A bash script that works on your Linux build server misbehaves on macOS: `sed -i 's/a/b/' notes.txt` fails with a cryptic message about an undefined label, and `date -d '2 days ago'` is rejected outright. Why do identically named commands behave differently, and how would you make the script work on both?

## Two different programs wearing the same name `sed` on a Debian or RHEL host is GNU sed. `sed` on macOS is a BSD sed descended from the FreeBSD tree. They are separate codebases that both implement the POSIX `sed` specification and then add their own options on top. The same is true of `date`, `stat`, `grep`, `readlink`, `awk` and most of the text-processing toolkit. Nothing about your bash script changed — the programs it calls did. POSIX is the contract you can rely on. Anything outside it is an extension, and extensions are exactly where the two lineages diverge. ## The specific traps worth memorising **`sed -i`** is not in POSIX at all. GNU sed accepts `-i` alone, or a suffix attached with no space (`-i.bak`). BSD sed defines `-i extension`, taking the extension as a *separate argument* that may be an empty string. The consequences are symmetrical and both are confusing: ```bash sed -i 's/a/b/' f # BSD: 's/a/b/' becomes the backup suffix, then f is read as the script sed -i '' 's/a/b/' f # GNU: '' becomes the script, then 's/a/b/' is read as a filename sed -i -e 's/a/b/' f # BSD: -e is taken as the backup suffix ``` **`date`.** GNU has `-d 'STRING'`, a full natural-language-ish date parser: `date -d '2 days ago' +%F`. BSD has no such thing; it has `-v` adjustments: `date -v-2d +%F`. Worse, `-r` exists on both and means different things — GNU's `-r FILE` prints that file's modification time, BSD's `-r SECONDS` interprets an epoch timestamp. **`stat`** has entirely different format syntax: GNU `stat -c '%s' f` versus BSD `stat -f '%z' f`. **`readlink -f`** is a GNU extension; older macOS `readlink` has no `-f`, which is why so many scripts hand-roll a `cd "$(dirname "$0")" && pwd` idiom. **`grep -P`** (PCRE) is a GNU extension and is not available on macOS's grep. `grep -E` is POSIX and safe. **`awk`** is three different programs in practice — gawk, mawk, and the BSD one-true-awk — so gawk-only features such as `gensub()` or `IGNORECASE` are not portable either. ## The strategies, from best to worst **1. Avoid the extension.** Most in-place edits do not need `-i`. Writing to a temp file and moving it into place is portable, and it is also atomic — readers never see a half-written file: ```bash tmp=$(mktemp) sed 's/foo/bar/g' config.txt > "$tmp" && mv "$tmp" config.txt ``` Similarly, prefer `date -u +%s` arithmetic over relative date strings where you can. Be aware this only takes you so far: converting an epoch back to a formatted date is `date -d @EPOCH` on GNU and `date -r EPOCH` on BSD, so some branch is unavoidable. **2. Feature-detect, don't guess the OS.** Testing behaviour beats testing `uname`, because a Mac with Homebrew's `coreutils` and `gnu-sed` installed has both userlands available under `g`-prefixed names: ```bash if date --version >/dev/null 2>&1; then stamp=$(date -d '2 days ago' +%F) # GNU else stamp=$(date -v-2d +%F) # BSD fi ``` Do the detection once at the top and store the answer in a variable; do not sprinkle `uname` checks through the script. **3. Fix the environment instead of the script.** If the script is infrastructure rather than a developer convenience, run it in a container image where the userland is pinned. That converts an open-ended portability problem into a build-time dependency, and it is usually the cheapest answer for anything that runs in CI or production. **4. Move the logic somewhere uniform.** If the script is doing enough text manipulation that every second command has a portability branch, a single POSIX `awk` program — or a real program in another language — replaces a dozen fragile invocations. ## What to say in an interview The insight interviewers want is *the shell is not the portability boundary; the userland is*. A script can be perfectly POSIX shell code and still be completely unportable because of the options it passes to `sed`. Naming one concrete trap precisely — the `sed -i` argument shape is the canonical one — plus the temp-file-and-`mv` fix and feature detection over `uname` sniffing, covers the ground well.