Week 10: Shell Environments / Programming / Makefiles Flashcards
The shell environment consists of a set of _____ with _____
Variables with values
Shell variables are used by putting _ in front of their names
$ in front of their names:
$HOME
The two kinds of shell variables are:
Environment variables: available in the current shell and the programs invoked by the shell
Regular shell variables: not available by programs invoked in this shell
In bash, REGULAR variables are defined by:
varname=varvalue
In bash, environment variables are called
“exported variables”
Environment variables are defined by:
MYENVVAR=”env var”
export MYENVVAR
or
export MYENVVAR=”env var”
To clear an environment variable, simply use…
unset
ex: unset varname;
In csh and tcsh to create REGULAR variables, use…
To delete them….
set
ex: set VARNAME=”var”
unset
ex: unset VARNAME
In csh and tcsh to create environment variables, use
setenv WITH NO EQUALS SIGN
ex: setenv VARNAME “var”
unsetenv
ex: unsetenv VARNAME
What do the following shell variables mean:
SHELL
PATH
LANG
USER
HOME
TERM
DISPLAY
HOSTNAME
SHELL: the name of the shell being used
PATH: where to find executables to execute
LANG: the locale you are using
USER: the user name of the user logged in
HOME: the user’s home directory
TERM: the kind of terminal the user is using
DISPLAY: where X program windows are shown
HOSTNAME: the name of the host logged on to
In UNIX, single quotes do what
Stop the expansion of a variable:
echo “Welcome $HOME”
Welcome /home/Connor
echo ‘Welcome $HOME’
Welcome $HOME
In UNIX, backwards quotes do what
Replace the variable with what is returned from the execution of the command
Unix automatically searches for executables in…
Whatever is specified by the variable PATH
In Unix, do executables automatically execute from the current durectory?
NO
use: ./ to specify the current directory using the .
If there are multiple versions of a command, Unix executes them in the order of …
Left to right
How does the alias command work, syntax?
alias alias-name=’real-command’
Where alias-name is one word and real-command can be separated by spaces
A shell script is basically
A UNIX file with shell commands in it
Why write shell scripts?
To automate difficult processes
and to avoid repetition
In order to run a shell script, you must first
chmod the files in order to make them executable
chmod u+x myencrypt mydecrypt
The syntax of a for loop in bash is:
for var in value1 value2 …
do
command_set
done
The syntax of an if statement in bash is
if
then
…
fi
An if, elif script looks like…
Semicolons in scripts are used to…
Seperate statements on the same line:
if grep “UNIX” myfile; then echo “Got it”; fi
A colon in shell scripts are meant for…
nothing, they do nothing
What is the syntax of the test command?
test -(file type) file
test –f file does file exist and is a regular file?
test -d file does file exist and is a directory?
test –x file does file exist and is executable?
test –s file does file exist and is longer than 0 bytes?
What does “test -d” do?
Does the file exist and is it a directory?
What does “test -r” do?
Does the file exist and is it a regular file?
What does “test -x” do?
Does the file exist and is it an executable?
What does test -s do?
Does the file exist and is it longer than 0 bytes?
What are the three types of string tests?
test -z
- Test whether or not the string has 0 characters in it
test str1 = str2
- Is str1 equal to str2
test str != str2
- Not equal?
What are the following options for integer tests?
-eq, -ne, -lt, -le, -gt, -ge
-eq, -ne, -lt, -le, -gt, -ge
equal, not equal, less than, less than or equal to, greater than, greater than or equal to
In shell scripts, how many command line arguments can be taken?
$1 up to $9
In unix shell commands, the shift command…
Shifts all the command arguments to the left
In shell scripts, the read command…
Reads iput from the terminal:
read var1 var2 var3
Action: reads a line of input from standard input
Assign first word to var1, second word to var2, …
The last variable gets any excess words on the line.
In shell scripts, the case syntax is in the form of…
