Double quotes
The text will be displayed in the form of characters, and part of the expansion will be filtered, but the parameter expansion, arithmetic expansion and command substitution are still valid ($,``,\)
apostrophe
Suppress all expansion
Backslash
Escape characters, which can limit the expansion of some double quotes
Example 1:
Variable nesting: can be achieved by suppressing the outer expansion and eval command
[portal@ccodtk test]$ cat test1.sh
#!/bin/bash
set -xueo pipefail
a_config="{a1,a2}"
app1_config=\${${1}_config}
echo $app1_config
eval app_config=\${${1}_config}
echo $app_config
--------------------------
[portal@ccodtk test]$ bash test1.sh a
+ a_config='{a1,a2}'
+ app1_config='${a_config}'
+ echo '${a_config}'
${a_config}
+ eval 'app_config=${a_config}'
++ app_config='{a1,a2}'
+ echo '{a1,a2}'
{a1,a2}
analysis:
app1_configAfter adding the transfer character to the outer layer of the variable , it refers to the expansion of the outer layer, so as to realize theapp1_configcontent of the variable form.- Through the command eval, the content of this variable in text form is parsed by bash to show the final content
Example 2:
Single quotes suppress all transfers, double quotes do not suppress transfers of parameter expansion
[root@qcteam-ciserver resources]# echo $USER
root
[root@qcteam-ciserver resources]# pwd
/home/docker/Cubes/dcmsSecurity/src/main/resources
[root@qcteam-ciserver resources]# ssh [email protected] 'pwd; echo $USER'
/home/portal
portal
[root@qcteam-ciserver resources]# ssh [email protected] "pwd; echo $USER"
/home/portal
root
[root@qcteam-ciserver resources]# ssh [email protected] "pwd; echo \$USER"
/home/portal
portal
analysis:
- Through the first two steps, you can see the current environment information;
- The third step shows the suppression and expansion function of single quotation marks, which will pass all the content inside the quotation marks as they are;
- The fourth step shows the function of the inconsistent parameter expansion of double quotes,
$USERwhich has been parsed as the current user's value before being passed. - The fifth step shows the suppression of expansion of backslashes in double quotes