Raveen Kumar

Bash

Bash

Reading materials

Select statement

select opt in y n; do
	echo $opt
done

This creates a loop where the user can select the option. Use case to break out of the loop.

select opt in y n; do
	echo $opt selected
	case $opt in
		y)
			break 
			;;
		n)
			break 
			;;
		*)
			echo wrong option
			;;
	esac
done

Bash variables

$USER
$HOSTNAME
$PS1

To Find or List all environment variables

env

This is useful when we want to manage the shell variables properly

Paths

$PATH
$PWD
$REALPATH

Bash Prompt customization

https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux https://www.shellhacks.com/bash-colors/

Startup and initializations

bash
bash --norc
bash --login
bash --noprofile
# etc...

Shopts (Shell options) shopt opt

To print all the enabled options, print the variable $BASHOPTS.

echo $BASHOPTS

Extended globbing or Negative globbing

shopt extglob
ls
ls !(*.mkv)

Conditional statements

-eq     Equality operator for numbers
==      Equality operator for strings

If statements

# -eq, -ne, -lt, -le, -gt, or -ge

if [[ $var -eq $var2 ]]; then
    echo do something
else
    echo do something else
fi

# == for equality check; != for non equality check

if [[ $var == "string" ]]; then
    echo do something
else
    echo do something else
fi
# Note: if you are using [ .. ]; then use = sign instead of ==

Case statements

var="hello"
case $var in
    hello)
        echo var is hello
        ;;
    world)
        echo var is world
        ;;
    *)
        echo wrong case
        ;;
esac

Loops

# Syntax

loop statement; do
done

or

loop statement
do
done

For Loop

number=20
for ((i=0; $i<$number; i=$i+1)); do
    echo $i
done

VRX=10
for (( i=1; i<=$VRX; i++)); do echo $i; done

# for file in `ls`; do; something with $file; done;

While Loop

declare cnt=0;
while [ true ]; do
    cnt=$((cnt+1));
    echo $cnt;
    [ -f done.txt ] && echo done || sleep 2;
    sleep 5;
done

For loop - lists

names=(Apple Bat Cat Dog Elephant Zebra)
for name in "${names[@]}"; do
    echo  $name
done

Variables

https://bash.cyberciti.biz/guide/Create_an_integer_variable

Sequence

To generate a sequence of variable names

touch test{0..9}.txt

hg remove adc_capture_data_spray_rx{0..9}.json

Strip some text from a variable

https://tldp.org/LDP/abs/html/string-manipulation.html https://tldp.org/LDP/abs/html/string-manipulation.html

some_string=/home/raveen/tools/scripts/run_cmds/poll_and_plot
echo ${some_string%plot}
echo ${some_string#/home}

Remove last few characters from a string in bash

https://reactgo.com/bash-remove-last-n-characters/

country="portugal"
modified=${country::-3}
echo $modified
modified=${country::-4}
echo $modified # "port"
modified=${country%????}
echo $modified # "port"

Operators

>
>>

Lists

FILENAMES=("file1.txt" "file2.txt" "file3.txt")
echo ${FILENAMES[0]}
echo ${FILENAMES[1]}

Subshell

You can make bash invoke subshell using parenthesis

echo hello && ( echo world ) # echo world runs in a subshell
run_main () {
    echo run in main shell
}
run_sub () (
    echo run in subshell
    )

Reading user input from command line

https://alvinalexander.com/linux-unix/shell-script-how-prompt-read-user-input-bash/

read -p "Enter input: " input
echo $input

Remove set functions and variables

unset fun

CREATE a new file or truncate a file

> file.txt
touch file.txt

Disposing of Unwanted Output

ls -l /bin/usr 2> /dev/null

File descriptors and redirection, reading files

0 - standard input, 1 - standard output, 2 - standard error

ls -l /bin/usr > ls-output.txt 2>&1
ls -l /bin/usr &> ls-output.txt
ls -l /bin/usr &>> ls-output.txt

Reading file as input stream, line by line

for line in $(< filename.txt); do
    echo $line
done

or

while read -re line; do
    echo $line
done < "filename.txt"

Bash Arithmetic operations

echo $((2+3))

You can also use c code in bash

count=1
((count++))
echo $count

Debugging bash scripts / Stepping thorugh the script line by line

https://stackoverflow.com/questions/9080431/how-execute-bash-script-line-by-line

#!/usr/bin/env bash
set -x
trap read debug

< YOUR CODE HERE >

Print commands executed similar to `-x`. Print standard inputs with `-v` option.

set -v

Ignore aliases and use original command

command bc

To auto cd into a directoty

shopt -s autocd

Bash color codes

<https://misc.flogisoft.com/bash/tip_colors_and_formatting>

Play beep sound

echo -e '\a'

Gotchas

Bash doesnot expand ~ inside quotes

~/file is not same as "~/file"

General

Better way to cd into a directory?

Beware that this could cause bugs.

Write a function for cd

cd () {
    builtin cd $1;
    clear;
    pwd;
    ls --color=auto -1Ah --color=auto
}

If you are anything like me and want to instantly ls after cd, this will save you some keystrokes. clear and pwd are optional.

listing files/directories along with absolute paths

realpath *

or

readlink -f *

Check for file / directory exists

https://linuxize.com/post/bash-check-if-file-exists/

[ -f file_path      ] && echo file exists      || echo file does not exist
[ -d directory_path ] && echo directory exists || echo directory does not exist

Current bash shell process id

echo $$

#Bash #Programming #Linux