converting hex/decimal/bianry with bash

put this functions in the ~/.bashrc

# convert a number into decimal format
n2d() {
perl -e "printf \"%d\n\", $@"
}

# convert a number into hex format
n2h() {
perl -e "printf \"0x%X\n\", $@"
}

# convert a number into binary format
n2b() {
perl -e "printf \"0b%b\n\", $@"
}

Interesting set intersection by using 'uniq'

I was asked to find the intersection of two IP lists yesterday. I write a simple program with Perl. The program iterates the two lists to find the repeated IPs, which is a O(n^2) algorithm.

foreach $a (@A) {
    foreach $b (@B) {
        if ($a eq $b) {
        ...
        }
    }
}

When I wake up this morning, another way to do this pops onto my mind.

cat file-A | sort | uniq > tmp
cat file-B | sort | uniq >> tmp
cat tmp | sort | uniq -d

Basically, it is a O(nlog(n)) algorithm. (the sort operation).

kill all child process

killtree () {
for child in $(ps -o pid= --ppid $1)
do
killtree $child
done
echo "kill -9 $1"
kill -9 $1 2>/dev/null
}
killtree {some pid}

read line in bash script

It is very easy to read "words" in a bash script. But what if you want to read a line in a text file? I have this problem today, After some googling, I found a interesting way to do this.

while read l
do
echo $l
done < xxx.txt

It use the xxx.txt as input, and the read command in bash will read the input line by line.