Practice of shell Special extended variables

Bo main name: Li Changming

Blog Address: http://keep88.blog.51cto.com

This note is from the old boy books: Learn Linux OPS Shell programming with old boys

Special extension variables

1. ${parameter:-word} Functional Practice

The role of ${parameter:-word} is to return the value of the word string substitution variable if the value of the parameter variable is empty or unassigned

For example:

[[email protected] ~]# echo $test #>== This time the test variable is not assigned [[email protected] ~]# echo ${test:-word} #>== can see the test The variable is not assigned a value, output word, indicates that the test variable is empty, and returns the character defined after "-", but be aware that Word will not be assigned to the test variable, just an identity. Word[[email protected] ~]# test= "ABC" #>== assign the test variable ABC to view the output [[email protected] ~]# echo $testabc [[Email protecte D] ~]# Echo ${test:-word} #>== This time the test variable is assigned, so the value of the test variable is output. Abc[[email protected] ~]#

Comments:

The colon ":" in ${parameter:-word} can be omitted. No difference from the above results

2. ${parameter:=word} Functional Practice:

Determine if the parameter variable has a value, if there is a value output variable value, if not assigned, then "-" after the character word (custom) assigned to the parameter variable

For example:

[[email protected] ~]# unset test[[email protected] ~]# echo $test #>== test variable not assigned [[email protected] ~]# A=${test:=wor D}[[email protected] ~]# echo $A #>== unassigned, assign Word to the variable Testword if the test variable has an assignment, output the variable of test directly [[email protected] ~]# test=  "5678" [[email protected] ~]# echo $test 5678[[email protected] ~]# b=${test:=word}[[email protected] ~]# echo $B #>== The test variable has an assignment that outputs the value directly and does not assign word to $test5678

The difference between the above two special variables:

${parameter:-word}: If parameter has an assignment, output value, no assignment, output "-" after the defined character, will not be assigned to the variable parameter, just display information

${parameter:=word}: If parameter has an assignment, output value, no assignment, the character defined after "=", assigns a value to the variable parameter

3. ${parameter:?word} Functional Practice:

If the parameter variable is not assigned a value, the error message is "?". After the defined character, if the value is assigned, the output value is directly

For example:

[[email protected] ~]# unset test #>== Cancel assignment of test variable [[email protected] ~]# echo ${test:? is not Value}-bash:test:is Not value #>== can be seen, not assigned to the case, output? Post-defined error message

What if there is a value assigned? View results:

[[email protected] ~]# test= "ABCDEFG" #>== assign value to test variable [[email protected] ~]# echo $TESTABCDEFG [[email protected] ~]# echo ${test:? Test is not having VALUE}ABCDEFG #>== test variable has value, direct output value

4. ${parameter:+word} Functional Practice

If the parameter is not assigned, the output is empty, and if the parameter variable has an assignment, the output + post-defined information is not assigned to the parameter variable

For example:

[Email protected] ~]# unset test[[email protected] ~]# echo $test [[email protected] ~]# echo ${test:+word} #>== The test variable is assigned, the output is empty [[email protected] ~]# Test=aaaaaaa[[email protected] ~]# echo ${test:+word} #>== output Word, indicating that the test variable has an assignment, However, word is not assigned to test, only output information that prompts Word[[email protected] ~]# echo $test #>== test variable or originally defined value AAAAAAA

Production Case:

Delete outdated data backups from 7 days ago

If you forget to define the path variable, and you do not want the path value to be null, you can define the return value of/tmp instead of the path null value.

For example:

[[email protected] ~]# cat delete.sh find ${path-/tmp}-name *.tar.gz-type f-mtime +7 | Xargs rm-f[[email protected] ~]# sh-x delete.sh + xargs rm-f+ find/tmp-name ' *.tar.gz '-type f-mtime +7 If the path variable is not set, Empty, the following content is returned: [[email protected] ~]# cat delete.sh Find ${path}-name *.tar.gz-type f-mtime +7 | Xargs rm-f[[email protected] ~]# sh-x delete.sh + xargs rm-f+ find-name ' *.tar.gz '-type f-mtime +7 #>== Execute the script at this time, You'll get an error.

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Practice for Shell-specific extended variables