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shell script

Bash Shell: Find Out If a Variable Is Empty Or Not

My shell script depends upon user input. How do I find out if a variable called $_JAIL path is empty under Linux / Apple OS X / Unix like operating systems?
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I work for a small ISP (Internet Service Provider) and we are using Linux and Unix-like operating system with bash shell. I want to write a shell script to parse the csv file line by line. Then line must be parse again field by field. The sample input file is as follows:

example.com,username,groupname,homedir,md5password,permission,secondarygroup

I need to extract each of these example.com,username,groupname,homedir,md5password,permission,secondarygroup fields and passed to the different system utilities. How do I write a shell script to automate this task and use the bash shell to parse a text file?
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I'm writing a bash wrapper script that will pass arguments to the command. I need to find out the last argument if I call the wrapper as follows:

./wrapper -a -b --longarg=foo thisfilename.txt
./wrapper -a -b thisfilename.txt
./wrapper -a --next=true thisfilename.txt

Where,
=> $@ is all of them.
=> $0 is script name.
=> $1 is first arg.

I want thisfilename.txt stored in a shell variable called $last. How do I find the last argument passed to a shell script written in bash or ksh under Unix like operating systems?
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Linux / Unix: Generate A MD5 String

I need to generate a md5 hash for given string. It will be used by shell script to generate keys for remote web service or cryptographic application. How do I generate a md5 hash based on any input string under Linux or Unix like operating systems?
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I'm writing a shell script for automation purpose. The output the path of the current working directory is stored in $PWD or it can be obtained using the pwd command. How do I find out find out 3rd field separated by the forward slash (/) delimiter using $PWD under Unix like operating systems?
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Linux/Unix: Bash Set Shell Variable Command

How do I set or create variables within my shell script or at command line under Linux and Unix like operating systems?
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How do I add my RSA or DSA keyfile in shell script itself for the connection so that I need to carry only one file on my USB pen drive instead of $HOME/.ssh/id_rsa file under Unix / Linux operating systems?
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