Diff for "BashFAQ/090"

Differences between revisions 7 and 11 (spanning 4 versions)

How do I prepend a text to a file (the opposite of >>)?

You cannot do it with bash redirections alone; the opposite of >> does not exist....

To insert content at the beginning of a file, you can use an editor, for example ex:

ex file << EOF
0a
header line 1
header line 2
.
w
EOF

or ed:

printf '%s\n' 0a "line 1" "line 2" . w | ed -s file

ex will also add a newline character to the end of the file if it's missing.

Or you can rewrite the file, using things like:

{ echo line; cat file ;} >tmpfile && mv tmpfile file
echo line | cat - file > tmpfile && mv tmpfile file

Some people insist on using the sed hammer to pound in all the screws:

sed "1iTEXTTOPREPEND" filename > tmp &&
mv tmp filename

With bash version >= 4 , this can also be done using here-docs either on command line or inside a script.

PS: Has not been tested in other shells.

newline="This is line -1"
cat <<EOF >a.txt 
$newline       #variables are accepted
This is line 0 #fixed string
$(cat a.txt)   #the old file - same name is ok
EOF

There are lots of other solutions as well.

-  ⇤ ← Revision 7 as of 2010-05-26 05:12:24 → 
  Size: 681
  Editor: 99-190-22-125
  Comment:
+   ← Revision 11 as of 2017-02-14 16:07:10 → ⇥
  Size: 1243
  Editor: peir-sab-antaeustravel-peer
  Comment: here-doc method improved
-Deletions are marked like this.
+Additions are marked like this.
 Line 2:
-Line 5:
+Line 6:
-To do it in 2 commands with sed:
sed "1iTEXTTOPREPEND" filename > tmp
mv tmp filename
+To insert content at the beginning of a file, you can use an editor, for example `ex`:
-Line 9:
+Line 8:
-To insert content at the beginning of a file, you can use an editor:
-Line 19:
+Line 17:
-ex will also add a newline character to the end of the file if it's missing.
+or [[http://bash-hackers.org/wiki/doku.php?id=howto:edit-ed|ed]]:

{{{
printf '%s\n' 0a "line 1" "line 2" . w | ed -s file
}}}
`ex` will also add a newline character to the end of the file if it's missing.
-Line 22:
+Line 25:
-Line 26:
+Line 30:
+Some people insist on using the `sed` hammer to pound in all the screws:
-Line 27:
+Line 32:
-or lots of other solutions.
+{{{
sed "1iTEXTTOPREPEND" filename > tmp &&
mv tmp filename
}}}

With bash version >= 4 , this can also be done using here-docs either on command line or inside a script.  

PS: Has not been tested in other shells.

{{{
newline="This is line -1"
cat <<EOF >a.txt 
$newline       #variables are accepted
This is line 0 #fixed string
$(cat a.txt)   #the old file - same name is ok
EOF
}}}

There are lots of other solutions as well.