On 1/30/22 17:12, H wrote:
I am writing a long bash script under CentOS 7 where perl is used for manipulating some external files. So far I am using perl one-liners to do so but ran into a problem when I need to append text to an external file.
Here is a simplified example in the bash script where txt is a bash variable which I built containing a longish text with multiple newlines:
txt="a b$'\n'cd ef$'\n'g h$'\n'ij kl"
A simplified perl one-liner to append the text in the variable above to some file in the bash script would be:
perl -pe 'eof && do{print $_'"${txt}"'; exit}' someexternalfile.txt
This works when fine when $txt does /not/ contain any spaces but falls apart when it does.
I would like to keep the above structure, ie using bash variables to build text strings and one-liners to do the text manipulation. Hopefully there is a "simple" solution to do this, I have tried many variations and failed miserably... Note that I also want to use a similar pattern to do substitutions in external files, I would thus like to use the same code pattern.
Thanks.
On 2/2/22 17:54, H wrote:
I am still having a problem. The following (where $txt is an
arbitrary string) works:
perl -e 'print '""${txt}""';'
The following does not work (I want to append the content of the $txt
to the end of an existing file in-place):
perl -i -pe 'eof && do{print $_''"aaa"''; exit}' somefile.txt
but this does:
perl -i -pe "eof && do{print $_"""${txt}""'; exit}' somefile.txt
as does:
perl -i -pe "eof && do{print $_"""${txt}"""; exit}" somefile.txt
The difference is that the last two perl command strings use " rather
than '.
My questions are:
- Why would not using single-quotes for parts of the perl command
string work?
- Is there any reason I should fight this or should I just go with
double-quotes for all parts of the perl command string? Any downside? Remember, these are all in bash scripts and I am looking for a "pattern" to use for other, more complicated text substitutions, hence the use of perl.
On 2/2/22 18:55, H wrote:
I see I made a mistake, the line:
perl -i -pe 'eof && do{print $_''"aaa"''; exit}' somefile.txt
should be:
perl -i -pe 'eof && do{print $_''"${txt}"''; exit}' somefile.txt
Related question, if the $txt string contains eg $ or another special
character, what would be the best way of escaping it so it is not substituted by perl?
AIUI you are looking for a Bash shell scripting (programming) technique that allows you to append content to a file using a Perl one-liner with data that is dynamically generated from Bash variable values, all inside a Bash script (?).
Perhaps it would be simpler if you put the data into a file and then invoked the Perl one-liner with the data file filename as an argument:
2022-02-02 22:08:34 dpchrist@tinkywinky ~ $ cat somefile.txt this is the contents of somefile. some more contents.
2022-02-02 22:08:37 dpchrist@tinkywinky ~ $ cat centos-h.sh #!/bin/bash txt="... foo ..." echo "$txt" > tmp.txt perl -pe 's/foo/bar/g' tmp.txt >> somefile.txt
2022-02-02 22:08:46 dpchrist@tinkywinky ~ $ bash centos-h.sh
2022-02-02 22:08:53 dpchrist@tinkywinky ~ $ cat somefile.txt this is the contents of somefile. some more contents. ... bar ...
David