1, the redirect -tr (data input and output)
procedures = commands (instructions) + data (variables)
in the program, how the data input and output
1) Input Data: keyboard - the standard input, but not the only way to input
echo " 123456 "| passwd --stdin" username " // password
./useradd.sh <user.txt // user.txt the redirection file to the useradd.sh
2) output data: display - standard output, but the output is not the only way.
LS / etc /> a.txt
FD file identifier 0-9: classification corresponds to the file
0: Standard Output
1: Standard Input
2: Error input-output
common redirection symbol:
1) Standard Output
> coverage redirection
set -C Close covering the redirection function set + C: recovery
> | forced redirection
>> append redirection
2) standard input
<a <b: to perform a to b
tr replacement file content (tr set1 [set2]) char ( character : a one transducer)
TR the ABC ABC <
<< The multiple rows of data simultaneously input
3) Error output
2> 2 >>
expansion: no output content, only the output state;
echo $: a command to determine whether the correct (0 correct, 1-255 other is not correct? )
2, pipeline -tee (one input, two outputs)
command1 | command2 | Command3 | ......: the results of the previous command to a command to perform
free -m | grep "^ Mem" | -d Cut '' -f19
as Free -m | grep "^ Mem" | awk '{3} $ Print'
TEE /tmp/tee.out // If no file is created, the default file if there is content, will be covered;
exercise:
1) the content of the first 5 rows / etc / passwd file conversion saved as a file in uppercase to /tmp/passwd.out
head -5 / etc / passwd | tr [az] [AZ]> / tmp / passwd the .out
2) login to the system after the user information on the summary of information into the current three saved as a file in uppercase to /tmp/who.out
who | tail -3 | cut -d ' ' -f1 | tr [az] [AZ] tee /tmp/who.out
tail end of the file to see how many lines (10 lines)
-n tail -n / etc / passwd == -5 short tail / etc / passwd
-f file to view real-time updates
tail -f / var / log / meddage : real-time view the log file
See file header head how many rows (10 rows)
# pre -n #,
example: take the / etc / passwd file line 10-20
head -20 /etc/passwd | tail -10