Monthly Archives: June 2017


Bash: Remove the last character from each line 1

The following script, uses rev and cut to remove the last character from each line in a pipe.
rev utility reverses lines character-wise.
cut removes sections  from each of line.
It is a very simple script where we reverse the line once, remove the first character (which was the last one in the original form of the line) and finally we reverse the line back with the last character missing.

echo -e "hi\nHI" | rev | cut -c 2- | rev;

# Will produce:
h
H

 

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Bash: Switch positions between all characters in odd positions with characters in even positions

The following awk script allowed us to switch position of all characters placed in odd numbered positions with their next neighboring even numbered position characters.
In detail what it does is to create a for loop that skips one character every time and then it prints each pair in reverse order (it will print the second character first, then the first one, then the fourth and so on).

echo "123456789" | awk -vFS= '{for (i = 1; i <= NF; i+=2) {printf $(i+1)$i""} printf "\n"}';

# Will produce 214365879

echo "1234567890" | awk -vFS= '{for (i = 1; i <= NF; i+=2) {printf $(i+1)$i""} printf "\n"}';

# Will produce 2143658709

Please note that we set the built-in variable FS (The input field separator which is a space by default) to the empty string so that each character is treated like a different field by NF (The number of fields in the current input record).

 


Bash: Print time stamp in front of every line in a pipe

Recently, we received a binary that collected data from a web service and it printed them on screen.
The binary did not print a time stamp in front of each line so we had to improvise of a way to add the time stamp to the logs without modifying the binary.

The solution we came to was to use awk to prepend the time stamp in front of every line using a pipe.
Specifically, our solution was the following:

server_application 2>&1 | awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0; fflush(); }'

What we did there was to start our binary server_application, redirect stderr to stdout (using 2>&1) so that we will have only one stream and then we read the lines one by one using awk and printed the time stamp right before the line ($0) using strftime.
The strftime() function formats the broken-down time according to the format specification format.
fflushforces a write of all user-space buffered data for the given output or update stream via the stream’s underlying write function. We call it at each line to make sure that we do not cause additional delay in presenting the data due to buffering limitations caused by our prints.

Example

$ echo -e "hi\nHI" 2>&1 | awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0; fflush(); }'
2017-06-21 20:33:41 hi
2017-06-21 20:33:41 HI


How to execute `find` that ignores .git directories

Trying to find a source code file by its content using find and -exec grep, can some times result in getting results from the repository .git folders as well.

This behavior not only does it provide results you do not need but it also makes your search slower.
Below, we propose a couple of solutions on how to make a more efficient search.

Example 1: Ignore all .git folders no matter where they are in the search path

For find to ignore all .git folders, even if they appear on the first level of directories or any in-between until the last one, add -not -path '*/\.git*' to your command as in the example below.
This parameter will instruct find to filter out any file that has anywhere in its path the folder .git. This is very helpful in case a project has dependencies in other projects (repositories) that are part of the internal structure.

find . -type f -not -path '*/\.git/*';

Note, if you are using svn use:

find . -type f -not -path '*/\.svn/*';

Example 2: Ignore all hidden files and folders

To ignore all hidden files and folders from your find results add -not -path '*/\.*' to your command.

find . -not -path '*/\.*';

This parameter instructs find to ignore any file that has anywhere in its path the string /. which is any hidden file or folder in the search path!


Linux on life issues

Why can GNU/Linux users always have love ?

Because they can install it from the repositories!

$ love
 bash: love: command not found...
 Install package 'love' to provide command 'love'? [N/y] y

What is the opinion of your PC on love ?

That is love is not something appropriate!

#When you do not have the love package installed, you will get the following message
$ whatis love
 love: nothing appropriate.