&& tasks before and after the success to be successful
|| There is a before and after the mission success even if successful
[ $USER == root ] && echo "ok" || echo "no"
[A == a] && echo "equal" || echo "unequal"
a=
[-Z $ a] && echo "empty" || echo "not empty"
#!/bin/bash
read - p "Enter account:" u
[-Z $ a] && echo "account can not be empty" && exit
useradd $u
stty -echo
read -p "Enter password:" p
stty echo
echo $p | passwd -- stdin $u106
-eq equal
-ne not equal
-gt greater than
greater than or equal -ge
-le less
-lt less than
[-Z $ a] // determines whether empty, not empty true
[! -Z $ a] determines whether the non-empty, empty true
To test the file
Whether -e file exists
-D if file exists and is a directory
-F if file exists and is a regular file
-r whether the current user has read access to the file
-w whether the current user has write access to the file
-x whether the current user has permission to execute the file
[-E / opt / 123] 123 test whether there is, do not care about file types
echo $?
Use if the judge sentences
1. Single branch
if the test condition; then
Executing instructions
be
2. Dual Branch
if the test condition 1; then
1 instruction execution
else
2 instruction execution
be
3. Multi-Branch
if the test condition 1; then
1 instruction execution
elif condition test 2; then
2 instruction execution
else
Executing instructions 3
be
