Bash

Perform diff on two folders

 in Bash  tagged diff / recursion by

To perform a recursive diff on all the files of two folders we just need to add the -r (or --recursive) parameter that recursively compares any subdirectories found.

To avoid needless messages from the tool, we can also use the -q (or --brief) parameter that reports only when files differ.

Example of performing diff on two folders recursively while preventing needless messages.


diff -rq aFolder someOtherFolder;

How to find lines that contain only lowercase characters

 in Bash  tagged grep / lower case / lowercase / regex / regular expression by

To print all lines that contain only lower case characters, we used the following regular expression in grep:


egrep '^[[:lower:]]+$' <file>;
#If you do not have egrep, use
grep -e '^[[:lower:]]+$' <file>;

Breakdown of the above regular expression:

  • ^ instructs the regular expression parser that the pattern should always start with the beginning of the line
  • [[:lower:]] this special instruction informs us that only lower case characters can match it
  • + the plus sign causes the preceding token to be matched one or more times
  • $ signifies the end of the line

Fedora 26: C++: static linking cannot find -lstdc++ -lm and -lc

 in Bash / GNU/Linux / Programming  tagged C++ / dnf / fedora / g++ / glibc / glibc-static / lc / libstdc / libstdc++-static / lm / lstdc / static by

Recently, we were trying to compile a C++ application with the following compilation command on a Fedora 26 64bit :


g++ -static -O2 -lm -Wall -Wno-unused-result -std=c++14 -DCS_ACADEMY -DONLINE_JUDGE 510152025.cpp -o 510152025;

unfortunately, we got the following errors:

 /usr/bin/ld: cannot find -lstdc++
 /usr/bin/ld: cannot find -lm
 /usr/bin/ld: cannot find -lc
 collect2: error: ld returned 1 exit status

To resolve the issues, we performed the following installations to install the static versions of the glibc and libstdc libraries:


sudo dnf install glibc-static libstdc++-static -y;

 

Bash: After redirected input file is done, allow user to control application via STDIN 1

 in Bash  tagged cli / concatenate / end of file / eof / keyboard / mix / stdin by

Recently, we needed to start an application using a script, which application had its own CLI.
After starting it, we had to feed it with some input to configure it before handing it over to the end user.

The application we used was named dog. We wrote into a plain text file (named food) the commands that we needed to feed the application and then we started the execution using a plain input redirect like so dog < food;.
Doing so, resulted into properly feeding the dog application the food data  but it would cause the application to terminate immediately after the food was done as it would also feed the EOF (End Of File) directive to dog.
Apparently, the application after receiving the EOF, it would then terminate without allowing the end user to take control.

To mitigate the problem, we used the cat command to concatenate the input file along with the stdin stream, permitting us to first feed all the data in the food file and then allow the user to input data using the standard input stream.
Our final command resulted in the following solution as below

cat <(cat food) - | dog;

Where - is the special sign for standard input stdin.
cat food can be of course replaced with other commands that produce output on the standard output (stdout).

A bad side-effect of this solution, is that we lost some functionality of the terminal including, but not limited to, using the backspace and navigation.