look shell info

    1.cat /etc/shells
        look linux installed shell
    2.echo $SHELL

write

        echo "$USER"
        echo "today is \c";date
        echo "number of login:\c";who|wc -l
        echo "Calandar"
        cal

diff with "" ,'',``

    1."" can know $
    2.'' just output as the same
    3.`` exec command

diff with >, >>, <

    1.> overwrite to a file
    2.>> append to a file
    3.< read from a file

if

    #!/bin/sh
    if [ $1 -gt 0 ];then
    echo "$1 > 0"
    elif [ $1 -eq 0 ];then
    echo "$1 = 0"
    else
    echo "$1 < 0"
    fi

    -gt >
    -lt <
    -le <=
    -gs >=
    -eq =
    -ne !=

    # file empty?
    -s file
    # file not a directory,exist and is a noraml file
    -f file
    # is a directory
    -d file
    # file can read
    -r file
    # file can write
    -w file
    # file can execute
    -x file

    ! expression
    expressiona -a expressionb
    expressiona -o expressionb

loop

    for i in 1 2 3 4 5
    do
    echo welcome $i time
    done