command meaning usage example
    date out put the date date
    echo print out its arguments echo hello
    echo hello/ world
    echo "hello world"
    $PATH is an environment variable that lists which directoried the shell should search for program when it is given a command echo $PATH
    which find out which file is executed for a given program name which npm
    pwd the absolute pathname of the current working directory pwd
    cd change the current working directory cd /
    cd ./xx/
    cd ../..
    . current directory
    .. parent directory
    ls list files ls
    ls /
    ls -l
    mv move file mv a.txt b.txt
    mv a.txt ../b.txt
    cp copy file cp a.txt b.txt
    cp a.txt ../b.txt
    rm remove file rm a.txt
    rm -rf ./xxDirecotry
    rmdir remove directory rmdir ./xxEmptryDirectory
    mkdir make directory mkdir directoryName
    man manual page man cp
    ctrl L clear and go back to the top
    < file rewire the input for this program to be the contents of this file cat < hello.txt
    > file rewire the output of the preceding program into this file echo hello > hello.txt
    cat < hello.txt > hello2.txt
    cat file print the content of the file cat hello.txt
    >> append instead of overwrite cat < hello.txt >> hello2.txt
    &#124; take the output of the program to the left and make it the input of the program to the right ls -l / &#124; tail -n1
    ls -l / &#124; tail -n1 > ls.txt
    curl --head --silent taobao.com &#124; grep -i content-length &#124; cut -d ' ' -f2
    tail prints the last n lines of its input tail -n1
    curl
    grep
    cut
    sudo do as super user
    sudo su get the shell as the super user
    tee take it input and write it to a file but also to standard out echo 1060 &#124; sudo teee brightness
    find
    xdg-open

    https://www.youtube.com/watch?v=Z56Jmr9Z34Q

    https://missing.csail.mit.edu/2020/course-shell/