on 05-Feb-2019 (Tue)

In linux, when running two or more commands together, what is the difference between && vs ||, i.e. the following two line:

command1 && command2
command1 || command2

In linux, write one-line script to create new directory ktest (in current directory) ONLY if ktest does not already exist. Do it by finishing following code snippet:

ls ktest >& /dev/null [...]

|| mkdir ktest

^^ note use of || to denote only if command 1 (i.e. ls ktest >& /dev/null) fails, command 2 (i.e. mkdir ktest) runs

In linux, write one-line script to spit out content of file kevin.txt to stdout ONLY if kevin.txt file exists. Do it by finishing following code snippet:

ls kevin.txt > /dev/null [...]

&& cat kevin.txt

^^ note use of && to indicate only if command 1 (i.e. ls kevin.txt > /dev/null) succeeds, then run command 2 (i.e. cat kevin.txt)

echo $#

^^ note that $ denotes variable expansion, and # is special variable to represent number of command-line arguments to current script

echo $?

^^ note that $ denotes variable expansion, and ? is special variable to represent exit code of last command/program executed in foreground.

echo $$

^^ note that $ denotes variable expansion, and $ (the second one:) is special variable to represent PID of current shell

echo $!

^^ note that $ denotes variable expansion, and ! is special variable to represent PID of last background process executed

Brace expansion

^^ syntax is simple: {}

touch kevintest{1..10}.txt

^^ note the use of {}, called brace expansion

touch kevin{one,two,three,four}.txt

^^ note the use of {}, called brace expansion

echo "hello ${NAME:-there}"

^^ note the special use of parameter expansion above

echo ${TEXT:2:3}

^^ note the special use of parameter expansion above

^^^ note that the first number, 2, is the starting index of string to start the substring from, and the second number, 3, is the length of substring. THIS IS DIFFERENT THAN PYTHON.

^^^^ To go to the end of substing, if you don't know length, just choose a very large length value, e.g. echo ${TEXT:2:20000}

echo ${#TEXT}

^^ note the special use of parameter expansion above

In linux, in shell scripting, what is the equivalant to python's "pass" statement, i.e. syntax to do nothing, for example:

if <command> ; then
    [...OCULSION...]
else
    echo "<command> was unsuccessful"
fi

:

^^ the colon syntax

It executes a shell script IN THE CURRENT SHELL, therefore preseving the variables/environment variables set by that script in the current/originating shell.

e.g. if you do "source test.sh" and test.sh sets variable X=2, then in the originating/current shell, X is now set, even after test.sh finishes running.

In linux, if you have shell script called test.sh and you want it executed IN THE CURRENT SHELL, such that the variables/environment variables set by that script are preserved in the current/originating shell, you can either do:

source test.sh

or, alternatively

[...] test.sh

.

^^ single dot, followed by space

^^^ the use of this is e.g. if you do ". test.sh" and test.sh sets variable X=2, then in the originating/current shell, X is now set, even after test.sh finishes running.

alias lsl="ls -l"

^^ note this only creates lsl alias in the current shell, i.e. once shell is exited (or new child shell created) alias no longer exists

alias -p

NOTE: just "alias" works too, without the -p