HemingwayLee / shell-cheatsheet

shell-cheatsheet

• sh is a specification, not an implementation
• bash is an implementation of sh (other implementations: ksh, zsh, dash, ...)
• /bin/sh used to point to /bin/bash on most GNU/Linux systems

Which shell I am using

echo $SHELL
echo $0

zsh vs bash

• Mac Catalina will default to zsh instead of bash
  • The version of bash shipped with macOS is VERY OLD, because it is GPLv2 license and Apple is allowed to distribute it
  • The current v5 bash is under the GPLv3 license that is poison to Apple, so they cannot give you an up-to-date bash with bug fixes
  • zsh has a better community supports
  • bash is still there, nothing will change for you. All that changes is that zsh will be the default login shell for new accounts, and we can select bash instead
  • Scripts are not affected. What changes is the shell for interactive use, i.e. the shell in terminals. If you have a script in a file starting with #!/bin/bash or #!/bin/sh or #!/usr/bin/env bash, it'll keep working exactly as before.

Reference

https://apple.stackexchange.com/questions/361870/what-are-the-practical-differences-between-bash-and-zsh

source command not found in sh shell

• /bin/sh is usually some other shell trying to mimic The Shell. Many distributions use /bin/bash for sh, it supports source. On Ubuntu, /bin/dash is used which does not support source. Most shells use . instead of source.

. filename

The first line of the file

• we need #!/bin/bash at the first line of the .sh file if we want to run .sh file in some docker environment
  • When we use #!/bin/bash then we tell the environment os to use bash as a command interpreter.
  • If we install many versions of Python, then #!/usr/bin/env ensures that the interpreter will use the first installed version on your environment's $PATH

direnv

• It is an extension for your shell
• It augments existing shells with a new feature that can load and unload environment variables depending on the current directory
  • when we get into the folder, the .envrc will be loaded

Cheatsheet

ssh (<user> is your_user_name in GoogleCloud and it is ubuntu in AWS)

ssh -i ~/.ssh/<my.pem> <user>@<ip>

ssh -F /dev/null <user>@<ip>

cp rm everything (folder and files)

sudo rm -fr *
sudo cp -R /home/abc/* .

scp

Use -r option to copy the folder

scp -i ~/.ssh/xxx.pem ~/Desktop/util.js [e-mail]:/home/foldername

ls

makes it human readable (e.g., GB, MB, ...)

ls -lh

order by time

ls -lt

Count how many files in folder

ls | wc -l

tmux

tmux ls
tmux kill-session -t runserver
tmux new-session -d -s runserver "python3 manage.py runserver 0.0.0.0:8000"

standard output

history > h.txt  # override
ls >> abc.txt    # append

Kill the process listening on port 3000

kill -9 $(lsof -i:3000 -t)

find folder

find . -type d -name 'foldername'

. means current directory

find and delete with wildcard

• delete files in current folder which contain aaa in filename

find . -type f -name '*aaa*' -delete

check file information

$ file -I README.md 
README.md: text/plain; charset=us-ascii

mkdir

• Create folders if they do not exist

mkdir -p /usr/share/jenkins/ref/

export

• Change environment variable

$ export abc=10
$ echo $abc
10

• Add variables /home/mytest to PATH

export PATH=$PATH:/home/mytest

• List all environment variables

export -p

• Check if environment variable exist

if [[ -z "${DEPLOY_ENV}" ]]; then
  MY_SCRIPT_VARIABLE="Some default value because DEPLOY_ENV is undefined"
else
  MY_SCRIPT_VARIABLE="${DEPLOY_ENV}"
fi

open

open from command line

open hello.html
open -a "Google Chrome" hello.html
open -a "Sublime Text 2" README.md

or add it to ~/.bashrc

alias chrome='open -a "Google Chrome"'
alias sublime='open -a "Sublime Text 2"'

Chain bash commands

Sometimes we need to chain the bash command when we run docker build, because each command in the Dockerfile (e.g. FROM, ENV, RUN) is a step in the build process. Docker processes each step in an intermediate container.

A; B    # Run A and then B, regardless of success of A
A && B  # Run B if and only if A succeeded
A || B  # Run B if and only if A failed
A &     # Run A in background

curl

Read JSON in bash

curl -s 'https://api.github.com/users/lambda' | python3 -c "import sys, json; print(json.load(sys.stdin)['name'])"

loop until web server available

until $(curl --output /dev/null --silent --head --fail http://myhost:myport); do
    printf '.'
    sleep 5
done

--fail: wait until return 200

Read random line

cat file.txt | shuf -n 1

shuffle

shuf words.txt
gshuf addresses.csv > addresses2.csv

Run process in the background

https://unix.stackexchange.com/questions/74520/can-i-redirect-output-to-a-log-file-and-background-a-process-at-the-same-time

colon operator in variable

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Disk

• See disk details

lshw -class disk

• Free space

df -h

• In which filesystem:

df -h /home/lee/projects

Note

service account and user account

Reference

https://stackoverflow.com/questions/5725296/difference-between-sh-and-bash