common/*: update pages of POSIX special Built-In utilities (#11858)

* eval, exec, exit, export, readonly, set, shift, trap: use Open Group links

* exec, export, trap: split Linux and POSIX

pages/linux/exec.md

@@ -0,0 +1,20 @@
+# exec
+
+> Replace the current process with another process.
+> More information: <https://linuxcommand.org/lc3_man_pages/exech.html>.
+
+- Replace with the specified command using the current environment variables:
+
+`exec {{command -with -flags}}`
+
+- Replace with the specified command, clearing environment variables:
+
+`exec -c {{command -with -flags}}`
+
+- Replace with the specified command and login using the default shell:
+
+`exec -l {{command -with -flags}}`
+
+- Replace with the specified command and change the process name:
+
+`exec -a {{process_name}} {{command -with -flags}}`

pages/linux/export.md

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+# export
+
+> Command to mark shell variables in the current environment to be exported with any newly forked child processes.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#index-export>.
+
+- Set a new environment variable:
+
+`export {{VARIABLE}}={{value}}`
+
+- Remove an environment variable:
+
+`export -n {{VARIABLE}}`
+
+- Mark a shell function for export:
+
+`export -f {{FUNCTION_NAME}}`
+
+- Append something to the PATH variable:
+
+`export PATH=$PATH:{{path/to/append}}`

pages/linux/trap.md

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+# trap
+
+> Automatically execute commands after receiving signals by processes or the operating system.
+> Can be used to perform cleanups for interruptions by the user or other actions.
+> More information: <https://manned.org/trap>.
+
+- List available signals to set traps for:
+
+`trap -l`
+
+- List active traps for the current shell:
+
+`trap -p`
+
+- Set a trap to execute commands when one or more signals are detected:
+
+`trap 'echo "Caught signal {{SIGHUP}}"' {{SIGHUP}}`
+
+- Remove active traps:
+
+`trap - {{SIGHUP}} {{SIGINT}}`