Untrack files from git using git bash

This will start ignoring the changes to the file:
git update-index --assume-unchanged path/to/file 

When you want to start keeping track again: 
git update-index --no-assume-unchanged path/to/file 

Now list files that are ignored: 
git ls-files -v 

And with a search options: 
git ls-files -v | grep "file_name_part"

HowTo Format Date For Display or Use In a Shell Script

How do I format date to display on screen on for my shell scripts as per my requirements on Linux or Unix like operating systemsYou need to use the standard date command to format date or time. You can use the same command with the shell script.
 The syntax is

1. date +FORMAT 
2. date +"%FORMAT"
3. date +"%FORMAT%FORMAT" 
4. date +"%FORMAT-%FORMAT"

Task: Display date in mm-dd-yy format

Open a terminal and type the following date command:
$ date +"%m-%d-%y"Sample output:

02-27-07

To turn on 4 digit year display:
$ date +"%m-%d-%Y"Just display date as mm/dd/yy format:
$ date +"%D"

Task: Display time only

Type the following command:
$ date +"%T"Outputs:

19:55:04

To display locale's 12-hour clock time, enter:
$ date +"%r"Outputs:

07:56:05 PM

To display time in HH:MM format, type:
$ date +"%H-%M"Sample outputs:

00-50

How do I save time/date format to the shell variable?

$ NOW=$(date +"%m-%d-%Y"To display a variable use echo / printf command:
$ echo $NOW

A sample shell script

#!/bin/bash
NOW=$(date +"%m-%d-%Y")
FILE="backup.$NOW.tar.gz"
echo "Backing up data to /nas42/backup.$NOW.tar.gz file, please wait..."
# rest of script
# tar xcvf /nas42/backup.$NOW.tar.gz /home/ /etc/ /var
 

A complete list of FORMAT control characters supported by the date command

FORMAT controls the output. It can be the combination of any one of the following:

%FORMAT String	Description
%%	a literal %
%a	locale's abbreviated weekday name (e.g., Sun)
%A	locale's full weekday name (e.g., Sunday)
%b	locale's abbreviated month name (e.g., Jan)
%B	locale's full month name (e.g., January)
%c	locale's date and time (e.g., Thu Mar 3 23:05:25 2005)
%C	century; like %Y, except omit last two digits (e.g., 21)
%d	day of month (e.g, 01)
%D	date; same as %m/%d/%y
%e	day of month, space padded; same as %_d
%F	full date; same as %Y-%m-%d
%g	last two digits of year of ISO week number (see %G)
%G	year of ISO week number (see %V); normally useful only with %V
%h	same as %b
%H	hour (00..23)
%I	hour (01..12)
%j	day of year (001..366)
%k	hour ( 0..23)
%l	hour ( 1..12)
%m	month (01..12)
%M	minute (00..59)
%n	a newline
%N	nanoseconds (000000000..999999999)
%p	locale's equivalent of either AM or PM; blank if not known
%P	like %p, but lower case
%r	locale's 12-hour clock time (e.g., 11:11:04 PM)
%R	24-hour hour and minute; same as %H:%M
%s	seconds since 1970-01-01 00:00:00 UTC
%S	second (00..60)
%t	a tab
%T	time; same as %H:%M:%S
%u	day of week (1..7); 1 is Monday
%U	week number of year, with Sunday as first day of week (00..53)
%V	ISO week number, with Monday as first day of week (01..53)
%w	day of week (0..6); 0 is Sunday
%W	week number of year, with Monday as first day of week (00..53)
%x	locale's date representation (e.g., 12/31/99)
%X	locale's time representation (e.g., 23:13:48)
%y	last two digits of year (00..99)
%Y	year
%z	+hhmm numeric timezone (e.g., -0400)
%:z	+hh:mm numeric timezone (e.g., -04:00)
%::z	+hh:mm:ss numeric time zone (e.g., -04:00:00)
%:::z	numeric time zone with : to necessary precision (e.g., -04, +05:30)
%Z	alphabetic time zone abbreviation (e.g., EDT)

SEE ALSO:

• Shell Scripting: Creating report/log file names with date in filename
• Format Date For Display or Use In a Shell Script

Call and store response from a url using bash script

#!/bin/bash

## generate random string of length 18
function randpass
{
        echo `</dev/urandom tr -dc A-Za-z0-9 | head -c18`
}

## get current time stamp
timestamp=$(date  +%s);
sessionId="$(randpass).$(randpass).$timestamp";
privateKey='test';
 
## get substring of sessionId from 0 to 5 characters.
sessionIdPart=${sessionId:0:5};
systemKeyPart=${privateKey:0:4};

## reverse string and store in a variable
word="$(echo "$sessionIdPart" | rev)$(echo "$systemKeyPart" | rev)";
word="$word$sessionId$privateKey";
 
## make a string md5
hash=`echo -n "$word" | md5sum | awk '{print $1}'`;
hash=${hash:0:10};
breakRest="securedSessionKey=$sessionId&securedHash=$hash";
 
saveFile=$(date +"%Y-%m-%d %T"); 
## saveFile would be '2013-08-25 19:23:28'

## calling a url using 'curl' and store response to some file.
curl -s -L -o "$saveFile.html" "http://domain.com/getStudent?$breakRest" &
## "&" sign at the end means run curl in background.

exit 1;

bash split string into array

IFS=', ' read -a array <<< "$string" /* array is the variable name */

To access an individual element:

echo "${array[0]}"

To iterate over the elements:

for element in "${array[@]}"
do
    echo "$element"
done

To get both the index and the value:

for index in "${!array[@]}"
do
    echo "$index ${array[index]}"
done

The last example is useful because Bash arrays are sparse. In other words, you can delete an element or add an element and then the indices are not contiguous.

unset "array[1]"
array[42]=Earth

How to set a BASH variable equal to the output from a command?

In addition to the backticks, you can use $(), which I find easier to read, and allows for nesting.

OUTPUT=$(ls -1)
echo $OUTPUT

 

 

You can use eval to execute a string:

eval $illcommando

 

How to tell if a string is not defined in a bash shell script

#!/bin/bash

if [ -z $1 ]; then
        echo 'Enter something please.';
        exit 1;
fi;

Check if a program exists from a bash script

$ command -v foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ type foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ hash foo 2>/dev/null || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}