x. x is used in order to prevent the getline from overwriting the current line present in $0. The print statement prints the current line($0), record separator(RS) which is the newline, and the next line which is in x.
5. To print only the line following the pattern without the line matching the pattern:
$ sed -n '/Linux/{n;p}' file
Solaris
The n command reads the next line into the pattern space thereby overwriting the current line. On printing the pattern space using the p command, we get the next line printed.
6. Same using awk:
$ awk '/Linux/{getline;print;}' file
Solaris
Multiple lines after the pattern:
GNU grep may not available in every Unix box. Excluding grep option, the above examples are good only to print a line or two following the pattern. Say, if you have to print some 10 lines after the pattern, the command will get clumsy. Let us now see how to print n lines following the pattern along with the pattern:
7. To print multiple(2) lines following the pattern using awk:
$ awk '/Linux/{x=NR+2}(NR<=x){print}' file
Linux
Solaris
Aix
To print 5 lines after the pattern, simply replace the number 2 with 5. This above example is a little tricky. Once the pattern Linux is found, x is calculated which is current line number(NR) plus 2. So, we will print lines from the current line till the line number(NR) reaches x.
8. To print 2 lines following the pattern without the line matching the pattern:
$ awk '/Linux/{x=NR+2;next}(NR<=x){print}' file
Solaris
Aix
The next command makes the current line, which is the pattern matched, to get skipped. In this way, we can exclude the line matching the pattern from getting printed.
9. To print 2 lines following the pattern along with the pattern matching line in another way.
$ x=`grep -n Linux file | cut -f1 -d:`
$ awk -v ln=$x 'NR>=ln && NR<=ln+2' file
Using grep and cut command, the line number of the pattern in the file is found. By passing the shell variable to awk, we make it print only those lines whose line number is between x and x+2.
10. One more way using sed and awk combination. First we calculate the from and to line numbers in the variables x and y. Using sed printing range of lines, we can get the same. sed can not only deal with numbers, it can work with variables as well:
$ x=`awk '/Linux/{print NR}' file`
$ y=`awk '/Linux/{print NR+2}' file`
$ sed -n "$x,$y p" file
OR
$ x=`awk '/Linux/{print NR+2}' file`
$ sed -n "/Linux/,$x p" file
- See more at: http://www.theunixschool.com/2012/05/different-ways-to-print-next-few-lines.html#sthash.9JcTMhUL.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
In this article, we will discuss the
different ways for how to print a few lines before a pattern match in log files. You will do well to know how to print a
few lines AFTER a pattern match.
Let us consider a sample file as below. The requirement is to print 2 lines before the pattern 'Linux':
$ cat file
Unix
AIX
Solaris
Linux
SCO
1. grep will do it for you if your grep is a GNU grep.
$ grep -B2 Linux file
AIX
Solaris
Linux
The option '-B' will give n lines before the pattern. Hence '-B2 Linux' gives 2 lines before the pattern 'Linux' including the line matching the pattern.
2. awk way:
$ awk '/Linux/{for(i=1;i<=x;)print a[i++];print}
> {for(i=1;i<x;i++)a[i]=a[i+1];a[x]=$0;}' x=2 file
AIX
Solaris
Linux
In this, an array is used which will always contain 2 lines before the current pattern. Hence, when the pattern is matched, the array contents along with the current pattern is printed.
3. Perl way:
$ perl -ne 'push @arr,$_; shift @arr if(@arr>3);
> /Linux/ and print @arr;' file
AIX
Solaris
Linux
On reading a line, the line is pushed into an array named "arr". As and when the array size goes beyond 3, an element is removed from the front. In this way, the array will always contain 3 lines including the current pattern. On encountering the pattern 'Linux', the array is printed.
4. sed and tail combination:
$ sed -n '1,/Linux/p' file | tail -3
AIX
Solaris
Linux
Using sed, a
range of lines can be read. sed reads from 1st line till the pattern 'Linux'. From this result, we print the last 3 lines which is nothing but the line containing the pattern and 2 lines before the pattern.
5. awk and sed combination:
$ x=`awk '/Linux/{print NR-2","NR}' file`
$ sed -n "$x p" file
AIX
Solaris
Linux
Using awk, the line number range is retrieved using the pattern 'Linux'. NR gives the current line number which is the number of the line containing the pattern, NR-2 gives the gives the number of the line which is 2 lines before. Using this
line range in sed, the set of lines can be filtered.
6. Another awk way, only if the file is a small one:
$ awk '{a[++i]=$0;}/Linux/{for(j=NR-2;j<=NR;j++)print a[j];}' file
AIX
Solaris
Linux
In this, awk stores the entire file in memory using an array. On encountering the pattern 'Linux', it starts printing from 2 lines before using the for loop. Since awk stores the entire file in memory, this SHOULD not be used in case of large files since it will cause performance issues.
7. tac command (The least favored option)
$ tac file | grep -A2 Linux | tac
AIX
Solaris
Linux
tac command is reverse of cat command which prints the file in reverse order. On the reversed file, grep for 2 lines above the pattern, and again reverse the output and display. This is just an option, not the preferred one due to its performance. tac command itself is not good for performance, add to that, using it twice.
Note: tac command is not available in all flavors of Unix. - See more at: http://www.theunixschool.com/2012/08/print-lines-before-pattern-linux.html#sthash.hDKNCoh7.dpuf
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