Shell Programming Quick Start

Shell programming: For Hadoop programmers, it is often necessary to familiarize yourself with shell programming, because the shell can easily run program code.

1. Shell file format: xxx.sh

#!/bin/sh---The first line of the shell file must be written like this
#井号后面的内容是注释
Echo ' ABCD '----echo is output, echo

mkdir shellfile
Span style= "font-size:14pt" >CD shellfile
ls
vi test.sh
#!/bin/sh
echo ' ABCD '
ls
ll
tset.sh---Now perform discovery without permission to modify permissions to execute
chmod u+x test.sh
ll    ---Listing file details
test.sh---can do it now. , the ABCD

is output

VI test.sh
#!/bin/sh
I=0---Variables do not need to be declared, and initialization does not require a specified type. Variable names can only consist of letters, numbers, underscores, and cannot begin with a number.
echo $i---Display variable values using: Echo $ variable Name

2. Variable classification: Temporary variables and environment variables (/etc/profile:export XXX)

3, single quotation marks, double quotation marks, the use of floating sign:
VI test.sh
#!/bin/sh
I=0
echo ${i}1234---Output 01234, if you want the variable to be used independently, open with curly braces

VI test.sh
#!/bin/sh
I=0
echo ' $i '---output $i, single quote not parsed variable
echo "$i"---output 0, double quote resolution variable

VI test.sh
#!/bin/sh
I=0
Echo ' $i '
echo "$i"
echo $ (date)---output time
echo ' Date '---float is the execution content, similar to the $ (XXX) above. Here ' data ' ==$ (date)

4, positional variables: When the script is executed, the passed in parameters are used in sequence, The value of the variable is referenced in the order.
vi test.sh
#!/ bin/sh
i=0
echo "First param:$1"
echo "second param:$2"
execute: test.shabc123
output: first param:abc
        second param:123

5, using Date: (see date command uses document ")
1, You can execute the date directly, outputting the current time of the system.
Br> 3, date +%s---format%s represents the number of seconds since 1970-01-01 00:00:00
4, date--date= ' 2017-4-10 '---Specify time output--date= ' 2009-01-01 11:11:11 '
5, specified time output--date= ' 3 days ago '

6, standard input, output, error:
1, standard input, output, error are command line, use file descriptor 0, 1, 2 reference
2, use redirection to convert information to another location
---------
ls > A.txt--- Converting test.sh to a.txt outputs the
ls 1>b.txt---a.txt and test.sh to b.txt output

7, using crontab (similar to a timer in Java):
1, scheduled tasks under Linux
2, edit using CRONTAB-E 　　　　　　　　
* * * * * date>>/root/a.txt
There are six columns, namely: time-of-day weekly command
(default is output by the first minute of each hour)
to follow the 5th minute output per hour, it needs to be modified to: 5 * * * * date>>/root/a.txt

3, View using crontab-l

8, if judgment, for loop: (note the spaces between the symbols!) There is a space at the end of each line!!!
if [...]---if and [between, [and conditions, conditions, and] there is a space between.
then
...
fi

for ((i=0;i<10;i++))---for and (there is a space between


done
----------
vi if.sh
#!/bin/sh
if [' a ' = ' a ']
then
echo ' equal '
fi
chmod u+x if.sh
if.sh---output equal

VI for.sh
#!/bin/sh
For ((i=0;i<10;i++))
Do
Echo $i
Done
chmod u+x for.sh
for.sh

9. Custom functions:
Function name () {
....
}

The last line statement is the default return value

When referencing a custom function file, use the source func.sh
------------------
VI func.sh
#!/bin/sh
function dat () {
Date +%y-%m-%d
}
Dat
chmod u+x func.sh
func.sh

VI func.sh
#!/bin/sh
function dat () {
Date--date= "Days Ago" +%y-%m-%d
}
DAT 2
func.sh---Output date 2 days before the current time
-----------------

Refer to func.sh in test.sh:
VI test.sh
#!/bin/sh
SOURCE func.sh---Best to specify a path
Dat-2
test.sh---Output 2 days after the date

Shell Programming Quick Start