I want to read a string from file and print it as follow :
LINE=tr '\n' ' ' < $FILENAME
x1=$LINE
echo $LINE
but the command echo $LINE display an empty line ?
-
Now saying that would give error, won't it? Use command substitution.devnull– devnull2014-01-15 18:05:20 +00:00Commented Jan 15, 2014 at 18:05
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3 Answers
As already pointed out by the other posters, to embed the output of the tr command into another command (in this case LINE=...), surround the tr command by $(...). In bash this is known as command substitution.
LINE=$(tr '\n' ' ' < "$FILENAME")
In case you intend to use $LINE as a sequence of parameters for subsequent commands (in this case echo), then newlines are eventually replaced by space during word splitting. This would make tr superfluous; you might as well do this:
LINE=$(cat "$FILENAME")
or even better:
LINE=$(< "$FILENAME")
Word splitting is not effective inside double quotes; so echo "$LINE" would still require tr to remove newlines.
5 Comments
mklement0
$(...) does NOT replace newlines with spaces - the only thing it does is trim trailing newlines (however many). Your alternate solutions therefore do not work as intended. Also, please double-quote the $FILENAME references.
Ruud Helderman
@mklement0: You are right; the truth about disappearing newlines is a bit more complicated. I made an edit to rephrase that.
mklement0
Thanks for the edit, but it is important to note that word splitting is NOT involved during variable assignment - if the contents of
$FILENAME has interior newlines, they will end up in the value of $LINE. It is only when you later reference $LINE without quoting that word splitting occurs - along with other shell expansions such as globbing, which may or may not be desired.Ruud Helderman
Thanks, I hope my edits sufficiently reflect your comments. Whether or not word splitting is applicable, depends entirely on the context (how will
$LINE be used). OP was not explicit about that, so everybody will have to make up his own mind there.mklement0
Thanks - between your edits and this comment thread I think we've got it covered.
You should be using command substitution like this:
LINE=$(tr '\n' ' ' < "$FILENAME")
But this will store tr's output into LINE variable.
4 Comments
user3137879
the problem was fixed , but what is the diffrence ?
anubhava
Command substitution has a syntax which is either back-tick OR $(..)mklement0
@user3137879: If you assign without
$(...) (or backticks), you assign a string literal to LINE. Command substitution means that the string enclosed in $(...) is interpreted as a command and executed, and its result is then assigned to LINE.anubhava
Thanks @mklement0 for your edit and explanation. OP: Sorry I was on phone with my client so couldn't explain it properly. See this link for ore details: en.wikipedia.org/wiki/Command_substitution
I think you need to put the call to the tr command with backquotes:
LINE=`tr '\n' ' ' < $FILENAME`
1 Comment
mklement0
Works, but use of backquotes is deprecated in favor of the
$(...) syntax - the latter has the advantage that it can be nested.