linux shell program summary

from:Sep 23 2016

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mathematical operation:

floating number,bc calculator:

we can also use bc in shell scripts:

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if - then - else

number comparison:

n1 -eq n2

n1 -ge n2

n1 -gt n2

n1 -le n2

n1 -lt n2

n1 -ne n2

if - then - elif, notice that the second elif won't be executed, though it's also right.

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strings comarison:

str1 = str2

str1 != str2

str1 < str2

str1 /> str2

-n str1

-z str1

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using file comparisons:

　　                -d file    checks if file exists and is a dictory

-e file    checks if file exists

-f file     checks if file exists and is a file

-r file     checks if file exists and is readable

　　               -s file     checks if file exists and is not empty

-w file    checks if file exists and is writable

-x file     checks if file exists and is executable

-o file     checks if file exists and is owned by the current user

-G file    checks if file exists and the default group is the same as the current user

file1 -nt file2   checks if file1 is newer than file2

file1 -ot file2   checks if file1 is older than file2

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considering compound testing

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working with advanced if-then features

((expression))

var++

var--

++var

--var

!              logical negation

~              bitwise negation

**             exponentiation

<<

>>

&

|

&&

||

notice the '++' of the screenshot.

it's writen of var++ not $var++

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using double brackets

[[ expression ]]

the double bracket command provides advanced features for string comparisons---pattern matching.

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Considering the case command:

case variable in

pattern1 | pattern2) command1;;

pattern3) command2;;

*) default commands;;

esac

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The for command:

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The while command:

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The until command: