Friday, 22 July 2022

Grokking grep

A colleague was tinkering with grep and, thanks to him, I discovered a bit more about the trusty little utility.

I had not really explored the -e switch: -

     -e pattern, --regexp=pattern

             Specify a pattern used during the search of the input: an input line is selected if it matches any of the specified patterns.  This option is

             most useful when multiple -e options are used to specify multiple patterns, or when a pattern begins with a dash (‘-’).

but he pointed out that that switch can have problems when parsing, say, a list of strings that are similar but different.

As an example, I have a set of files: -

-rw-r--r--   1 hayd  wheel    0 22 Jul 17:34 dave
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:34 dave_1
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:36 dave_2
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:36 dave_3

If I want to e.g. query a directory for all files containing the word 'dave' I could do this: -

ls -al | grep dave

or, to be more specific: -

ls -al | grep -e Dave

both of which return: -

-rw-r--r--   1 hayd  wheel    0 22 Jul 17:34 dave
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:34 dave_1
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:36 dave_2
-rw-r--r--   1 hayd  wheel    0 22 Jul 17:36 dave_3

However, if I only want to return the file that's named 'dave' and ignore the rest, I'm somewhat stymied.

However, grep -w comes to the rescue: -

     -w, --word-regexp
             The expression is searched for as a word (as if surrounded by ‘[[:<:]]’ and ‘[[:>:]]’; see re_format(7)).  This option has no effect if -x is
             also specified.


so I can run: -

ls -al | grep -w dave

and just get this: -

-rw-r--r--   1 hayd  wheel    0 22 Jul 17:34 Dave

Isn't that nifty ?

Wednesday, 20 July 2022

To start, press any key .... hey, where's the [Any] key ?

So I was writing some scripts for a demo that I delivered earlier today...

One script will run on my Mac, using the Zsh shell, the other will run on an Ubuntu box, using Bash.

In both cases, I wanted to pause the script and wait for user input before proceeding.

On Ubuntu, I did this: -

read -p "Press [Enter] to continue"

Sadly, however, on macOS 12.4 via zsh, that didn't work ...

read: -p: no coprocess

Thankfully, there's a better / different way: -

read "?Press [Enter] to continue"

which is nice

Today I Learned - Shorter redirection, what's not to like ?

So I write a lot of scrips for Bash and Zsh, and often make use of redirection where, for example, I generate a variable by running another command within parentheses.

As a specific example, I'd query my Floating IP and persist it to a text file for future use: -

ic is floating-ips --output JSON | jq -r '.[].address' > ~/floating_ip.txt

Later on, I'd look to use that Floating IP for, for example, a SSH connection, as follows: -

export floating_ip=$(cat floating_ip.txt)

ssh root@$floating_ip

Authorized uses only. All activity may be monitored and reported.

root@163.61.95.34's password:

However, one of my colleague's did point out that I could reduce the number of characters typed when setting the floating_ip variable: -

export floating_ip=$(< floating_ip.txt)

See, that saves a whole two characters ....

Mind you, I can also reduce things further via: -

ssh root@$(cat floating_ip.txt)

I love shelling !

More fun with pip

Again with the Python and pip  fun, this time on my Mac, where commands such as:  pip3 list and: - pip3 install --upgrade pip were failing w...