Practice and production practice of shell variable substitution technology

${value:-word}

If the variable name exists and is not NULL, the value of the variable is returned, otherwise the word string is returned.

${value:=word}

If the variable name exists and is not NULL, the value of the variable is returned, otherwise, the value of this variable is word and its value is returned.

${value:?message}

If the variable name exists and is not NULL, the value of the variable is returned, otherwise the variable name is displayed, and the current command or shell is exited.

Example: ${value:? " Notdefined "} If value is undefined, the-bash:value:not defined is displayed and exited.

${value:+word}

If the variable name exists and is not NULL, Word is returned, otherwise null is returned.

Example: ${value:+word} if value is already defined, return Word (that is, true)

1, ${value:-word}

When the variable is undefined or the value is empty, the return value is the word content, otherwise the value of the variable is returned

[Email protected] ~]# Result=${cmz:-unset}#当cmz没有定义合作和是空会返回UNSET
[Email protected] ~]# echo $result
UNSET
[Email protected] ~]# echo $CMZ

==> here is empty.

Conclusion: When CMZ has no content, it returns to unset.

[Email protected] ~]# cmz= "Caimengzhi"
[Email protected] ~]# Result=${cmz:-unset} #之前必须声明cmz变量
[Email protected] ~]# echo $result
Caimengzhi
Conclusion: When CMZ has content, it returns the contents of CMZ variables.

This variable function solves the work string problem, and the variable is undefined.

2, ${value:=word}

[[email protected] ~]# Result=${test:=unset}
[Email protected] ~]# echo $result
UNSET

[Email protected] ~]# echo $test
UNSET

3, ${value:? " Not defined "}

[Email protected] ~]# RESULT=${LMT:? Not defined}
-bash:lmt:not defined

4, ${value-word} remove Colon

That is, when httpd does not exist, it assigns-the/usr/sbin/httpd of the back to the httpd in front of the equal sign.

To manipulate the path of a variable, it is best to first determine whether the path is non-null, especially the delete operation.

Path=/server/backup

Find ${path:=/tmp/}-name "*.tar.gz"-type F | Xargs rrm-f

RM-RF ${path}

It's important.

[Email protected] shell_scrpit]# vim d.sh
[Email protected] shell_scrpit]# sh-x d.sh
+ find/tmp/-name ' *.tar.gz '-type f
+ Xargs Rrm-f
Xargs:rrm:No such file or directory
+ RM-RF
[email protected] shell_scrpit]# cat d.sh
#path =/server/backup

Find ${path:=/tmp/}-name "*.tar.gz"-type F | Xargs rrm-f

#此时path没有定义 path will become/tmp/

RM-RF ${path}

Practice and production practice of shell variable substitution technology