I am writing a shell program to output another shell program to be evalled later. Is there some common shell program to print shell escaped for a string?
I'm not sure I understand you question. But the %q option of printf might be what you are looking for.
%q Output the corresponding argument in a format that can be reused as shell input
printf %q 'C:\ProgramFiles is a Windows path;'
outputs C:\\ProgramFiles\ is\ a\ Windows\ path\;
(In this example, simple quotes are needed – comment of Gordon Davisson – but this doesn't matter if you print from a variable or the output of a command.)
You could use single quoted string as this is evaluated without any substitution.
For example the following commands are equivalent
cat abc\ hi.txt
cat 'abc hi.txt'
Related
I would like to do a printf of variables, strings and new lines to a file in bash, and in one line. I would of used echo, but echo seems to be interpreted differently and on other questions have recommended printf.
NAME="karl"
printf %"s\n" this is $(NAME) hello! > my_file
When I do a cat my_file I'd like it to look as the following:
this
is
karl
hello!
I think you just want this:
name="karl"
printf "%s\n" this is "$name" hello! > my_file
The only issue with your code was that you were using $() (a command substitution) rather than a standard parameter expansion. That said, your format specifier looked strange, as it is normally written %s (it still works written your way, because the shell consumes the quotes).
Quotes are always a good idea to prevent word splitting (e.g. spaces in the name would go on separate lines) and glob expansion (a name like a* would expand to a list of paths starting with a).
try replacing the printf line with
sed 's/[[:space:]]/\n/g ' <<< "this is ${NAME} hello!" > my_file
I am new in linux and AWK as well. I have a file in my home folder called testing.txt and i am trying to read the file using this awk command:
**arjun#arjun-Aspire-4741:~$ awk ´{print $1}´ testing.txt¨**
And I am getting this as output
**¨awk: ´{print
awk: ^ invalid char '�' in expression
arjun#arjun-Aspire-4741:~$ ¨**
The problem is that you've used forward ticks instead of quotes (in this case only single quotes are appropriate):
awk '{print $1}' testing.txt
instead of
awk ´{print $1}´ testing.txt
In shell, strings in double quotes " can contain expressions with special meaning (such as backticks, variables) which will be expanded before the string is processed as part of the full shell command. Strings in single quotes ' are fully escaped; to put it another way, the string is passed literally without any interpretation. That's why you should use single quotes when writing awk scripts, because the awk variable dereference operator $ is the same as in shell. There are no other valid string-delimiting characters*.
I initially thought you'd used backticks (thanks to Andras Deak for spotting my error).
Backticks have a special meaning in shell (equivalent to wrapping something in $(...)): execute this string as a command, and evaluate to its output (stdout). This is done before your main command is executed.
So, if I do
cat `echo myfile`
this turns into
cat myfile
which then executes.
You can read more about shell behaviour in a few places:
http://www.grymoire.com/Unix/Sh.html
http://www.tldp.org/LDP/Bash-Beginners-Guide/html/index.html
* ignoring that spaces are also technically string-delimiters
I have a string:
{2013/05/01},{2013/05/02},{2013/05/03}
I want to append a { at the beginning and a } at the end. The output should be:
{{2013/05/01},{2013/05/02},{2013/05/03}}
However, in my shell script when I concatenate the curly braces to the beginning and end of the string, the output is as follows:
{2013/05/01} {2013/05/02} {2013/05/03}
Why does this happen? How can I achieve my result? Am sure there is a simple solution to this but I am a unix newbie, thus would appreciate some help.
Test script:
#!/usr/bin/ksh
valid_data_range="{2013/05/01},{2013/05/02},{2013/05/03}"
finalDates="{"$valid_data_range"}"
print $finalDates
The problem is that when you have a list in braces outside quotes, the shell performs Brace Expansion (bash manual, but ksh will be similar). Since the 'outside quotes' bit is important, it also tells you how to avoid the problem — enclose the string in quotes when printing:
#!/usr/bin/ksh
valid_data_range="{2013/05/01},{2013/05/02},{2013/05/03}"
finalDates="{$valid_data_range}"
print "$finalDates"
(The print command is specific to ksh and is not present in bash. The change in the assignment line is more cosmetic than functional.)
Also, the brace expansion would not occur in bash; it only occurs when the braces are written directly. This bilingual script (ksh and bash):
valid_data_range="{2013/05/01},{2013/05/02},{2013/05/03}"
finalDates="{$valid_data_range}"
printf "%s\n" "$finalDates"
printf "%s\n" $finalDates
produces:
ksh
{{2013/05/01},{2013/05/02},{2013/05/03}}
{2013/05/01}
{2013/05/02}
{2013/05/03}
bash (also zsh)
{{2013/05/01},{2013/05/02},{2013/05/03}}
{{2013/05/01},{2013/05/02},{2013/05/03}}
Thus, when you need to use the variable $finalDates, ensure it is inside double quotes:
other_command "$finalDates"
if [ "$finalDates" = "$otherString" ]
then : whatever
else : something
fi
Etc — using your preferred layout for whatever you don't like about mine.
You can say:
finalDates=$'{'"$valid_data_range"$'}'
The problem is that the shell is performing brace expansion. This allows you to generate a series of similar strings:
$ echo {a,b,c}
a b c
That's not very impressive, but consider
$ echo a{b,c,d}e
abc ace ade
In order to suppress brace expansion, you can use the set command to turn it off temporarily
$ set +B
$ echo a{b,c,d}e
a{b,c,d}e
$ set -B
$ echo a{b,c,d}e
abe ace ade
Thank you very much in advance for helping.
The title says everything: what's the difference between using:
echo `basename $HOME`
and
echo $(basename $HOME)
Please notice that I know what the basename command does, that both syntax are valid and both commands give the same output.
I was just wondering if there is any difference between both and if it's possible, why there are two syntaxes for this.
Cheers
Rafael
The second form has different escaping rules making it much easier to nest. e.g.
echo $(echo $(basename $HOME))
I'll leave working out how to do that with ` as an exercise for the reader, it should prove enlightening.
They are one of the same.
please read this.
EDIT (from the link):
Command substitution
Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed like this:
$(command)
or like this using backticks:
`command`
Bash performs the expansion by executing COMMAND and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting.
$ franky ~> echo `date`
Thu Feb 6 10:06:20 CET 2003
When the old-style backquoted form of substitution is used, backslash retains its literal meaning except when followed by "$", "`", or "\". The first backticks not preceded by a backslash terminates the command substitution. When using the $(COMMAND) form, all characters between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backticks with backslashes.
If the substitution appears within double quotes, word splitting and file name expansion are not performed on the results.
They are alternative syntaxes for command substitution. as #Steve mentions they have different quoting rules and th backticks are harder to nest with. On the other hand they are more portable with older version of bash, and other shells eg csh.
This question already has answers here:
Command to escape a string in bash
(5 answers)
Closed 4 years ago.
Is there a command that not just echos it's argument but also escapes them if needed (e.g. if a argument contains white space or a special character)?
I'd need it in some shell magic where instead of executing a command in one script I echo the command. This output gets piped to a python script that finally executes the commands in a more efficient manner (it loads the main() method of the actual target python script and executes it with the given arguments and an additional parameter by witch calculated data is cached between runs of main()).
Instead of that I could of course port all the shell magic to python where I wouldn't need to pipe anything.
With bash, the printf builtin has an additional format specifier %q, which prints the corresponding argument in a friendly way:
In addition to the standard printf(1) formats, %b causes printf to expand backslash escape sequences in the corresponding argument (except that \c terminates output, backslashes in \', \", and \? are not removed, and octal escapes beginning with \0 may contain up to four digits), and %q causes printf to output the corresponding argument in a format that can be reused as shell input.
So you can do something like this:
printf %q "$VARIABLE"
printf %q "$(my_command)"
to get the contents of a variable or a command's output in a format which is safe to pass in as input again (i.e. spaces escaped). For example:
$ printf "%q\n" "foo bar"
foo\ bar
(I added a newline just so it'll be pretty in an interactive shell.)