hmmk99/shell_scripting

some training and useful files for shell scripting

Shell Scripting

some training and useful files for shell scripting

To make simple hello world bash script writ the following commands

create the file with:
touch hello_world.sh
decide the shell with:
echo "#! /bin/bash" >> hello_world.sh
echo "echo 'hello world'" >> hello_world.sh
then we have to change the permissions with:

NOTE: it's not good to add a permission to all users you may use 'chmod u+x' to change for the user only

chmod +x ./hello_world.sh
then call the file in the terminal with:
./hello_world.sh

Piplines

pipelines are where the input of a state is the output of the previous one and so on.
you can use the pipeline as follows cmd1 | cmd2
so imagin we have a file and want to get the unique values of it we can do this
sort [file name] | uniq

Some commands, such as tr, only accept "standard input" as input (not strings or filenames)
we use pipline for them as follows:
echo "Linux and shell scripting are awesome!" | tr "aeiou" "_"
OUTPUT: L_n_x _nd sh_ll scr_pt_ng r w_s_m!

It's also pretty useful filtering text or o/p of curl commands with grep as follows:
curl -s --location --request GET https://www.example.com | grep -oE ""price":\s*[0-9]?.[0-9]"

Metacharacters

# for comments
; for ending one command to start another in the same line
* indicates any number of charecters as in 'ls /bin/bas*'
? indicates 1 character as in 'ls /bin/?ash'
\ this is used before metacharactes to interpret them as text
"" must use \ before metacharacters while '' interpret all text as text

> redirect output to file and create one if not existed and override if it exists
>> append output to file
2> catchs error and writes them to a file
2>> catchs error and apend it to a file
< to pass file content to a standard i/p command

&(command) or `command` is used for replacing command with its o/p as in $ variable=$(pwd)
./random_file arg1 arg2 passing arguments to shell script

; runs commands sequentially
& runs commands parrelel

Scheduling jobs

cron allows u to do that
crond is the cron deamon which checks 'crontab files' every minute and submits job to cron
crontab is a table contains jobs and scedule data crontab -e opens the editor
U add commands describing jobs as follows:
m h dom mon dow command
all 5 position must have numeric or * for any

dow 0:sunday and so on

crontab -l gets the jobs and scedules