on 06-Feb-2019 (Wed)

Brace expansion

^^ syntax is simple: {}

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}

In linux, the [...] command is used when you have a command stored in a variable and you want to execute that command stored in the string.

For example:

[kevin@linux-box ~]$ COMMAND="ls -l"

[kevin@linux-box ~]$ [...] $COMMAND

In linux, the [...] built-in command is used when you want to run a command/program where the shell is replaced with that command, instead of a separate process being run (e.g. if you want user to run an applicaton but have no access to the shell, you use this build-in command to run the application from within the ~/.profile startup script, as follows):

[...] appln-program

Assume that buyer preferences are of the special type: u(q, T , θ) = θ i v(q) − T (Consumer Surplus),

v(q) is interpreted as [...] for q

with v 0 (q) > 0, v 00 (q) < 0

Assume that buyer preferences are of the special type: u(q, T , θ) = θ i v(q) − T ([...]),

v(q) is interpreted as Willingness to Pay for q

with v 0 (q) > 0, v 00 (q) < 0

Benchmark case: Suppose types are known. What package should be offered if we are interested in maximizing the gains from trade (not the profits of the monopolist) ?

This is equivalent to [...]

\({\theta_H}{v '}(\tilde{q}_H)=c\)

\({\theta_L}v'(\tilde{q}_L)=c\)

local x=2

^^ note the use of the local shell built-in command

pushd and popd

^^ this is nice way to navigate directories with keeping track of history (note: will only last in current shell)

^^^ example usage:

pushd /etc
pushd ~/ktest
popd <-- you are now back in /etc

printf "%0.2f\n" $x

^^ note the use of the printf build-in shell command

In linux, if you have a given command, how do you find out if that command is an alias (e.g. alias lsl="ls -l"), a system executable (e.g. cp), or a built-in shell command (e.g. source or printf)? Complete:

[...] <command>

ulimit -a

^^ note the use of the ulimit built-in shell command

^^^ note the -a flag to list all the resource limits, you can also look at individual ones with other flags

unset x

^^ note that unset is a shell built-in command

wait 123

^^ note that wait not only waits until background process 123 has finished but it also returns the exit code of process 123

trap

^^ the way you use trap is first create a function (e.g. called on_exit ) to react to the desired signal, then run trap like so, to bind the function to the signal: trap on_exit SIGINT

In linux, complete the following shell script snippet, to ensure on_exit function is run whenever the SIGINT signal is sent (i.e. when user presses ctr-c):

#!/bin/sh
 
function on_exit ()
{
    echo 'Bye bye'
}
 
[...]
 
sleep 500

trap on_exit SIGINT

^^ note the use of trap built-in command to bind the signal SIGINT to function on_exit

^^^ note the order of command line arguments given to trap, i.e. the binding functon is given first, then the signal

+

^^ e.g. set +h, to disable the caching of the locations of the binaries set in the PATH

set -u

^^ use the set built-in command, at top of script.

^^^ note the u stands for unset (i.e. you are enabling guarding against unset variables)