Consider a scenario where we are having a huge
number of lines in a text file. We want to do some pattern matching and need to
delete that pattern matched line as well as some lines immediately preceding
that line. In this case we can follow the below steps to achieve that using SED & AWK shell commands.
Step 1: Reverse
the order of the lines of the file.
This reversal is required as we
are going to use the “sed” to do pattern matching and deletion of lines. As
“sed” doesn’t back track, once a line is processed it is done. We cannot delete
the lines preceding the pattern matched line. By reversing the order of the lines, we can easily delete the lines
following the pattern matched line instead of preceding lines.
Step 2: Use
the “sed” to do pattern matching and to delete the required lines.
In
this operation, for each line deleted a blank line will get generated.
Step 3: Using
“awk” we can remove the empty lines introduced due to the operation performed
in step 2.
Step 4: Again
reverse the order of the lines of the file so that we can get the original file
with unwanted lines removed.
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Steps
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Command
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Purpose of command
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Step 1
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tac input_file
> output_file1
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Reversing the order of line in input file and
redirecting the output to another file “output_file1”
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Step 2
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sed –e ‘/<pattern_to_match>/,+nd’
output_file1 > output_file2
Where n = number of lines to delete following the
pattern matched line.
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This command will delete the pattern matched line
and ‘n’ number of lines following that in the file “output_file1”. The output
is redirected to the file “output_file2”
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Step 3
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awk NF output_file2 > output_file3
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This command will remove the blank lines from
“output_file2” and output is getting redirected to “output_file3”
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Step 4
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tac output_file3 > output_file4
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Reversing the order of lines in the file
“output_file3” so that we can get the original file “input_file” with
unwanted lines removed.
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Illustrative
Example:
Input File:
Objective: In
this file, we need to remove the commands which are getting failed along with
its failure message.
line 1
mac-address-table static unicast
00:00:00:00:08:01 vlan 10 interface extreme-ethernet 0/10
mac-address-table static unicast
00:00:00:00:08:02 vlan 10 interface extreme-ethernet 0/10
mac-address-table static unicast
00:00:00:00:08:03 vlan 10 interface extreme-ethernet 0/10
mac-address-table static unicast
00:00:00:00:08:04 vlan 10 interface extreme-ethernet 0/10
mac-address-table static unicast
00:00:00:00:08:05 vlan 10 interface extreme-ethernet 0/10
mac-address-table static unicast
00:00:00:00:08:06 vlan 10 interface extreme-ethernet 0/10
