What's the Difference Between: ". [Script]" or "Source [Script]", "Bash [Script] or $Shell [Script]", and "./ [Script]" or "[Script]"

What's the difference between: . [script] or source [script], bash [script] or $SHELL [script], and ./ [script] or [script]?

. script and source script execute the contents of script in the current environment, i.e. without creating a subshell. On the upside this allows script to affect the current environment, for example changing environment variables or changing the current work directory. On the downside this allows script to affect the current environment, which is a potential security hazard.

bash script passes script to the bash interpreter to execute. Whatever shebang is given by script itself is ignored. ("Shebang" referring to the first line of script, which could e.g. read #!/bin/bash, or #!/usr/bin/perl, or #!/usr/bin/awk, to specify the interpreter to be used.)

$SHELL script passes script to whatever is your current shell interpreter to execute. That may, or may not, be bash. (The environment variable SHELL holds the name of your current shell interpreter. $SHELL, if running bash, is evaluated to /bin/bash, with the effect detailed in the previous paragraph.)

./script executes the contents of a file script in the current work directory. If there is no such file, an error is generated. The contents of $PATH have no effect on what happens.

script looks for a file script in the directories listed in $PATH, which may or may not include the current work directory. The first script found in this list of directories is executed, which may or may not be the one in your current work directory.

Using dot or source while calling another script - what is the difference?

There is no difference.

From the manual:

source

source filename

A synonym for . (see Bourne Shell Builtins).

source command not found in sh shell

/bin/sh is usually some other shell trying to mimic The Shell. Many distributions use /bin/bash for sh, it supports source. On Ubuntu, though, /bin/dash is used which does not support source. Most shells use . instead of source. If you cannot edit the script, try to change the shell which runs it.

No such file or directory but it exists

This error can mean that ./arm-mingw32ce-g++ doesn't exist (but it does), or that it exists and is a dynamically linked executable recognized by the kernel but whose dynamic loader is not available. You can see what dynamic loader is required by running ldd /arm-mingw32ce-g++; anything marked not found is the dynamic loader or a library that you need to install.

If you're trying to run a 32-bit binary on an amd64 installation:

• Up to Ubuntu 11.04, install the package ia32-libs.
• On Ubuntu 11.10, install ia32-libs-multiarch.
• Starting with 12.04, install ia32-libs-multiarch, or select a reasonable set of :i386 packages in addition to the :amd64 packages.

How to check if an environment variable exists and get its value?

[ -z "${DEPLOY_ENV}" ] checks whether DEPLOY_ENV has length equal to zero. So you could run:

if [[ -z "${DEPLOY_ENV}" ]]; then
  MY_SCRIPT_VARIABLE="Some default value because DEPLOY_ENV is undefined"
else
  MY_SCRIPT_VARIABLE="${DEPLOY_ENV}"
fi

# or using a short-hand version

[[ -z "${DEPLOY_ENV}" ]] && MyVar='default' || MyVar="${DEPLOY_ENV}"

# or even shorter use

MyVar="${DEPLOY_ENV:-default_value}"

Read a variable in bash with a default value

You can use parameter expansion, e.g.

read -p "Enter your name [Richard]: " name
name=${name:-Richard}
echo $name

Including the default value in the prompt between brackets is a fairly common convention

What does the :-Richard part do? From the bash manual:

${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Also worth noting that...

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

So if you use webpath=${webpath:-~/httpdocs} you will get a result of /home/user/expanded/path/httpdocs not ~/httpdocs, etc.

Difference between source and ./ execution of linux scripts

check if variable "a" is set in your current shell:

set | grep '^a='

Remember that once you source script to your current shell,
all it's global variables are there until unset or you exit the current shell.
You may want to start a new shell, source the script, end exit shell to perform valid tests.

I don't know the context of your problem, but you may want to export some key variables to have their copies in every subprocess.

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