Miscellanea 2020

01/22/2020

NFS network file system, list nfs port: https://serverfault.com/questions/377170/which-ports-do-i-need-to-open-in-the-firewall-to-use-nfs
1
rpcinfo -p | grep nfs
It depends on the version of the protocol you intent to use. NFS 4 only require 2049 while older versions require more.
setup nfs cluster to prevent single point of failure https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/high_availability_add-on_administration/ch-nfsserver-haaa

02/26/2020

如果不想让一个executable 执行多次，可以在每次run的时候在当前或固定文件夹create一个hidden file, for example: .commad.lock，然后写入当前正在run的command 参数等。通过检查这个file是否存在去决定是不是正在执行。
redhat 一个很不错的网站: https://www.redhat.com/sysadmin/
command uuidgen 可以用来generate random unique number.

04/07/2020

find softlink file, use type l:

1	find . -type l -name dsenv

or use -L, then the -type predicate will always match against the type of the file that a symbolic link points to rather than the link itself (unless the symbolic link is broken).

1	find -L / type f -name dsenv

类似于readlink, 会去softlink directory里面寻找original file.

04/09/2020

> /dev/null 2>&1 can be written as &> /dev/null
su vs su -. 都是login to another user, - 表示normal login，login后会完全变成当前user的初始环境，比如在当前user的home dir且$PATH也是当前user的。没有-, 则会继承上个user的环境，比如dir还是上次的。2个login都会执行~/.bashrc.

04/18/2020

在pod container中，如果script process不是init process (pid 1)，那么script中的trap 内容不会被执行。还不太清楚为什么（这是docker container的一个特性）

04/26/2020

declare -F show function name in current shell
declare -f show function definition in current shell, declare -f <f name> get just that definition

06/16/2020

很多build可以用Makiefile + make command去实现，应该是一个通用的工具。

06/21/2020

What is the difference between “#!/usr/bin/env bash” and “#!/usr/bin/bash”? Using #!/usr/bin/env NAME makes the shell search for the first match of NAME in the $PATH environment variable. It can be useful if you aren’t aware of the absolute path or don’t want to search for it.

06/24/2020

bash -x ./script, no need set -x

06/25/2020

script performance comparison strace -c ./script. 也就是说，多用bash internal command from help. -c will output show system time on each system call.

06/26/2020

Create custom systemd service: https://medium.com/@benmorel/creating-a-linux-service-with-systemd-611b5c8b91d6. Interesting. 印象里面和当时设置K8s systemd 有点类似.

07/05/2020

shopt -s nocasematch, set bash case-insensitive match in case or [[ ]] condition. 这个是从bash tutorial 中文版中学到的, shopt is bash built-in setting, unlike set is from POSIX.

07/09/2020

vim can directly operate on file in tarball: vim xx.tgz then save the changes

07/12/2020

Database SQL course online from Stanford or Edx.com

07/29/2020

!! re-execute last command.
!<beginning token> re-execute last command start with this token.
$_ last token in last command line

08/22/2020

Interesting: top 10 most used CLI: history | awk {'print $2'} | sort | uniq -c | sort -nr | head -n 10
same idea to check my blog theme: ls -ltr | awk 'NR>1 {print $NF}' | cut -d'-' -f1 | sort | uniq -c | sort

09/09/2020

Mac netstat list listening port, the netstat on Mac is not that powerful as on Linux:

## -p: protocol
## -n: numerical address
## -a: show all connections
netstat -an -ptcp | grep -i listen
## or
## -i: lists IP sockets
## -P: do not resolve port names
## -n: do not resolve hostnames
lsof -i -P -n | grep -i "listen"

09/18/2020

Interesting project https://github.com/hubotio/hubot
seq commands, i.e. seq 1 9 generate sequence 1 to 9, used in shell for loop as counter.

10/25/2020

Interesting, ed editor: https://sanctum.geek.nz/arabesque/actually-using-ed/
Dash is a Mac app browser API
Typora markdown editor

11/03/2020

Known from IBM channel, docker security talk

docker security scan
k8s cilium: securing the network connectivity between application

11/20/2020

shell nop command: :, synopsis is true, do nothing, similar to pass in python
1
2
3
4
5
## : as true
while :; do
sleep 1
echo 1
done
shell mutliline comment, can use heredoc
1
2
3
: <<'COMMENT'
...
COMMENT

11/24/2020

python shebang:

1 2	#!/usr/bin/env python # -- coding: utf-8 --

11/27/2020

ls -1 每行只输出一个文件名.
mv -v verbose

12/06/2020

bash read file line by line

cat $file | while read line
do
  echo $line
done

# or
input="/path/to/txt/file"
while IFS= read -r line
do
  echo "$line"
done < "$input"

12/09/2020

sudo to edit restricted permission file:

# executing as non-root user
# wrong, because redirection is done by the shell which doesn't has write permission.
sudo echo "sth" >> /root/.bashrc

# correct
echo "sth" | sudo tee -a /root/.bashrc
# or
sudo sh -c 'echo sth >> /root/.bashrc'
# or mutli-line
sudo tee -a /root/.bashrc << EOF
dexec()
{
  docker exec -it \${1} sh
}
EOF

12/10/2020

semantic versioning major.minor.patch

12/25/2020

cat jsonfile | python -m json.tool 和jq 类似，一个json的format工具，但只能pretty print.
sudo: unable to resolve host: 我不太理解为什么sudo 会涉及到hostname resolution?