Logical judgment in the shell, if judgment file, directory attribute, some special usage of if judgment

logical judgment in the shell
Format 1: if condition; then statement; fi // If the condition is satisfied, then execute the statement
[[email protected] shell] # cat if1.sh
#! / bin / bash
a = 5
if [$ a -gt 3]
then
echo ok
fi
[[email protected] shell] # sh -x if1.sh
+ a = 5
+ ‘[‘ 5 -gt 3 ‘]’
+ echo ok
ok
Format 2: if condition; then statement; else statement; fi // If the condition is satisfied, then execute the statement, otherwise execute the else statement
[[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
Format 3: if…; then…; elif…; then…; else…; fi // If the condition is satisfied, the statement is executed, otherwise the elif condition is continued and the statement is executed. elif condition can write multiple
[[email protected] shell] # cat if3.sh
#! / bin / bash
a = 1
if [$ a -gt 3]
then
echo ok
elif [$ a -lt 3]
then
echo "less than 3"
else
echo nook
fi
[[email protected] shell] # sh -x if3.sh
+ a = 1
+ ‘[‘ 1 -gt 3 ‘]’
+ ‘[‘ 1 -lt 3 ‘]’
+ echo $ ‘\ 345 \ 260 \ 217 \ 344 \ 272 \ 2163’
Less than 3
Logical judgment expression: if [$ a -gt $ b]; if [$ a -lt 5]; if [$ b -eq 10]. -gt (>); -lt (<); -ge (> =); -le (<=); -eq (==); -ne (! =) Note that there are spaces everywhere. Can also be written as if ((a> b)), but this is not standard

[[email protected] shell] # cat if1.sh
#! / bin / bash
a = 5
b = 1
if ((a> b))
then
echo ok
fi
[[email protected] shell] # sh -x if1.sh
a = 5
b = 1
((a> b))
echo ok
ok
You can use && || to combine multiple conditions
[[email protected] shell] # cat if1.sh
#! / bin / bash
a = 5
if [$ a -gt 3] && [$ a -lt 10]
then
echo ok
fi
[[email protected] shell] # sh -x if1.sh
+ a = 5
+ ‘[‘ 5 -gt 3 ‘]’
+ ‘[‘ 5 -lt 10 ‘]’
+ echo ok
ok
[[email protected] shell] # vim if1.sh
[[email protected] shell] # cat if1.sh
#! / bin / bash
a = 3
if [$ a -gt 3] && [$ a -lt 10]
then
echo ok
elif [$ a -gt 3] || [$ a -lt 10]
then
echo "a is greater than 3 or less than 10"
else
echo no
fi
[[email protected] shell] # sh -x if1.sh
+ a = 3
+ ‘[‘ 3 -gt 3 ‘]’
+ ‘[‘ 3 -gt 3 ‘]’
+ ‘[‘ 3 -lt 10 ‘]’
+ echo $ ‘a \ 345 \ 244 \ 247 \ 344 \ 272 \ 2163 \ 346 \ 210 \ 226 \ 350 \ 200 \ 205 \ 345 \ 260 \ 217 \ 344 \ 272 \ 21610’
a is greater than 3 or less than 10
if judges file and directory attributes
[-f file] Determine whether it is a normal file and exists
[-d file] Determine if it is a directory and exists
[-e file] Determine whether a file or directory exists
[-r file] determines whether the file is readable
[-w file] Determine if the file is writable
[-x file] Determine if the file is executable
[[email protected] shell] # cat file1.sh
#! / bin / bash
f = / tmp / akuilinux
if [-f $ f]
then
echo ok
else
touch $ f
fi
[[email protected] shell] # sh -x file1.sh
+ f = / tmp / akuilinux
+ ‘[’ -F / tmp / akuilinux ‘]’
+ touch / tmp / akuilinux
[[email protected] shell] # sh -x file1.sh
+ f = / tmp / akuilinux
+ ‘[’ -F / tmp / akuilinux ‘]’
+ echo ok
ok
It can also be used in a script like this, file1.sh and file2.sh have the same meaning.! Means reverse
[[email protected] shell] # cat file1.sh
#! / bin / bash
f = / tmp / akuilinux
if [-f $ f]
then
rm -f $ f
fi
[[email protected] shell] # cat file2.sh
#! / bin / bash
f = / tmp / akuilinux
[-f $ f] && rm -f $ f
[[email protected] shell] # cat file1.sh
#! / bin / bash
f = / tmp / akuilinux
if [! -f $ f]
then
touch $ f
fi
[[email protected] shell] # cat file2.sh
#! / bin / bash
f = / tmp / akuilinux
[-f $ f] || touch $ f
Some special uses of if judgment
if [-z "$ a"] ?? This shows what happens when the value of variable a is empty
if [-n "$ a"] means when the value of variable a is not empty
When judging whether a variable is empty, [] needs to be enclosed in double quotes. -Z and -n can only judge whether the variable is empty, not whether the file is empty.
[[email protected] shell] # cat if4.sh
#! / bin / bash
#First determine if the file exists, exit if there is no, so as to avoid bugs
if [! -f / tmp / lalal]
then
echo "/ tmp / lalal not exist."
exit
fi
n = `wc -l / tmp / lalal`
#Exit if the value is empty
if [-z "$ n"]
then
echo error
exit
#Print if there is a value
elif [-n "$ n"]
then
echo $ n
fi
[[email protected] shell] # cat / tmp / lalal
dadhaj
dahdajhj
127676
adadahj
[[email protected] shell] # sh if4.sh
4 / tmp / lalal
if grep -wq ‘user1’ 1.txt; then means what happens if 1.txt contains the line ‘user1’
-w means match a word
-q does not print what grep matches
case judgment in shell
format
case variable name in
  value1)
     command
     ;;
  value2)
     command
     ;;
  *)
    command
     ;;
esac
In case programs, you can use | in conditions to indicate or mean, such as
2 | 3)
command
     ;;
case
#! / 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
fi
if [$ n -lt 60] && [$ n -ge 0]
then
tag = 1
elif [$ n -ge 60] && [$ n -lt 80]
then
#! / bin / bash
#readInteract with the user and assign the value entered by the user to n
read -p "Please input a number:" n
#If n is empty, the user has not entered
if [-z "$ n"]
then
echo "Please input a number."
exit 1
fi
#Determine if the input is a number, prompt and exit if it is not a number
n1 = `echo $ n | sed‘ s / [0-9] // g’`
if [-n "$ n1"]
then
echo "Please input a number."
exit 1
fi
#If it is a number, if it is less than 60, mark it, if it is greater than 60 and less than 80, mark 2 and so on
if [$ n -lt 60] && [$ n -ge 0]
then
#Determine if the input is a number, prompt and exit if it is not a number
n1 = `echo $ n | sed‘ s / [0-9] // g’`
if [-n "$ n1"]
then
echo "Please input a number."
exit 1
fi
#If it is a number, if it is less than 60, mark it, if it is greater than 60 and less than 80, mark 2 and so on
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
#Use case to judge the mark and give the result
case $ tag in
1)
    echo "not ok"
    ;;
2)
    echo "ok"
    ;;
3)
    echo "ook"
    ;;
4)
    echo "oook"
    ;;
*)
    echo "The number range is 0-100."
    ;;
esac
Logical judgment in the shell, if to judge file and directory attributes, some special uses of the if judgment