09-21-2019
Not sure I'm an expert nor a guru, but this is what I'd do: refer to the respective man pages.
man bash:
Quote:
Parameter Expansion
${parameter#word}
${parameter##word}
Remove matching prefix pattern.
${parameter%word}
${parameter%%word}
Remove matching suffix pattern.
CONDITIONAL EXPRESSIONS.
An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex(3))
Of course you need to learn the difference between shell's pattern matching and regex matching.
man regex:
Quote:
An atom is a regular expression ... '^' (matching the null string at the beginning of a line)
A bracket expression is a list of characters enclosed in "[]". It normally matches any single character from the list (but see below). If the list begins with '^', it matches any single character (but see below) not from the rest of the list.
Admittedly the ambiguous use of the caret is something you need to accustom to.
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RBASH(1) General Commands Manual RBASH(1)
NAME
rbash - restricted bash, see bash(1)
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted. A restricted shell is
used to set up an environment more controlled than the standard shell. It behaves identically to bash with the exception that the follow-
ing are disallowed or not performed:
o changing directories with cd
o setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
o specifying command names containing /
o specifying a file name containing a / as an argument to the . builtin command
o specifying a filename containing a slash as an argument to the -p option to the hash builtin command
o importing function definitions from the shell environment at startup
o parsing the value of SHELLOPTS from the shell environment at startup
o redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
o using the exec builtin command to replace the shell with another command
o adding or deleting builtin commands with the -f and -d options to the enable builtin command
o using the enable builtin command to enable disabled shell builtins
o specifying the -p option to the command builtin command
o turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed, rbash turns off any restrictions in the shell spawned to execute the script.
SEE ALSO
bash(1)
GNU Bash-4.0 2004 Apr 20 RBASH(1)