How could I tail and append a text in a file, in one line, in Bash?

My failed attempt would be

tail file.txt > file.txt && date >> file.txt

2

3 Answers

This produces unexpected results:

tail file.txt > file.txt && echo $(date) >> file.txt

The first redirection truncates the file before the tail process starts, so tail is now operating on an empty file. (ref)

2 suggestions:

1. a temp file:

   f=file.txt
   tmp=$(mktemp)
   { tail "$f"; date; } > "$tmp" && mv "$tmp" "$f"

2. sponge from the moreutils package

   f=file.txt
   { tail "$f"; date; } | sponge "$f"

Note that echo $(date) is redundant: you don't need echo to send the output of date to stdout -- date does this by default.

Another way without using temporary files or installing additional tools (but not efficient for large files):

<<<"$(<file)" tail >file && date >>file

This reads file in a command substitution, guaranteeing that the reading of file will take place before the other redirections are resolved.

You can use Vim in Ex mode:

ex -sc '1,-10d|x' file.txt

1. 1,-10 select all but last 10 lines

2. d delete

3. x save and close

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