So I'm currently trying to reverse-engineer a batch of scripts passed down from our last SysAdmin.
There's a line in one of the shell scripts that I (for the life of me) am unable to understand. Here it is:-
sed -n 'H;${x;s/^\n//;s/-jar\n/Dcom.sun.management.jmxremote.ssl=false\n&/;p;}' "$File"
GNU Bash 4.2.46(2) RHEL6 64bit.
The command requires GNU sed and seems to insert the string Dcom.sun.management.jmxremote.ssl=false followed by a newline before the first occurance of -jar at the end of a line.
Example:
something
something
something-jar
something
is transformed into
something
something
somethingDcom.sun.management.jmxremote.ssl=false
-jar
something
The sed command reads the whole file into memory by running H for each line of the file (appends the current line into the "hold space" with a newline character at the start). When it reaches the last line ($), it swaps the hold space with the pattern space (x) and deletes the first newline (put there by running H for the first line of the file). It then substitutes -jar\n with Dcom.sun.management.jmxremote.ssl=false\n-jar\n (the & is whatever was matched by the expression). It then prints the collected lines (p).
Another way of doing the same thing with awk:
awk 'skip==0 && sub("-jar$", "Dcom.sun.management.jmxremote.ssl=false\n-jar") { skip=1 } { print }' "$File"
This tries to do the same substitution on each line, and when it succeeds it sets skip=1 which will cause it to not try again. Every line is printed regardless of whether a substitution occurred or not.
This does not store the whole file in memory.
