CS469/569 - Linux and Unix Administration and Networking Spring 2022

Bash Scripting - Part 2

Parameters / variables:

By default variables are strings, but can be declared with the "declare" builtin to be 64 bit ints (-i), single dimensional arrays (-a), or associative arrays (-A).

Variables are declared with name=[value]. If no value is specified, the default is the empty string.

let expression [expression...]

• Defines integer variables, performs C like arithmetic expressions.

readonly variable
declare -r variable

• built-ins that make the variables read-only.

local expression

• makes variables local to a function, otherwise variables are global by default.

Arrays:

name[index]=[value]
name=( values... );

Examples

x="string";
declare -i n=1235;
let n=5;
let n++;
declare -a a=("a" "b" "c");
a[0]="a";
declare -A aa=([a]="v1" [b]="v2")
aa[a]="v1";

Environment variables:

Each process maintains a set of variables called the environment, these are name/value pairs (strings only.)

In bash, variables that you want in the environment are "exported" using the 'export' built-in. i.e.:
export PATH=$PATH:.

Environment variable commands

> printenv [name]
> env

• Displays the current environment.

> unset [-vf] [name ...]

• unset the variable (default or -v) or function (-f).
• read-only vars cannot be unset.

Special variables:

Bracket and variable expansions:

• null pat matches all characters.

Command and arithmetic expansion:

$(command)
or
`command`

• command substitution. The output of command is substituted for the expansion.

$(( expression ))

• arithmetic expansion. Evaluates the expression as if it were a C arithmetic expression. Supports most C operators.

Functions:

Functions are like self-contained "scripts", i.e. arguments to the function are the positional parameters ($1 .. $n), and they return an integer exit value (via return.)

function name {
...
}

name () {
...
}

• Variables used in functions are global unless declared local.

Loops:

while list; do list; done

until list; do list; done

for name [ [ in [ word ...] ] ; ] do list ; done

• Basically a foreach type loop, places words into variable one at a time. If [in <words...>] are omitted it iterates over the positional parameters.

for (( expr1; expr2; expr3 )) ; do list ; done

• C style loop, each of the expressions are C arithmetic expressions. You can use {}'s in lieu of do and done when using this kind of loop.

break [n];

continue [n];

Conditionals:

if list; then list;
[elif list; then list;] ...
[else list;]
fi

case word in [ [(] pattern [ | pattern] ... ) list ;; ]
...
esac

select name [ in word ]; do
list ;
done

Bash Builtins:

alias name='value'

• Creates an command alias. is expanded (replaced with) .

bg [job specifier]

• Continues a stopped process in the background.

fg [job spec]

• Brings a job to the foreground.

jobs

• Lists jobs, both running and stopped.

echo [strings]

• Prints to the output, or a newline if strings is omitted.

printf [-vvar] format [arguments]

• Like the C printf function
  -v var assigns output to a variable.

read [-p prompt] [-s] [-n nchars] names

• Reads a line of text and splits it according to IFS and assigns words the variables listed.
  -p prompt - Print a prompt before reading input
  -s - Do not echo input characters
  -n nchars - Stop after reading n characters.
  ... Many more options.

test

• Same as [...] and mostly [[...]], use "help test" in bash to list file operators, such as:
  if [[ -f <file> ]]; then echo "is a regular file"; fi

source file [params]
or
. file [params]

• Reads and executes in the current shell. Use source not . (dot) as it is less confusing that way.