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This line makes no sense:

• and then echo the string to the file:

The following line makes no sense:

• echo the string to a file:

• This is in a function to isolate some variables – I'm assuming you don't want $prev, $dif, etc. set as global variables
• Not sure what $marker is for, so assuming you want it set as a global var
• ~/.bash_profile, ~/.bashrc are read directly by bash so shouldn't start with the line #!/bin/sh or have executable mode set
• You might want to consider making ~/days.log obsolete by changing the format for the first line of ~/ipy.log so that prev can be read from the first line/field of ipy.log

• This is in a function to isolate some variables – I'm assuming you don't want $prev, $dif, etc. set as global variables
• Not sure what $marker is for, so assuming you want it set as a global var
• ~/.bash_profile, ~/.bashrc are read directly by bash so shouldn't start with the line #!/bin/sh or have executable mode set
• You might want to consider making ~/days.log obsolete by changing the format for the first line of ~/ipy.log so that prev can be taken from the first line/field of ipy.log

I think the assignment to PREV wasn't working as ~/days.log would have been an existing but empty file, due to the mistake:

The assignment to PREV wasn't working as expected because ~/days.log would have been an existing but empty file, due to the mistake: