1, the position variable
$$ returns to the routine of the PID
$ 0 representative of the script itself the name of
the first parameter contact runtime $ 1 represents a script (common)
then $ 2 represents the script runs the second parameter
contact runtime $ 3 represents a script third parameters
$ * Represents all parameters
$ @ is on behalf of all parameters
$ # display the number followed by the parameters
$? execute a command on success, returns 0, failure of non-zero (common) returns
2, variable calculation
commonly used numerical order :
(()), the let, expr, BC, $ []
# 100 = A
# B = 200 is
# $ C = ((A + $ B $)) - arithmetic operation, calculates the integer only, two brackets, not less
# c = $ [$ a + $ b] - arithmetic operation, only calculates the integer
# let c = a + b - arithmetic, only calculates the integer
# expr $ a + $ b - arithmetic, only integer calculation, pay attention to the format, there are spaces on both sides of the equal sign
# a = 3.14
# b = 1.23
# echo "$ a + $ b" | bc - arithmetic, decimal integer can be calculated, bc is a calculator
4.37
bc is a calculator
# bc
Whether it is an integer or decimal BC can be used to calculate, with | piped to
3, and determines the logical sequence
; - perform End front, regardless of success or not, without performing the logic behind
&& - front successfully performed, was performed behind; failed front, behind not perform a logical aND
|| - front failed, before the implementation of the back; successful execution front, back or not execution logic
; these symbols are connected && || command
- correct determination method:
LS # / Test / && echo 'existed' || echo 'Not existed'
- error determination method:
# LS / testfsdafas / || echo 'existed' && echo 'Not existed'
two determination conditions.
1. condition judgment syntax
[ syntax]
format 1: test <test expression> - not used
Example: test 1.txt -f 0 && echo echo. 1 ||
2 format: [<test expression>] - common
Example: [-f 1. txt] && echo 1 || echo 0
format 3: [[<test expression>]] - not used
Example: [[-f 1.txt]] &&1 || 0 echo echo
2. Analyzing the various conditions
(1) and the file type of the file exists Analyzing
-e whether there are (exist)
-f if a regular file
-d whether the directory
-S socket file
-p pipe manage files
-c character-character file
-b block block file
-L soft link
# test -f 1.txt && echo 0 || echo. 1
. 1
# [-f 1.txt] && echo echo. 1 || 0 - Note expressions [] at the front and a space
. 1
# [[-f 1.txt]] && echo. 1 || echo 0 - Note expressions [] at the front and a space
. 1
# [[-f 1.txt]!] && echo echo. 1 || 0
0
inputs a file, it is determined what type
# / bin /! the bash
Read -p "Please enter a file name:" filename
[-e $ filename] || echo "file does not exist"
[-f $ filename] && echo "$ {filename} is an ordinary file"
[-d $ filename] && echo "$ {filename} is a directory"
[$ -S filename] && Echo "$ {filename} is the socket file"
[$ -p filename] && Echo "$ {filename} pipeline File"
[-C $ filename] && echo "$ {filename} is the character file"
[$ filename -b] && echo "$ {filename} is the block file "
[$ -L filename] && echo" $ {filename} is a soft link file "
------------------------------- -------------
! # / bin / the bash
[-e $. 1] || echo "file does not exist"
[-f $. 1] && echo "$ {} is an ordinary file. 1"
[- d $ 1] && echo "$ {1} is a directory"
[-S $. 1] && echo "$ {. 1} is the socket file"
[-p $. 1] && echo "$ {. 1} is a pipe file"
[-C $. 1] && echo "$ {1} is a character file"
[$. 1 -b] && echo "$ {1} is a block file"
[-L $. 1] && echo "$ {1} is a soft link file"
(2) determine the file permissions associated
-r which the current user is readable - valid for the average user, root user inaccurate (rw allowed)
-w which the current user is writable
-x whether its current user execution - common
Are there suid -u
-g whether sgid
whether there is -k t bits
(. 6) Analyzing logic
-s is blank file - not empty is true, empty is false
#! / bin / SH
[-e $ 1] || echo "File does not exist"
[-x $ 1] && SH $ 1 || echo "no execute permissions"
(3) compare two files to determine the
file1 -nt file2 compare file1 whether than file2 (new new Within last)
file1 file2 -ot comparison whether file1 file2 older than (old Within last)
file1 file2 -ef whether the comparison for the same file
(4) is determined between integers
-eq equal
-ne unequal
-gt greater than
-lt less than
-ge greater than or equal
-le or less
(5) is determined between strings
-z string is empty - is determined whether the variable is empty
-n is a non-empty string
string1 = string2 are equal
string1 =! string2 unequal
# A =
# [the -Z $ A] && echo echo. 1 || 0
. 1
# A. 1 =
# [the -Z $ A] && echo echo. 1 || 0
0
-a two conditions are met, it is true
-o both meet First, it is to true
! Non
(7) determines the difference between the three formats
used (arithmetic) in (()) are used in the [] and [[]] in the (arithmetic) Description
== -eq equal, equal
! = -Ne not equal , is not equal to
> -gt greater than, greater than
> = -ge greater equal, greater than or equal
<-lt less than, less than
<= -le less equal, less than or equal
in [] is the logical operator [[]] in using logical operators instructions
-a && with
-o or ||
!! non
# [-e install.log -a -f install.log] && cat install.log || echo " file does not exist"
# [-e 1.sh -a -x 1.sh] && ./1.sh || echo " file does not exist or file not authorized to perform"
# [[-e 1.sh && -x 1.sh]] && ./1 .sh || echo "file does not exist or file not authorized to perform."
if condition (condition true or false)
the then - either a separate line, or later if, to add a semicolon
Statement
Fi
or
IF for condition Condition; the then
Statement
Fi
IF (if / is assumed) conditions (true / false) - the condition is true, the code is executed
the then (so)
codes;
Fi
(2) double-branched structure
if condition (condition true or false)
the then
Statement - condition is true, execution portion of
the else
Statement - condition is false, execution section
Fi
(. 3) a multi-branched structure
if condition (condition true or false)
the then
Statement
elif for condition condition
the then
Statement
elif for condition condition
the then
Statement
the else
Statement
Fi
A document input, determines whether there is, the file type
#! / Bin / the bash
[-e $. 1] && echo "file exists" || echo "file does not exist"
[-f $. 1] && echo "$ {}. 1 is a general file "
[-d $. 1] && echo" $ {. 1} is a directory "
[-S $. 1] && echo" $ {. 1} is the socket file "
[-p $. 1] && echo" $ {. 1} is a pipe file "
[ -c $ 1] && echo "$ {1} is a character file"
[$. 1 -b] && echo "$ {}. 1 is a block file"
[-L $. 1] && echo "$ {}. 1 is a soft link file"
- ---------------------------
input a file determines if the file type of a single limb:
! # / bin / the bash
if [-e $. 1] ; the then
"file exists" echo
Fi
# logical operator to determine the file does not exist
IF [-e $. 1!]; the then
echo "file does not exist"
Fi
# determines the file type
IF [-f $. 1]; the then
echo "$. 1 is an ordinary file"
Fi
IF [-d $. 1]; the then
echo "$. 1 is a directory file"
Fi
IF [-S $. 1]; the then
echo "$. 1 is a socket file"
Fi
IF [-p $. 1]; the then
echo "$. 1 is a pipe file"
Fi
IF [ -C $. 1]; the then
echo "$. 1 is a character file"
Fi
IF [-b $. 1]; the then
echo "$. 1 is a block file"
Fi
IF [-L $. 1]; the then
echo "$. 1 is a link file"
Fi
---- ------------------------
two-branch and multi-branch
# determine whether a file exists
#! / bin / bash
whether a file exists # judge
if [-e $ 1 ]
the then
echo "file exists"
the else
echo "file does not exist."
Fi
# file type is determined
if [-f $ 1]; then
echo "$ {1} is an ordinary file"
elif [-d $. 1]; the then
echo "$ {. 1} is a directory"
elif [-S $. 1]; the then
echo "$ {. 1} is the socket file"
elif [-p $. 1]; the then
echo "$ {. 1} is pipe file "
elif [-C $. 1]; the then
echo" $ {. 1} is a character file "
elif [-b $. 1]; the then
echo" $ {. 1} is the block file "
the else
echo" $ {. 1} is a soft link file "
fi
-------------------------------------------
how to determine a host Are online?
! # / bin / bash
the ping -c $ 1 & 2> / dev / null
IF (($ == 0)?); the then
echo "$ 1 online"
the else
echo "$ 1 is not online"
fi
-------- ---
#! / bin / bash
the ping -c $ 1 & 2>
IF [$ -eq 0?]; the then
echo "$ 1 online"
the else
echo "$ 1 is not online"
fi