BSD sed vs. GNU sed, and -i

bsdgnused

Unix iMac shell terminal 

sed -i 's/original/new/g' maths.tx

Message returned: sed: -i may not be used with stdin

Best Answer

Macs use the BSD version of utilities such as sed and date, which have their own idiosyncrasies.

In this specific case, the BSD build of sed mandates the extension for the backup file with -i, rather than it being optional, as in GNU sed.

As such:

sed -i .bak 's/needle/pin/g' haystack

The shown command will replace all instances of needle with pin in the file haystack, and the original file will be preserved in haystack.bak.

From the manual for the implementation of sed on a Mac:

-i extension
         Edit files in-place, saving backups with the specified extension.  If a zero-length extension is given, no backup will be saved.
         It is not recommended to give a zero-length extension when in-place editing files, as you risk corruption or partial content in
         situations where disk space is exhausted, etc.

As opposed to on a Linux host:

  -i[SUFFIX], --in-place[=SUFFIX]

          edit files in place (makes backup if SUFFIX supplied)

Note that "a zero-length extension" is distinct from "no extension". You can eschew the backup entirely, then, with:

sed -i '' 's/needle/pin/g' haystack
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