mjhjmtu

I want to print 3 lines above a matched parameter in a log file. I am using following command, but it gives me an error.
Is there any alternative to this.

grep -A 3 "exception" Services.log

It gives the following error:

grep: Not a recognized flag: A
Usage: grep [-r] [-R] [-H] [-L] [-E|-F] [-c|-l|-q] [-insvxbhwyu] [-p[parasep]] -e pattern_list...
 [-f pattern_file...] [file...]

The following awk command would give you the line that contains the string exception along with three lines of "before context" (-B 3 with GNU grep and some other grep implementations):

awk 'BEGIN bc=3 lines[NR%(bc+1)] = $0 /exception/ for (i=1; i<=(bc+1); ++i) print lines[(NR+i)%(bc+1)] ' file

This keeps a "circular buffer", lines, of bc+1 lines, where bc is the number of lines of "before context" you want. When a line matches the pattern exception, the contents of this buffer is printed.

This does not properly handle the case where a match occurs within the "before context" of another match, or where the first match in the file occurs less than bc lines into the file.

Generalized into a script that gives you a configurable amount of context before and after for some pattern:

#!/bin/sh

# Usage:
# ./script [ -A n ] [ -B n ] PATTERN FILE ...

after=0
before=0

while getopts 'A:B:' opt; do
 case $opt in
 A)
 after=$OPTARG
 ;;
 B)
 before=$OPTARG
 ;;
 *)
 echo 'error in command line parsing' >&2
 exit 1
 esac
done
shift "$(( OPTIND - 1 ))"

pattern=$1
shift

pattern=$pattern awk -v bc="$before" -v ac="$after" '
 lines[NR%(bc+1)] = $0 
 $0 ~ ENVIRON["pattern"] 
 for (i=1; i<=(bc+1); ++i) print lines[(NR+i)%(bc+1)]
 print_after=ac
 next
 
 print_after > 0 print; print_after-- ' "$@"

Testing it:

$ cat file
1
2
3
4
5
exception
6
7
8
9
0
exception

$ sh script.sh -B 3 exception file
3
4
5
exception
8
9
0
exception

$ sh script.sh -A 3 exception file
exception
6
7
8
exception

$ sh script.sh -A 1 -B 1 exception file
5
exception
6
0
exception

Simple but not necessarily efficient:

tac Services.log | awk '/exception/ L = NR + 4 NR < L' | tac