How can I grep a list of names from case? - bash

Question

So as an example, I have a bunch of apps that are constantly writing to /var/log/app//nonsence.file there's nothing else those folders, just logs from this one set of apps. so I can easily do:
cat /var/log/app/*/nonsence.file
and I'll get a nice stream of the app logs.
Mixed into this stream are periodic references to people. I'd like to build a script to trigger when certain names appear in the stream.
I can do this easily enough:
cat /var/log/app/*/nonsence.file | grep 'greg|john|suzy|stacy'
and I can put THAT into a simple script thusly:
#!/bin/sh
NAME=`cat /var/log/app/*/nonsence.file | grep 'greg\|john\|suzy\|stacy'`
case "$NAME" in
"greg" ) echo "I found greg!" >> ~/names.meh ;;
"john" ) echo "I found john!" >> ~/names.meh ;;
"suzy" ) echo "I found suzy!" >> ~/names.meh ;;
"stacy" ) echo "I found stacy!" >> ~/names.meh ;;
* ) echo "forever alone..." >> ~/names.meh ;;
esac
easy peasy!
the trouble is, the list of names change from time to time and I would really like a neater list.
After some thinking I believe what I REALLY want to do is add each name into the case section only. so what do I need to do in the NAME variable section to tell the command to grep the name referenced in the case section?

Answer 1

cat file | grep is a useless use of cat. Just grep file.
Command in a pipe are by default block buffered.
The >> ~/names.meh is just repetition. Just specify it once for the whole block.
The backticks ` are discouraged. It's preferred to use $(..) instead.
Each time NAME=... is assigned the file is read, while you seem to want to want:
... I'd like to build a script to trigger when certain names appear in the stream.
which suggest you want to react when the name appears in the script, not after some time.
You may try:
patterns=(greg john suzy stacy)
printf "%s\n" /var/log/app/*/nonsence.file |
# tail each file at the same time by spawning for each a background process
xargs -P0 -n1 tail -F -n+1 |
# grep for the patterns
# pass the patterns from a file
# the <(...) is a process substitution, a bash extension
grep --line-buffered -f <(printf "%s\n" "${patterns[#]}") -o |
# for each grepped content execute different action
while IFS= read -r line; do
case "$line" in)
"greg") someaction; ;;
# etc
*) echo "Internal error - unhandled pattern"; ;;
esac
done >> ~/names.me
Because specyfing patterns twice is lame, you could do an associative function to map the patterns to function names, or just use unique function names and geenerate from them the pattern list:
pattern_greg() { echo "greg"; }
pattern_kamil() { echo "well, not greg"; }
patterns=($(declare -F | sed 's/declare -f //; /^pattern_/!d; s/pattern_//'))
... |
while IFS= read -r line; do
if declare -f pattern_"$line" >/dev/null 2>&1; then
pattern_"$line"
else
echo "Internal error occured"
fi
done
alternatively, but I like the functions better:
greg_function() { echo do something; }
kamil_callback() { echo do something else; }
declare -A patterns
patterns=([greg]=greg_function [kamil]=kamil_callback)
... | grep -f <(printf "%s\n" ${!patterns[#]}) ... |
while IFS= read -r line; do
# I think this is how to check if array element is set
if [[ -n "${patterns[$line]}" ]]; then
"${patterns[$line]}"
else
echo error
fi
done