Centralized BASH history with timestamps

For every Unix user, there comes a point where shell history suddenly becomes very relevant. You learn to consult it, then start recovering the last command, then switch to searching past commands history to save precious time normaly taken typing.
Shortly after such a point in your life, you'll probably want to enhance your shell history in two very common ways:
  1. Make sure every terminal window can update AND access your centralized shell history. So you run a command or two in one window, then type "history" anywhere else and see them two commands right there.
  2. Provide meanigful timeline, this is done with timestamps. Very simple and powerful change helps you see exactly when each command occured.

Here's how you achieve both of these massive improvents to your history in BASH. Just add this to /etc/bashrc on your Linux system:

export HISTTIMEFORMAT="%d.%m.%y %T "
export HISTCONTROL=ignoredups:erasedupsshopt -s histappend
export PROMPT_COMMAND="${PROMPT_COMMAND:+$PROMPT_COMMAND$'\n'}history -a; history -c; history -r;"
export HISTCONTROL=ignoreboth

Fixed calculations in Unix scripts

Although I've already shown you how to sum numbers up in bash, I only covered the bash way of doing it. I really like scripting with bash, but when it comes to calculations, there's quite a few important features missing from bash, and fixed point (thanks for the correction, Azrael Tod!) calculations is one of them. Fortunately, bc command comes as a standard in most Unix distros, and can be used for quite complex calculations.

Basic calculations with bc

bc is a very simple command. It takes standard input as an expression and then evaluates this, performing all the necessary calculations and showing you the result. Thus, to quickly sum numbers up or get a result of some other calculation, simply echo the expression and then pipe it out to the bc command:

ubuntu$ echo "1+2" | bc

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Another way to use math expressions in shell scripts

Today I'd like to expand a bit more on the basic calculations in Unix scripts.

Use Parenthesis to simplify math expressions

In Basic Arithmetic Operations post I've shown you how expression evaluation can be used to calculate simple math expressions in your Unix shell:

ubuntu$ START=1
ubuntu$ FINISH=10
ubuntu$ ELAPSED=$(($FINISH - $START))
ubuntu$ echo $ELAPSED

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How To Parse Text Files Line by Line in Unix scripts

I'm finally back from my holidays and thrilled to be sharing next of my Unix tips with you!

Today I'd like to talk about parsing text files in Unix shell scripts. This is one of the really popular areas of scripting, and there's a few quite typical limitations which everyone comes across.

Reading text files in Unix shell

If we agree that by "reading a text file" we assume a procedure of going through all the lines found in a clear text file with a view to somehow process the data, then cat command would be the simplest demonstration of such procedure:

redhat$ cat /etc/redhat-release
Red Hat Enterprise Linux Client release 5 (Tikanga)

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Using variables in Unix shell scripts

Any Unix shell script longer than a line will most likely involve using variables. Variables are used to store temporary values to simply using them in various Unix commands of your script. The beauty of using variables is that they can be evaluated and set ones, but then reused as many times as you like without your shell interpreter having to re-evaluate them again.

Defining a variable in Unix shell

To specify a value for a variable, you need to decide on the variable name – can be any word or combination of English alphabet symbols and digits, and specify the value.

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Perl: Searching Through Directory Trees

I had a need to scan a huge directory tree today, identifying the users and Unix groups owning all the files. The problem I faced was too long usernames and group names which meant the

find /directory -ls

command which I normally use for such tasks wasn't terribly useful because there was no space delimiter between a username and a group. Results of such scan of the directory tree will have to later be parsed by other tools, and that's why proper splitting of the output into separate fields is so important.


This issue was motivational enough to refresh my Perl skills and sketch the following script (based entirely on this Never Run Unix Find Again article).

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