notes/IT/Scripting.md

Marking a script as executable

Sometimes, you may need to mark a script as an executable to run it. On Linux, this can be accomplished by chmod +x [FILE], where [FILE] is the file you want to mark as executable. To unmark a file as executable, use chmod -x

The Shebang

There are various shells with their own language syntax (sh, bash, fish, etc). Therefore, more complicated scripts will indicate a particular shell by specifying the absolute path to the interpreter as the first line, prefixed by #! like this:

#!/bin/sh
echo "Hello, World!"

#!/bin/sh means that this script can be executed by the binary located at /bin/sh, and so the reader knows this is a sh(ell) script.

Variables

In bash, variable assignment is done with the = operator. Variables are conventionally named with SCREAMING_SNAKE_CASE, and can be accessed by prefixing the variable name with a dollar sign ($):

#!/bin/bash
# There may not be any spaces used during assignment, `A = B` means something different than `A=B`
# Assignment is done with the equal sign (=) operator:
MY_VARIABLE="Hi Mom!"
echo $MY_VARIABLE

Quotes

In Bash, different styles of quotes mean different things:

Quote	Description
Single Quotes (')	Enclosing characters in single quotes (‘'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Commands in single quotes will not be evaluated.
Double Quotes (")	Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘’, ‘\’, and, when history expansion is enabled, ‘!’. When the shell is in POSIX mode (see [Bash POSIX Mode](https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html)), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. The characters ‘$’ and ‘’ retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: ‘$’, ‘’, ‘"’, ‘\’, or newline`. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ‘!’ appearing in double quotes is escaped using a backslash. The backslash preceding the ‘!’ is not removed.

Commands

Command	Description
exit [CODE]	Make the script process exit, where [CODE] is the exit code the process will terminate with.