Input/Output

Heredocs

Everybody knows Heredocs:

$ rotix <<Here
> Lorem ipsum dolor sit amet, consectetur adipiscing elit,
> sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
> Here
Yberz vcfhz qbybe fvg nzrg, pbafrpgrghe nqvcvfpvat ryvg,
frq qb rvhfzbq grzcbe vapvqvqhag hg ynober rg qbyber zntan nyvdhn.

Heredoc Indentation

Contrary to popular belief, heredocs don't need to be outdented within blocks, by prefixing the End pattern with a -.

#!/usr/bin/env bash

if true
then
        rotix <<-Here
        Lorem ipsum dolor sit amet, consectetur adipiscing elit,
        sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
        Here
fi

Heredoc Quoting

$ cat <<Here
> My name is $LOGNAME
> Running on $(uname -s)
> Here
My name is martin
Running on Linux

$ cat <<"Here"
> My name is $LOGNAME
> Running on $(uname -s)
> Here
My name is $LOGNAME
Running on $(uname -s)

Here Strings

Bash also knows here strings, for passing input without echoing through a pipe:

$ rotix <<<'Lorem Ipsum'
Yberz Vcfhz

$ figlet <<<'Lorem Ipsum'
 _                               ___
| |    ___  _ __ ___ _ __ ___   |_ _|_ __  ___ _   _ _ __ ___
| |   / _ \| '__/ _ \ '_ ` _ \   | || '_ \/ __| | | | '_ ` _ \
| |__| (_) | | |  __/ | | | | |  | || |_) \__ \ |_| | | | | | |
|_____\___/|_|  \___|_| |_| |_| |___| .__/|___/\__,_|_| |_| |_|
                                    |_|

Process Substitution

Process substitution allows a process’s input or output to be referred to using a filename.

Extremely useful where you would usually pass a file but only have an input stream.

$ diff /etc/services <(ssh 192.168.1.35 cat /etc/services)

Splitting input

Don't: Split input by using external commands such as awk or cut.

#!/usr/bin/env bash

while read -r LINE
do
    # Uses 2 subprocesses each:
    servicename=$(echo "$LINE" | cut -f 1)
    serviceport=$(echo "$LINE" | awk '{print $2}')
    printf "%s on %s\n" "$serviceport" "$serviceport"
done < /etc/services

Do: Use read to split directly into fields of an array and work from there.

#!/usr/bin/env bash

while read -a fields -r 
do
    # Uses no subprocess at all
    servicename=${fields[0]}
    serviceport=${fields[1]}
    printf "%s on %s\n" "$servicename" "$serviceport"
done < /etc/services