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Some systems have special conversion tools available to do this automatically. `dos2unix`, `recode, and `fromdos` are some examples. Some systems have special conversion tools available to do this automatically. `dos2unix`, `recode`, and `fromdos` are some examples.

How do I convert a file from DOS format to UNIX format (remove CRs from CR-LF line terminators)?

Carriage return characters (CRs) are used in line ending markers on some systems. There are three different kinds of line endings in common use:

  • Unix systems use Line Feeds (LFs) only.
  • MS-DOS and Windows systems use CR-LF pairs.
  • Old Macintosh systems use CRs only.

If you're running a script on a Unix system, the line endings need to be Unix ones (LFs only), or you will have problems.

Testing for line terminator type

A simple check is to simply look at the output of cat -e:

cat -e yourscript

If you see something like this, then you're dealing with CRLF style newlines:

another command^M$

Another method is to use the file utility to guess at the file type:

file yourscript

The output tells you whether the ASCII text has some CR, if that's the case. Note: this is only true on GNU/Linux. On other operating systems, the result of file is unpredictable, except that it should contain the word "text" somewhere in the output if the result "kind of looks like a text file of some sort, maybe".

imadev:~$ printf 'DOS\r\nline endings\r\n' > foo
imadev:~$ file foo
foo:            commands text
arc3:~$ file foo
foo: ASCII text, with CRLF line terminators

In a script, it's more difficult to say what the most reliable method should be. Anything you do is going to be a heuristic. In theory a non-corrupt file created by a non-broken UNIX utility should only contain LFs, and by a DOS utility, there should be no bare LFs not preceded by a CR. You can check this with a PCRE.

# Bash / Ksh

# [[ $(</dev/fd/0) == ~(P)(?<!$'\r')$'\n' ]] # ksh93 PCRE
if [[ $(</dev/fd/0) == $pattern ]] <<<$'foo\r\nbar\r\nbaz\r'; then
    print 'File contains only CRLFs'
    print 'File contains at least one newline not preceded by a CR'

Converting files

ex is a good standard way to convert CRLF to LF, and probably one of the few reasonable methods for doing it in-place from a script:

ex -sc $'%s/\r$//e|x' file

Of course, Any of the more powerful dynamic languages to do this with relative ease.

perl -pi -e 's/\r\n/\n/' filename

Some systems have special conversion tools available to do this automatically. dos2unix, recode, and fromdos are some examples.

It be done manually with an editor like nano:

nano -w yourscript

Type Ctrl-O and before confirming, type Alt-D (DOS) or Alt-M (Mac) to change the format.

Or in Vim, use :set fileformat=unix and save with :w. Ensure the value of fenc is correct (probably utf-8).

To simply strip all CRs from some input stream, you can use tr -d '\r' <infile >outfile. Of course, you must ensure these are not the same file.

BashFAQ/052 (last edited 2022-01-30 01:59:53 by larryv)