More Bash features and their composability

Objectives

Recognize and read advanced shell features

To follow along

Please navigate to the examples/composability directory.

More streams and redirections

Use a file as `stdin` for a process: `<`

The < redirection operator takes a file and put its contents in the stdin of a process, as if we were typing it ourselves:

$ wc -l < myfile
17

`stderr` into `stdout` or vice versa

&> can be used to redirect both streams
1>&2 redirects stdout to stderr
2>&1 does the opposite

Process substitution: `<()`

Create a temporary file containing the output (stdout) of the list of commands inside the brackets

Compound commands

Grouping a list commands together with { }.

The exit code of the compound command is the one of the last command executed.

Bash functions

Functions can be created with the following syntax:

function f(){
  FIRST_ARG="$1"
  echo $FIST_ARG 
}

Function is actually not necessary, so this would work:

f(){
  FIRST_ARG="$1"
  echo $FIST_ARG 
}

Warning

Bash functions do not define a scope

Any operation made on variables visible by the functions will affect the shell where the function is executing.

This is actually by design. What about the module command?

Subshells instead do define a scope.

Arrays

Arrays are a bash feature that allows to store a list of elements.

They are generated with the syntax

A=( first_element
    second_element
    ...
    )

The list of elements can be generated in various ways.

As an example, we will now reate a variable named “outputfile” that is composed of 3 strings:

Environment variable $LOGNAME
Arbitrary string of 4 characters generated in subshell via: mktemp -u XXXX
First 2 characters of the current month (→ use „date“) using a bash array

A possible solution reads:

array=($(date))
month=${array[1]:0:2}

declare -r outputfile="${LOGNAME}_$(mktemp -u XXXX)_${month}.log"
echo ${outputfile}

# Try changing output file
outputfile="new"