Can I make `cut` change a file in place?
The moreutils package from ubuntu (and also debian) has a program called sponge
, which sort-of also solves your problem.
From man sponge:
sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file. This allows constricting pipelines that read from and write to the same file.
Which would let you do something like:
cut -d <delim> -f <fields> somefile | sponge somefile
You can't. Either use ed or GNU sed or perl, or do what they do behind the scenes, which is to create a new file for the contents.
ed
, portable:
ed foo <<EOF
1,$s/^\([^,]*\),\([^,]*\),\([^,]*\).*/\1,\3/
w
q
EOF
GNU sed
:
sed -i -e 's/^\([^,]*\),\([^,]*\),\([^,]*\).*/\1,\3/' foo
Perl:
perl -i -l -F, -pae 'print @F[1,3]' foo
cut
, creating a new file (recommended, because if your script is interrupted, you can just run it again):
cut -d , -f 1,3 <foo >foo.new &&
mv -f foo.new foo
cut
, replacing the file in place (retains the ownership and permissions of foo
, but needs protection against interruptions):
cp -f foo foo.old &&
cut -d , -f 1,3 <foo.old >foo &&
rm foo.old
I recommend using one of the cut
-based methods. That way you don't depend on any non-standard tool, you can use the best tool for the job, and you control the behavior on interrupt.
I don't think that is possible using cut
alone. I couldn't find it in the man or info page. You can do something such as
mytemp=$(mktemp) && cut -d" " -f1 file > $mytemp && mv $mytemp file
mktemp
makes you a relatively safe temporary file that you can pipe the cut
output into.