Daily Archives: 21 June 2017

Bash: Remove the last character from each line 1

 in Bash  tagged cut / last character / remove / rev by

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

 

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

 in GNU/Linux  tagged awk / input field separator / NF / number of fields / vFS by

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

 in Bash  tagged awk / fflush / stamp / strftime / time / timestamp by

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