Shell scripting tutorial

A first script

File: my-script.sh
Run: ./my-script.sh
The first line tells Unix that the file is going to be executed by /bin/sh
Commend line is marked with "#"

Variables

File: var.sh
File: var2.sh
Declaration VAR=value
There is no space between. VAR = value doesn't work
The shell does not care about the type of variables, they may store strings, integers, real number or anything. They are all stored as strings.
You can interactively set variable names using read command Run:

$ MYVAR=hello
$ ./myvar2.sh
MYVAR is:
MYVAR is: hi there

When myvar2.sh is called, a new shell is spawned to run the script, for MYVAR to be understood by another program, we need export. \

$ export MYVAR
$ ./myvar2.sh
MYVAR is: hello
MYVAR is: hi there

When myvar2.sh exits, MYVAR keeps its value of hello. If we want it to receive new change from the script, we must source the script. We source a script via "." (dot) command.

$ MYVAR=hello
$ echo MYVAR
hello
$ . ./myvar2.sh
MYVAR is: hello
MYVAR is: hi there
$ echo $MYVAR

Change has now been made it out into our shell. This is how .profile or .bash_profile file works

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called $USER_NAME_file"
touch $USER_NAME_file

This will lead to error because there is no variable USER_NAME_file

Fix by user.sh

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called ${USER_NAME}_file"
touch "${USER_NAME}_file"

The quotes around ensure that if user enter "Steve Parker", the the argument pass to touch would be Steve and Parker file, which produce 2 separated files.

Wildcards

You can use wildcard character to select everything. For example, you want to copy all files from /tmp/a to /tmp/b. You can use:

$ cp /tmp/a/* /tmp/b/
$ cp /tmp/a/*.txt /tmp/b/
$ cp /tmp/a/*.html /tmp/b

Escape characters

Acknowledgement

I followed the tutorial from: Link

AustrianOakvn / Shell_scripting_learning