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bash_linux.md

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Bash&Linux

Mostly linux-specific stuff.

Sections

Text processing

  • Print field 3 if it starts from some_string:

    $ awk '$3 ~ /^ *some_string/ {print $3}'

  • Get unique IPs connected to local port 2222:

    $ netstat -nap | grep 2222 | awk '{print $5}' | cut -d: -f1 | sort -u

  • Get a value of the status key in a json:

    $ echo '{"jobId":"1","status":"RUNNING"}'|python3 -c 'import sys, json; print(json.load(sys.stdin)[sys.argv[1]])' status

  • Get top5 CPU consumers:

    $ ps aux | sort -nrk 3,3 | head -n 5

  • Get environment of running process:

    $ tr '\0' '\n' < /proc/<pid>/environ

  • Print specific line:

    $ sed -n -e 192997p access.log

  • Add line numbers:

    $ cat access.log |awk '{ print NR,$(NF-1)}'|sort -k 2 > sorted.txt

  • Remove newlines:

    $ tr '\n' ' ' < test.txt

  • See connection number status:

    $ netstat -ant| awk '{print $6}' | sort | uniq -c | sort -n
$ netstat -ant| awk '{print $5}' |cut -d ':' -f1 | sort | uniq -c | sort -n

  • Get top 10 residential memory consumers with RSS column in Mb:

    $ ps aux --sort +rss | tail -10|awk 'NR==1 {o=$0; a=match($0,$11);}; NR>1 {o=$0;$6=int(10*$5/1024)/10"M";}{ printf "%-8s %6s %-5s %-5s %9s %9s %-8s %-4s %-6s %-5s %s\n", $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, substr(o, a);}'

GDB magic

  • Dump nginx config from running process:

    # Set pid of nginx master process here
pid=1234
# generate gdb commands from the process's memory mappings using awk
cat /proc/$pid/maps | awk '$6 !~ "^/" {split ($1,addrs,"-"); print "dump memory mem_" addrs[1] " 0x" addrs[1] " 0x" addrs[2] ;}END{print "quit"}' > gdb-commands
# use gdb with the -x option to dump these memory regions to mem_* files
gdb -p $pid -x gdb-commands
# look for some (any) nginx.conf text
grep worker_connections mem_*
grep server_name mem_*

  • Get stdout of running process:

    $ gdb -p <PID_OF_CAT_PROCESS> /bin/cat
(gdb) p close(1)
$1 = 0
(gdb) p creat("/tmp/foo3", 0600)
$2 = 1
(gdb) q

Misc

  • Get inotify watches consumers:

    $ lsof -K | grep inotify | (less||more||pg)

    hotfix:

    $ echo 1048576 > /proc/sys/fs/inotify/max_user_watches

  • Bash for-loop cpp-style

    for ((x=; x<=; x++))
{
  echo $x
}

  • Ad-hoc network speed test:

    # myserver.local.net
nc -v -v -l -n 2222 >/dev/null
# client
dd if=/dev/zero bs=1024K count=512 | nc -v -v -n myserver.local.net 2222 >/dev/null

  • Send a file over netcat # server nc -l -p 1234 -q 1 > something.zip < /dev/null # client cat something.zip | netcat server.ip.here 1234

  • Simulate packet loss with iptables:

    # for randomly dropping 10% of incoming packets:
iptables -A INPUT -m statistic --mode random --probability 0.1 -j DROP
# and for dropping 10% of outgoing packets:
iptables -A OUTPUT -m statistic --mode random --probability 0.1 -j DROP

  • Get your local sshd host fingerprint:

    $ ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub
$ ssh-keygen -E md5 -lf /etc/ssh/ssh_host_ed25519_key.pub

  • Get your ssh pubkey fingerprint:

    $ ssh-keygen -l -E sha256 -f ~/.ssh/id_rsa.pub

  • Watch with pipe (memcached connections example):

    $ watch -n 1 'lsof -i :11211|wc -l'

  • Get a timestamp from dmesg for time 600711.395348:

    $ ut=`cut -d' ' -f1 </proc/uptime` 
$ ts=`date +%s` 
$ date -d "70-1-1 + $ts sec - $ut sec + 600711.395348 sec" +"%F %T"