2018-4-18 17 weeks 1 lessons Shell logic judgment, File directory attribute judgment, if, case

20.5 logical Judgments in shell scripts

• format 1:if conditions; Then statement; Fi

Example: a=5

If [$a-gt 3]; then echo OK; Fi

[Email protected] shell]# sh If1.shok

• format 2:if conditions; Then statement; else statement; Fi

Example: a=5

If [$a-gt 3]; then echo OK; else Echo Nook; Fi

[[email protected] shell]# sh if1.sh
ok
[[email protected] shell]# sh if1.sh
not ok
[[email protected] shell]# sh -x if1.sh
+ a=2
+ '[' 2 -gt 3 ']'
+ echo not ok
not ok

• Format 3:if ...; Then ...; Elif ...; Then ...; else ...; Fi

The first condition, how it is, the second condition, how; Otherwise, other conditions

[[email protected] shell]# sh -x if1.sh
+ a=3
+ '[' 3 -gt 4 ']'
+ '[' 3 -lt 6 ']'
+ echo '<6 && >1'
<6 && >1

[[email protected] shell]# sh -x if1.sh
+ a=5
+ '[' 5 -lt 4 ']'
+ '[' 5 -gt 6 ']'
+ echo not ok
not ok

• Logical judgment expression:

If [$a-gt $b]; -GT (>), greater Than

If [$a-lt 5]; -lt (<); less than

If [$b-eq 10] etc-eq (= =);

-le (<=);-ge (>=); -ne (! =) Note that there are spaces everywhere

• If you want to use > <, you can use (())

[[Email protected] shell]# if (($a >1)) then echo Ok;fi

Ok

• can use && | | Combine multiple conditions

if [$a-gt 5] && [$a-lt 10]; Then and

if [$b-gt 5] | | [$b-lt 3]; Then or

20.6 File Directory property judgment

· [-F file] Determines if it is a normal file, and there is

· [-D file] Determines if it is a directory and exists

· [-E file] to determine whether files or directories exist

· [-R File] to determine if the document is readable

· [-W file] Determines whether the file is writable

· [-X file] Determines whether the file is executable

· [-F file] Determines if it is a normal file, and there is

[[email protected] ~] # sh -x file1.sh
+ f = / tmp / alex
+ '[' -f / tmp / alex ']' ## Exists and is a file
+ touch / tmp / alex ## Create if not present
[[email protected] ~] # sh -x file1.sh
+ f = / tmp / alex
+ '[’-F / tmp / alex'] '## Is it a file?
+ echo / tmp / alex exist ##
/ tmp / alex exist

· [-D file] Determines if it is a directory and exists

[[email protected] ~]# sh -x file2.sh
+ f=/tmp/alex
+ '[' -d /tmp/alex ']'
+ touch /tmp/alex
[[email protected] ~]# sh -x file2.sh
+ f=/tmp/alex
+ '[' -d /tmp/alex ']'
+ touch /tmp/alex

· [-E file] to determine whether files or directories exist

[[email protected] ~]# sh -x file2.sh
+ f=/tmp/alex
+ '[' -e /tmp/alex ']'
+ echo /tmp/alex exist
/tmp/alex exist

· [-R File] to determine if the document is readable

[Email protected] shell]# sh file2.sh/tmp/alex readable

· [-W file] Determines whether the file is writable

[Email protected] shell]# sh File2.sh/tmp/alex writeable

· [-X file] Determines whether the file is executable

[[email protected] ~]# sh file2.sh
[[email protected] ~]# ll /tmp/alex
-rw-r--r-- 1 root root 0 April  18 21:21 /tmp/alex
#!/bin/bash
f="/tmp/alex"
if [ -f $f ]
then
rm -f $f
fi

You can use the If judgment to write

#!/bin/bash
f="/tmp/alex"
[ -f $f ] && rm -f $f

	


#!/bin/bash
f="/tmp/alex"
if [ -f $f ]
then
rm -f $f
else
touch $f
fi

The original script is preferable to write back as

#!/bin/bash
f="/tmp/alex"
if [ ! -f $f ]
then
touch $f
fi

You can use the If judgment to write

#!/bin/bash
f="/tmp/alex"
[ -f $f ] || touch $f

20.7 If special usage

if [-Z "$a"] This indicates what happens when the value of variable A is empty (the variable is quoted, the file is not quoted)

can be optimized to

[[email protected] shell] # sh -x file3.sh
++ wc -l / tmp / jojo
wc: / tmp / jojo: no such file or directory
+ n =
+ '[' -z '' ']'
+ echo not exist
not exist

Can be re-optimized to

[[email protected] shell] # sh -x file3.sh
++ wc -l / tmp / jojo
wc: / tmp / jojo: no such file or directory
+ n =
+ '[' -f ']'
+ echo 'not exist.'
not exist.
+ exit

if [-N "$a"] means that when the value of variable A is not empty

[[email protected] shell]# echo $b
[[email protected] shell]# if [  -n 01.sh ];then echo ok;fi
ok
[[email protected] shell]# if [  -n "$b" ];then echo "$b";else echo "b is null";fi
b is null

if Grep-q ' 123 ' 1.txt; Then what happens if the 1.txt contains a ' 123 ' row

[[email protected] shell] # if grep -w "user1" / etc / passwd; then echo "user1 exist"; fi
user1: x: 1011: 1011 :: / home / user1: / bin / bash
user1 exist
[[email protected] shell] # if grep -wq "user1" / etc / passwd; then echo "user1 exist"; fi
user1 exist ## grep -q silent output
[[email protected] shell] # if! grep -wq "user2" / etc / passwd; then useradd user2; fi

IF [!-e file]; Then what happens when the file doesn't exist?

if (($a <1)); Equivalent to if [$a-lt 1]; Then ...

· Symbols such as <,>,==,!=,>=,<= cannot be used in []

20.8/20.9 Case Judgment

• Format: Case variable name in

value1)

Command

;;

value2)

Command

;;

*)

Commond

;;

Esac

:: Indicates the end of a judgment, into the next judgment

• In a case program, you can use |, meaning, or means in a condition, such as

2|3)

Command

;;

· *) In addition to all of the above

Shell Script case

#! / bin / bash
read -p "Please input a number:" n ## Read input from standard input and assign value to variable n
if [-z "$ n"]
then
echo "Please input a number."
exit 1 ## 1 is the prompt when exiting
fi

n1 = `echo $ n | sed 's / [0-9] // g'` ## Replace the number from the variable n with empty. If n1 is not empty, it is not a pure number, then enter a number and drop out
if [-n "$ n1"]
then
echo "Please input a number."
exit 1
fi

if [$ n -lt 60] && [$ n -ge 0] ## Pure numbers continue to judge
then
tag = 1 ## Tag tag
elif [$ n -ge 60] && [$ n -lt 80]
then
tag = 2
elif [$ n -ge 80] && [$ n -lt 90]
then
tag = 3
elif [$ n -ge 90] && [$ n -le 100]
then
tag = 4
else
tag = 0
fi
case $ tag in
1)
echo "Fail"
;;
2)
echo "Pass"
;;
3 | 4)
echo "excellent"
;;
*)
echo "The number range is 0-100."
;;
esac

• Perform view results:

[[email protected] shell] # sh -x case.sh
+ read -p 'Please input a number:' n
Please input a number: 77
+ '[' -z 77 ']'
++ echo 77
++ sed 's / [0-9] // g'
+ n1 =
+ '[' -n '' ']'
+ '[' 77 -lt 60 ']'
+ '[' 77 -ge 60 ']'
+ '[' 77 -lt 80 ']'
+ tag = 2
+ case $ tag in
+ echo $ '\ 345 \ 217 \ 212 \ 346 \ 240 \ 274'
Pass
[[email protected] shell] # sh -x case.sh
+ read -p 'Please input a number:' n
Please input a number: 8sdafasdf ## Enter an impure number
+ '[' -z 8sdafasdf ']'
++ echo 8sdafasdf
++ sed 's / [0-9] // g'
+ n1 = sdafasdf
+ '[' -n sdafasdf ']'
+ echo 'Please input a number.'
Please input a number.
+ exit 1
[[email protected] shell] # sh -x case.sh
+ read -p 'Please input a number:' n
Please input a number: 123 ## Enter a number greater than 100
+ '[' -z 123 ']'
++ echo 123
++ sed 's / [0-9] // g'
+ n1 =
+ '[' -n '' ']'
+ '[' 123 -lt 60 ']'
+ '[' 123 -ge 60 ']'
+ '[' 123 -lt 80 ']'
+ '[' 123 -ge 80 ']'
+ '[' 123 -lt 90 ']'
+ '[' 123 -ge 90 ']'
+ '[' 123 -le 100 ']'
+ tag = 0
+ case $ tag in
+ echo 'The number range is 0-100.'
The number range is 0-100. 

2018-4-18 17 weeks 1 lessons Shell logic judgment, File directory attribute judgment, if, case