Shell New Added 8 September 2026

sed: -i may not be used with stdin / extra characters after command (macOS)

macOS ships BSD sed, where -i takes a mandatory backup suffix argument. A script written against GNU sed passes the expression where BSD sed expects the suffix, so the same command works on Linux and fails or silently creates odd files on a Mac.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# GNU (Linux)
sed -i 's/foo/bar/' file

# BSD (macOS): empty suffix must be given explicitly
sed -i '' 's/foo/bar/' file

# Portable across both
sed -i.bak 's/foo/bar/' file && rm file.bak

# Or install GNU tools and use them by name
brew install gnu-sed
gsed -i 's/foo/bar/' file

# Portable scripts are usually better off with perl
perl -pi -e 's/foo/bar/' file

How to diagnose Shell errors

Shell errors are usually about PATH, permissions, or quoting and expansion. The quoting class is the most damaging because it fails silently: an unquoted variable containing a space becomes two arguments, and an empty one disappears entirely. Running shellcheck over any script longer than a few lines catches most of these before they run.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Trace execution with bash -x script.sh to see each command after expansion. This makes quoting bugs immediately visible.
Start every script with set -euo pipefail so failures stop the script instead of cascading.
Quote every variable expansion: "$var", "${arr[@]}". This single habit prevents the majority of shell bugs.
Use command -v foo to check whether something is on PATH, and echo "$PATH" to see what the shell is actually searching.
In Makefiles, recipe lines must begin with a real tab character. "missing separator" always means spaces were used instead.

Tools worth reaching for

shellcheck
bash -x
set -euo pipefail
command -v
cat -A (to reveal tabs)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

gnu.org

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