Linux Basics IX

1. File descriptor     input: standard input  stdin,0    output: standard output  stdout,1              standard error  stderr,2     &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;2.I/O redirection      Input redirection  <     <<  Create files Here, commonly used in bash to create files or build menus     cat /tmp/a.txt <<  EOF        a:ls /etc/passwd         b:ls /etc/default/useradd    eof    3. Output redirection  >  >>    > Overwrite output     >> append output      4.set -C  prohibit overrides redirect to existing file   set +C  close-C attribute   >1  in set &NBSP;-C, force the use of overwrite redirection   /dev/null :bit bucket, bit bucket   5. Error redirection  2> ,2> >    2>  overlay     2>>  append     6. Simultaneous redirection of standard output and error output      command > /path/to/outfile 2> /path/to/outfile    command  & > /path/to/outfile    command > /path/to/outfile  2>&1  7. Pipe      Output two times results     cat /etc/rc.d/ Arithmetic operations in the Rc.sysinit |tee /tmp/a.out |wc -l    8.bash      declare        -i: Shaping Variables         &NBSP;-X: Environment Variables                   let varname= arithmetic expression   9.bash cannot perform floating-point operations, the result is rounded       varname=$[ Arithmetic Expression]      varname=$ ((arithmetic expression))       varname= ' expr   $num 1+ $num 2 '

Linux Basics IX