One more update to MacOS section.

#153

README.md

@@ -442,13 +442,13 @@ These are items relevant *only* on MacOS.
 
 - Package management with `brew` (Homebrew) and/or `port` (MacPorts). These can be used to install on MacOS many of the above commands.
 
-- Copy output of any command with `pbcopy` and paste input with `pbpaste`.
+- Copy output of any command to a desktop app with `pbcopy` and paste input from one with `pbpaste`.
 
 - To open a file with a desktop app, use `open` or `open -a /Applications/Whatever.app`.
 
 - Spotlight: Search files with `mdfind` and list metadata (such as photo EXIF info) with `mdls`.
 
-- Be aware MacOS is based on BSD Unix, and many commands (for example `ps`, `ls`, `tail`, `awk`, `sed`) have many subtle variations from Linux, which is largely influenced by System V-style Unix and GNU tools. You can often tell the difference by noting a man page has the heading "BSD General Commands Manual." In some cases GNU versions can be installed, too (such as `gawk` and `gsed` for GNU awk and sed).
+- Be aware MacOS is based on BSD Unix, and many commands (for example `ps`, `ls`, `tail`, `awk`, `sed`) have many subtle variations from Linux, which is largely influenced by System V-style Unix and GNU tools. You can often tell the difference by noting a man page has the heading "BSD General Commands Manual." In some cases GNU versions can be installed, too (such as `gawk` and `gsed` for GNU awk and sed). If writing cross-platform Bash scripts, avoid such commands (for example, consider Python or `perl`) or test carefully.
 
 
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