A. With expr
Format expr m + n or $ ((m + n)), the operator must pay attention to the space expr
Such as (3 + 2) * value of 4
1. calculation step
S=‘expr 2 + 3’
expr $ S \ * 4 ## * No need to escape
2. The calculation in one step
expr 'expr 2 + 3' \ * 4
echo 'expr \'expr 2 + 3\'\* 4'
II. With (()) // do not need space
((1+2))
(((2+3)*4))
count=1
((count++))
echo $ count result of the operation need to take references with $
a=$((1+2))
III. With $ []
a=$[1+2]
echo $ a
Four .if grammar
#！/bin/bash
read -p "please input your name:" NAME ## read commands for reading the input data from the console
## printf '%s\n' $NAME
if [ $NAME = root ]
then
echo "hello $(NAME), welcome!"
elif [ $NAME = itcast ]
then
echo "hello $(NAME), welcome!"
else
echo "Get out here!"
be
V. judgment condition
1. The basic syntax conditional
[Condition] (Note that condition must be a space before and after)
# Non-empty returns true, use $? Verification (0 is true,> 1 is true) [itcast]
# Null return false []
2. The composition determination condition
[Condition] && echo OK || echo notok condition is detected, the statement following &&; condition is not satisfied, the implementation of the latter statement ||
Note: [] difference between [[]] of: [[]] may be used in combination && || logic symbol, [] which can be used in combination logic -a -o
3. Analyzing common operator
String comparisons: = = -z -n!
-z returns true length of the string 0
-n string length is not zero return true
Integer Comparison: -lt -le less than equal to or less -eq -gt greater than -ne not equal to or greater -ge
It's the file: -d whether the directory
Whether -f file
-e whether there