Carding the basic knowledge of shell script & lt; VII & gt;: process Control until
Until syntax
The until loop executes a series of commands until the condition is true.
The until loop and the while loop are handled in the opposite way.
While loops are generally better than until loops, but until loops are more useful at some times-and only in rare cases.
Until syntax format:
Until test condition
Do
Instruction
Done
Condition is generally a conditional expression, and if the return value is false, the statement inside the loop continues to be executed, otherwise it jumps out of the loop.
Example 1: for the following example, we use the until command to output numbers from 0 to 9:
#! / bin/bash
Aplasy 0
Until [! $a-lt 10]
Do
Echo $a
A=expr $a + 1
Done
Output 0 1 2 3 4 5 6 7 8 9
Example 2
#! / bin/bash
#
# determine whether ip can reach ping. If not, wait for 60s.
Read-p "Enter IP Address:" ipadd
Until ping-c 1$ ipadd & > / dev/null
Do
Echo "${ipadd} ping."
Sleep 60
Echo "waited for 60s"
Done
Execution result
[root@localhost shell] # sh until-1.sh
Enter IP Address:192.168.1.123
192.168.1.123 ping.
Waited for 60s
192.168.1.123 ping.
Waited for 60s
Example 3
#! / bin/bash
#
# determine whether the user is in the system
Username=$1
If [$#-lt 1]
Then
Echo "Usage:'basename $0' []"
Exit 1
Fi
If grep "^ $username:" / etc/passwd > / dev/null
Then
:
Else
Echo "$username is not a user on this system."
Exit 2
Fi
Until who | grep "$username" > / dev/null
Do
Echo "$username is not logged on."
Sleep 5
Done
Execution result
[root@localhost shell] # sh until-2.sh
Usage:'basename until-2.sh' []
[root@localhost shell] # sh until-2.sh ab
Ab is not a user on this system.
