shell script: array

# space separated
alphabets=("a" "b" "c" "d")

#To access any element by index
${alphabets[1]}

# to loop
for f in "${alphabets[@]}"

shell script: file information

file=/foo/bar/myfile.txt

File modify date and time

file_modify_date=$(stat -c %y $file | cut -d" " -f1)

file_modify_time="$(stat -c %y $file | cut -d" " -f2 | cut -d":" -f1,2 | tr -d ":")

File directory, name and extension

filename="${file##*/}"  # get filename

extn="${filename##*.}"

filename="${filename%.*}" # removing extension

if [[ $filename == $extn ]]; then
    extn=""
else
    extn=.$extn
fi

dirname="${file%/*}" # get dirname

basename is a good command too

References:

man stat
man basename

http://www.thegeekstuff.com/2010/07/bash-string-manipulation/

linux: logging top processes by cpu or memory

I wanted to log the processes consuming cpu and memory

First thought was to use top in the batch mode (-b).
-c to show the process name.
-n 1 to capture for 1, to run for 1 frame
   
$ top -b -c -n 1 > top_$(date +"%Y-%m-%d_%H%M").log

But I was not getting the complete process/command name.

Later I  used the below technique and it was quite helpful

#!/bin/bash

log_file=top_$(date +"%Y-%m-%d_%H%M").log
echo 'user    %cpu    %mem    pid    elapsed time    command' > $log_file
echo ' =========== CPU ===========' > $log_file
ps -eo user,pcpu,pmem,pid,etime,command | sort -rn -k2 | head -11 > $log_file
echo ' ========== MEMORY ==========' >> $log_file
ps -eo user,pcpu,pmem,pid,etime,command | sort -rn -k3 | head -11 >> $log_file

Observation:

1. 'ps aux' is a wonderful command

2. ps command itself has --sort <fieldname>, but there is no reverse sorting

bash script: arithmetic comparison

Arithmetic in BASH is integer math only. You can't do floating point math in Bash; if you need that capability, see Bash FAQ #22.

Remember few tricks

1. use [[ .. ]] for strings and files

2. use (( .. )) for numbers

To compare arithmetic numbers use bc function

$(echo "1.4 < 2.5" | bc)

> and < is for ASCII comparison and so 100 > 75 is false

-gt, -lt is only integer comparison.

This works for me

    if (( $(echo "$mem_util > 75" | bc) == 1 ))
    then
        ...
    fi

I am wondering why there was no floating point support??

References:

http://mywiki.wooledge.org/ArithmeticExpression

http://mywiki.wooledge.org/BashFAQ/031

linux: complete command

I have my .ssh/config with a lot of connection names. What would be better than to have auto complete with ssh <TAB> <TAB>

Added this line to my bash profile and I was good to go.

complete -W "$(awk '/^\s*Host\s*/ { sub(/^\s*Host /, ""); print; }' ~/.ssh/config)" ssh

man complete

There is a lot that human can do, for everything else there is [shell] script :)

linux: RANDOM is so simple

I was impressed with $RANDOM in linux. Simply use $RANDOM to get a random number.