BASH - Run a program or script - Run a bunch of commands in sequence

To run a bunch of commands just put semicolons between the commands:

date ; free; date

returns:

Thu  4 Feb 13:30:42 GMT 2021
              total        used        free      shared  buff/cache   available
Mem:       65872344     7039056    47427116      929180    11406172    57177400
Swap:        999420           0      999420
Thu  4 Feb 13:30:42 GMT 2021

NOTE: As can be seen the same command can be run multiple times.

Here the date command is run twice.

ALERT: Running commands in sequence can be bad.

For example:

cd /foo; rm *

NOTE: The idea here is to erase all the files in directory foo (but not sub-directories; -r is needed for that).

But what if the cd fails? Then all the files in the current directory will be erased instead. Not good!

Far better to use:

cd /foo && rm *

NOTE: The && is just like in C; this is the AND operator.

Nearly all BASH programs return a 0 error status for true and anything else is false.

That allows programmers to return an error code for failure and which returns a false.

To test, this rather use the following (which does not include the rm for safety).

cd /foo && ls   # Not rm so we don't do anything bad by mistake.

If /foo exists, the cd command will return 0 which is true.

That means the result of the AND could be true and therefore execution continues with the ls.
However, if the cd fails, the result will be false.
If any input to an AND function is false, the other values does not matter.
Therefore, as soon as anything returns false, the whole thing stops.
So if there is no /foo, the ls command will not execute at all.

OR

There is also an OR operator || that quits as soon as anything returns true.

grep "dummy" /etc/passwd || echo No such user

NOTE: Try this command using an actual user instead of dummy to see what is returned when this succeeds.