SED variable substitution and Linux special symbols encyclopedia

1 SED supports the substitution of variables in two ways, it is recommended to use the following second, relatively simple

This allows the variable A to be assigned a number of special characters, such as Assignment a= '! @#￥% ... '

Sed-n ' s/echo/' "$a" '/P ' AA
Sed-n "s/echo/$a/P" AA

2 Linux special characters Daquan, see links

Shell Escape character
Character	Description
‘’	Single quotes, hard escapes, and all shell metacharacters and wildcards inside them are turned off. Note that ' (single quotation marks) are not allowed in hard escaping. The enclosed quotation marks are treated as a single string. The $ sign that represents the variable within the quotation marks does not work, that is, he is treated as a general symbol and prevents any variable substitution.
“”	Double quotes, soft escapes, which are only allowed to appear within the specific shell metacharacters ($, ', \): $ for Variable value substitution, ' for command substitution, \ \ For escaping a single character
\	Backslash, escaping, removes the special meaning of the meta-character or wildcard immediately followed. The content enclosed in double quotes is treated as a single string. It prevents wildcard expansion, but allows variable expansion. This is different from the single-argument approach

Reference

Use variable substitution in sed and execute external command

Linux Special Symbols Encyclopedia

SED variable substitution and Linux special symbols encyclopedia