Forem: Rehan Qadir

SEQ & PASTE: An effective way to add sequential numbers to the beginning of a file in bash

Rehan Qadir — Tue, 26 May 2020 14:28:16 +0000

Recently I explored seq and paste Linux commands and I was amazed how effectively we can accomplish our desired target of adding a sequential numbers to the beginning of a file without using any for loop or manipulating the data file line by line.

Lets dive in to see how we can leverage these two commands.

seq [n] generates sequential numbers starting from 1 to n.

seq 10
# Output
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 8
# 9
# 10

We can redirect the output of the command to a file, say, serial.

seq 1024 > serial

wc -l serial
# Output
# 1024 serial

The second command we are going to explore is paste. The command uses two or more files to produce an output separated by whitespace in columns.

Lets say we have a cities file with names of different cities in it

Prague
Montreal
Amsterdam
Rome
Barcelona

and a serial file with the following numbers

Lets use paste command on serial and cities files.

paste serial cities
# Output
# 1 Prague
# 2 Montreal
# 3 Amsterdam
# 4 Rome
# 5 Barcelona

As we can see the output is separated by a whitespace. We will see how we can modify the delimiter shortly.

Wind things up

Lets put 2 commands together to accomplish our target.

Suppose we have a CSV file mock.csv with the following content

Ryder,eget.metus.In@congueturpis.com,El Quisco
Keelie,vel.faucibus.id@libero.net,Temuka
Hamilton,enim.non.nisi@Maecenas.co.uk,Leeds
Lani,neque.sed.dictum@et.net,Largs
Gloria,montes.nascetur@nisl.co.uk,Vejalpur

and we wish to add sequential numbers to the beginning of the file. Here is the final bash script.

#!/bin/bash

# Generate sequence of numbers (1 to 5)
seq 5 > serial

# Use paste to combine two files with comma as a delimiter
paste -d, serial mock.csv > data.csv

cat data.csv
# Output
# 1,Ryder,eget.metus.In@congueturpis.com,El Quisco
# 2,Keelie,vel.faucibus.id@libero.net,Temuka
# 3,Hamilton,enim.non.nisi@Maecenas.co.uk,Leeds
# 4,Lani,neque.sed.dictum@et.net,Largs
# 5,Gloria,montes.nascetur@nisl.co.uk,Vejalpur

Viola!!! That was simple and elegant. Note that we have used -d, option with paste to indicate the delimiter to be , rather than a whitespace.

Note that we have hard coded 5 as an argument to seq command because we know beforehand that mock.csv contains 5 rows. What if we don't know the number of rows in the CSV file? Simply use wc -l command and extract the number of lines using awk or cut.

LINES=$(wc -l mock.csv | awk ' {print $1} ')
# LINES=$(wc -l mock.csv | cut -d ' ' -f1)

# Use LINES variable as an argument to seq command
seq $LINES

That's it.

I hope it will help you. Keep Coding!

Bash: All you need to know to work with bash arrays

Rehan Qadir — Fri, 15 May 2020 23:01:09 +0000

Indexed Arrays

We start with simple indexed arrays in bash. You can define an indexed array by using parentheses and assignment operator. Lets say you wish to store names of months in months variable as an array. Here is how you will do it in bash.

months=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")

Bash arrays start with the index 0 so to retrieve the value for October we will use index 9, as in the following code.

months=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")

echo ${months[9]}
# Output: Oct

index=5

echo ${months[$index]}
#Output: Jun

Make sure to use curly braces when accessing an element through index.

Bash allow use of special characters to get all values and indexes of an array at once. Use * or @ to get all values stored in an array and ! before array name to get all indexes. And to get length of an array use #.

months=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")

# Get all values from months array
echo ${months[*]}
# or echo ${months[@]}
# Output: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

# Get all indexes
echo ${!months[@]}
# or echo ${!months[*]}
# Output: 0 1 2 3 4 5 6 7 8 9 10 11

# Length of array
echo ${#months[@]}
# or echo ${#months[*]}
# Output: 12

You can easily append data to an existing indexed array using += operator or you can specify the index by your own.

months=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")

# Add to the months array
months+=("Raj" "Sha" "Ra1" "Ra2")

echo ${months[@]}
# Output: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Raj Sha Ra1 Ra2

# Add element to any specified index
months[20]="Muh"
echo ${months[@]}
# Output: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Raj Sha Ra1 Ra2 Muh

# Get all indexes
echo ${!months[@]}
# Output: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 20

# Initialize second array
animals=("Cow" "Goat" "Horse" "Lion" [11]="Brock Lesnar")

# Get all indexes for animals
echo ${!animals[@]}
# Output: 0 1 2 3 11

Notice that you can specify an index out of order as well and bash will take care of it. As it can be seen that we have added Muh at the 20th index and it worked just fine. Also, while initiating animals array we changed the indexing deliberately and it also worked without any error.

Keep note that negative indexes are not allowed.

Associative Arrays or Hashes

Bash also supports hashes that is storing data as key => value pair. You can all it associative array if you are coming from PHP background or dictionary in Python.

We can declare a variable to be an associative array by using declare -A <variable name> command. We can also initialize it using parentheses at the time of declaration.

declare -A stars=([ALP]="Cow" [BET]="Bow" [GAM]="Goat" [DEL]="Scorpion"
[EPS]="Balance" [ZET]="Queen" [ETA]="Lion" [THE]="Fish")

# OR
declare -A planets
planets["first"]="Mercury"
planets["second"]="Venus"
planets["third"]="Earth"
planets["Musk adventure"]="Mars"
planets["second last"]="Uranus"

We can access the keys and values of an associative array in a similar fashion as of indexed arrays.

We will print the key value pairs of an associative array using for loop to conclude this topic.

declare -A stars=([ALP]="Cow" [BET]="Bow" [GAM]="Goat" [DEL]="Scorpion"
[EPS]="Balance" [ZET]="Queen" [ETA]="Lion" [THE]="Fish")

for key in ${!stars[@]}; do
    echo "$key => ${stars[$key]}"
done

# Output
# GAM => Goat
# EPS => Balance
# ALP => Cow
# ZET => Queen
# DEL => Scorpion
# BET => Bow
# ETA => Lion
# THE => Fish

# Get length of stars
echo ${#start[@]}
# Output: 8

That's it for Arrays in bash from my side.

I hope it will help you. Keep Coding!

Bash: Some useful tricks with date command

Rehan Qadir — Thu, 14 May 2020 12:23:59 +0000

Lets get started

Just typing date will print the date and time on the standard output. Following are some useful options with the date command.

Seconds since UNIX epoch (01/01/1970)

Use %s to get number of seconds passed since UNIX epoch. Creating unique file names is one of the possible use case.

# + is use for formatting
date +%s
# Output: 1589455968

Number of days since the year's beginning

Use %j to get the number of days passed since the beginning of the year.

# + is use for formatting
date +%j
# Output: 135

Dates in the past or future

Use -d or --date option to get any date in the past or future.

# + is use for formatting
date +%F -d "1 day ago"
# Output: 2020-05-13

# More easily by using - sign
date +%F -d "-1 day"
# Output: 2020-05-13

date +"%D %T" -d "1 year ago"
# Output: 05/14/19 16:54:02

date +"%F" --date="1 month ago"
# Output: 2020-04-14

# For dates in future use + sign before number
date +%F -d "+1 day"
# Output: 2020-05-15

Get Time Zone

Use %Z option to get your time zone

# + is use for formatting
date +%Z
# Output: PKT

Get date from UNIX epoch

To extract date from the number of seconds passed since UNIX epoch, use -d with @.

date -d @1500000000
# Output: Fri 14 Jul 07:40:00 PKT 2017

Get date in ISO Format

Use -Iseconds option to get current date in ISO Format.

date -Iseconds
# Output: 2020-05-14T17:18:22+05:00

Get the last day of the previous month

While executing an SQL command to retrieve data for the starting and ending date of the previous month, it is always a point of concern to correctly determine whether the previous month was of 30 or 31 days. To resolve this issue, we can use date in the option argument of date command with %d option to get the current day of the month.

# + is use for formatting
# %d is used to get the day of the month

date +%F -d "$(date +%d) days ago"
# Output: 2020-04-30

I hope it will help you. Keep coding!

P.S. I will keep on adding to this post with time.

Nuxt: How to use nuxt-link-active and nuxt-link-exact-active classes with nuxt-link?

Rehan Qadir — Wed, 13 May 2020 20:06:43 +0000

Problem

Apply custom active class to active route in nuxt-link tag including all nested routes

Let's say you wish to apply custom active class to an active route in nuxt-link tag.

...
<nav>
    <nuxt-link to="/">Home</nuxt-link>
    <nuxt-link to="/explore">Explore</nuxt-link>
    <nuxt-link to="/about">About</nuxt-link>
</nav>
...

The first choice is to use nuxt-link-active class to style the active route.

.nuxt-link-active {
    font-weight: bold;
}

Discrepancy

If user navigates to /explore or /about route, the styling will get applied to / route as well which in practice is not desired.

Well, to resolve this issue one is prompted to use nuxt-link-exact-active along similar lines.

.nuxt-link-exact-active {
    font-weight: bold;
}

Apparently this approach resolves the issue very well. But hold on, what if there are some nested routes? What about /explore/express or /explore/ruby-on-rails?

In practice if user navigates to any of the nested routes /explore should continue to have the styling, right? but with just nuxt-link-exact-link class in hand it will fail to persist.

Solution

To get the best of both worlds, use nuxt-link-active class along with exact keyword in nuxt-link tag.

Use exact in the nuxt-link tag

...
<nav>
    <nuxt-link to="/" exact>Home</nuxt-link>
    <nuxt-link to="/explore">Explore</nuxt-link>
    <nuxt-link to="/about" exact>About</nuxt-link>
</nav>
...

with the following css

.nuxt-link-active {
    font-weight: bold;
}

For every nested explore routes, /explore will continue to have the styling. Furthermore, / and /about will have the styling only when the exact route matches.

I hope it will help you. Keep coding!