Logical judgments in Linux shell scripts, file directory attribute judgments, if special usages, case judgments

Logical judgments in shell scripts

• Format 1:if condition; Then statement; Fi (Common)

#Expressed by command
[[email protected] ~] # for i in `seq 1 5`; do echo $ i; done
1
2
3
4
5
[[email protected] ~] # for i in `seq 1 5`
> do
> echo $ i
> done
1
2
3
4
5
[[email protected] ~] # a = 5
[[email protected] ~] # if [$ a -gt 3]
> then
> echo ok
> fi
ok
[[email protected] ~] # if [$ a -gt 3]; then echo ok; fi
ok

#Script execution
[[email protected] ~] # cd shell /
[[email protected] shell] # vi if1.sh
[[email protected] shell] # cat if1.sh
#! / bin / bash
a = 5
if [$ a -gt 3]
then
     echo ok
fi
[[email protected] shell] # sh if1.sh
ok

• Format 2:if condition; Then statement; else statement; Fi

[[email protected] shell]# cp if1.sh if2.sh
[[email protected] shell]# vi if2.sh 
[[email protected] shell]# cat if2.sh 
#!/bin/bash
a=1
if [ $a -gt 3 ]
then
    echo ok
else
    echo nook
fi
[[email protected] shell]# sh -x if2.sh 
+ a=1
+ ‘[‘ 1 -gt 3 ‘]‘
+ echo nook
nook
[[email protected] shell]# sh if2.sh 
nook

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

[[email protected] shell] # vi filel.sh
[[email protected] shell] # cat filel.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-f $ f]
then
    echo $ f exist
else
    touch $ f
fi
[[email protected] shell] # sh -x filel.sh
+ f = / tmp / aminglinux
+ ‘[’ -F / tmp / aminglinux ‘]’
+ touch / tmp / aminglinux
[[email protected] shell] # sh -x filel.sh
+ f = / tmp / aminglinux
+ ‘[’ -F / tmp / aminglinux ‘]’
+ echo / tmp / aminglinux exist
/ tmp / aminglinux exist
[[email protected] shell] # cp filel.sh filel2.sh
[[email protected] shell] # vi filel2.sh
[[email protected] shell] # cat filel2.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-d $ f]
then
    echo $ f exist
else
    touch $ f
fi
[[email protected] shell] # sh -x filel2.sh
+ f = / tmp / aminglinux
+ ‘[’ -D / tmp / aminglinux ‘]’
+ touch / tmp / aminglinux
[[email protected] shell] # vi filel2.sh
[[email protected] shell] # cat filel2.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-e $ f]
then
    echo $ f exist
else
    touch $ f
fi
[[email protected] shell] # sh -x filel2.sh
+ f = / tmp / aminglinux
+ ‘[‘ -E / tmp / aminglinux ‘]’
+ echo / tmp / aminglinux exist
/ tmp / aminglinux exist

[[email protected] shell] # vi filel2.sh
[[email protected] shell] # cat filel2.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-r $ f]
then
    echo $ f readable
fi
[[email protected] shell] # sh -x filel2.sh
+ f = / tmp / aminglinux
+ ‘[‘ -R / tmp / aminglinux ‘]’
+ echo / tmp / aminglinux readable
/ tmp / aminglinux readable
[[email protected] shell] # vi filel2.sh
[[email protected] shell] # cat filel2.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-w $ f]
then
    echo $ f writeable
fi
[[email protected] shell] # sh -x filel2.sh
+ f = / tmp / aminglinux
+ ‘[’ -W / tmp / aminglinux ‘]’
+ echo / tmp / aminglinux writeable
/ tmp / aminglinux writeable
[email protected] shell] # ls -l / tmp / aminglinux
-rw-r--r-- 1 root root 0 Feb 3 14:52 / tmp / aminglinux
[[email protected] shell] # vi filel2.sh
[[email protected] shell] # cat filel2.sh
#! / bin / bash
f = "/ tmp / aminglinux"
if [-x $ f]
then
    echo $ f exeable
fi
[[email protected] shell] # sh filel2.sh // No output because it is not executable

• Logical judgment expression: if [$a-gt $b]; If [$a-lt 5]; If [$b-eq 10] et-gt (>); -lt (<); -ge (>=); -le (<=);-eq (= =); -ne (! =) Note that there are spaces everywhere

• can use && | | Combine multiple conditions
  If [$a-gt 5] && [$a-lt 10]; Then
  If [$b-gt 5] | | [$b-lt 3]; Then

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

[[email protected] shell] # vi if4.sh
[[email protected] shell] # sh -x if4.sh
++ wc -l / tmp / lalal
wc: / tmp / lalal: no such file or directory
+ n =
+ ‘[’ -Gt 100 ‘]’
if4.sh: line 3: [: -gt: expect unary expression
[[email protected] shell] # vi if4.sh
[[email protected] shell] # cat if4.sh
#! / bin / bash
n = `wc -l / tmp / lalal`
if [-z "$ n"]
then
    echo error
    exit
elif [$ n -gt 100]
then
    echo aladafaf
fi
[[email protected] shell] # sh -x if4.sh
++ wc -l / tmp / lalal
wc: / tmp / lalal: no such file or directory
+ n =
+ ‘[’ -Z ‘‘ ‘]’
+ echo error
error
+ exit
[[email protected] shell] # vi if4.sh
[[email protected] shell] # cat if4.sh
#! / bin / bash
if [! -f / tmp / lalal]
then
    echo "/ tmp / lalal not exist."
    exit
fi
n = `wc -l / tmp / lalal`
if [-z "$ n"]
then
    echo error
    exit
elif [$ n -gt 100]
then
    echo alsdflljk
fi
[[email protected] shell] # sh -x if4.sh
+ ‘[’ ‘!’ -F / tmp / lalal ‘]’
+ echo ‘/ tmp / lalal not exist.’
/ tmp / lalal not exist.
+ exit
[[email protected] shell] # sh if4.sh
/ tmp / lalal not exist.

If special usage

• If [-Z "$a"]?? This indicates what happens when the value of variable A is empty

[[email protected] shell]# ls
01.sh  filel2.sh  filel.sh  for1.sh  if1.sh  if2.sh  if3.sh  if4.sh
[[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 [-n ' $a '] means that the value of variable A is not empty

[[email protected] shell]# grep -w ‘user1‘ /etc/passwd
user1:x:1002:1002::/home/user1:/bin/bash
[[email protected] shell]# if grep -w ‘user1‘ /etc/passwd; then echo "user1 exist"; fi
user1:x:1002:1002::/home/user1:/bin/bash
user1 exist
[[email protected] shell]# if grep -wq ‘user1‘ /etc/passwd; then echo "user1 exist"; fi
user1 exist
[[email protected] shell]# if ! grep -wq ‘user1‘ /etc/passwd; then useradd user1; fi
[[email protected] shell]#

• If Grep-q ' 123 ' 1.txt; Then?? What happens if the 1.txt contains a ' 123 ' row
• if [!-e file]; Then what happens when the file doesn't exist?
• if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
• Symbols such as <,>,==,!=,>=,<= cannot be used in []

Case Judgment

• Example of case judgment for Vi/etc/init.d/network file can be viewed

• Case Judgment Script format

• Shell Script case

[[email protected] shell]# vi case.sh
[[email protected] shell]# cat case.sh 

#!/bin/bash
read -p "Please input a number: " n
if [ -z "$n" ]
then
    echo "Please input a number."
    exit 1
fi

n1=`echo $n|sed ‘s/[0-9]//g‘`
if [ -n "$n1" ]
then
    echo "Please input a number."
    exit 1
#elif [ $n -lt 0 ] || [ $n -gt 100 ]
#then
#    echo "The number range is 0-100."
#    exit 1
fi

if [ $n -lt 60 ] && [ $n -ge 0 ]
then
    tag=1
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 "not ok"
        ;;
    2)
        echo "ok"
        ;;
    3)
        echo "ook"
        ;;
    4)
        echo "oook"
        ;;
    *)
        echo "The number range is 0-100."
        ;; 
esac

[[email protected] shell]# sh case.sh 
Please input a number: 101
The number range is 0-100.
[[email protected] shell]# sh -x case.sh 
+ read -p ‘Please input a number: ‘ n
Please input a number: 101
+ ‘[‘ -z 101 ‘]‘
++ echo 101
++ sed ‘s/[0-9]//g‘
+ n1=
+ ‘[‘ -n ‘‘ ‘]‘
+ ‘[‘ 101 -lt 60 ‘]‘
+ ‘[‘ 101 -ge 60 ‘]‘
+ ‘[‘ 101 -lt 80 ‘]‘
+ ‘[‘ 101 -ge 80 ‘]‘
+ ‘[‘ 101 -lt 90 ‘]‘
+ ‘[‘ 101 -ge 90 ‘]‘
+ ‘[‘ 101 -le 100 ‘]‘
+ tag=0
+ case $tag in
+ echo ‘The number range is 0-100.‘
The number range is 0-100.

Logical judgments in shell scripts for Linux, file directory attribute judgments, if special usages, case judgments