Session 6 - Wrapping it up.

The Plan

  • Get user input
  • Internal Field Separator (IFS)
  • Execution: background, parallel, conditional
  • Reviewing existing code
  • Codequiz
  • Q and A

Get user input

Interactive from keyboard (user):

read -p "Message" var1 var2 ... varN

See: Getting User Input Via Keyboard (cyberciti.biz)

Hello Who?

read -p "What is your name? " NAME
echo "Hello $NAME."
read -p "What is your name? " FIRST LAST
echo "Hello $LAST $FIRST."

Read: More modes

  • read -p: Show a message (prompt)
  • read -a: Assign words to array
  • read '-d ': Set other delimiter (e.g. space)
  • read -t n: Timeout after n seconds
  • …

See: $ help read or this article on computerhope.com

Internal Field Separator ($IFS)

“A string treated as a list of characters that is used for field splitting, expansion of the ’*’ special parameter, and to split lines into fields with the read utility."

See: Shell Command Language (POSIX)

$IFS and lists

Changing the separator to ‘,’ (comma):

LIST="one,two,three"

IFS=','
for ITEM in $LIST; do
    echo "item: $ITEM"
done

…but it could be any character! :)

$IFS and read

Reading a textfile line by line:

I=1
while IFS= read -r LINE
do
  echo "$I: $LINE"
  I=$((I + 1))
done < mylist.txt

mylist.txt:

one
two something: \"
three and additional
four

Background Execution

Just add an ampersand at the end:

$ my_program &

Background / Foreground / Suspend

  • Ctrl+Z: Suspend Job
  • fg [JOB_ID]: Move to foreground
  • bg [JOB_ID]: Run in background
  • jobs: List background jobs

See: 5 Examples to Manage Unix Background Jobs

Parallel Execution

ffmpeg -i day.ts -f segment -segment_time 3600 \
    -c:a copy out_%02d.ts

for FILE in out*.ts; do
    MP3_OUT=$(basename "$FILE" .ts).mp3

    # Run FFmpeg separately for each segment.
    # In the background.
    ffmpeg -i $FILE -c:a mp3 -b:a 192k MP3_OUT &
done

Parallel Execution

…or GNU-parallel-ize it!

ls *.ts | parallel ffmpeg -i {} -c:a mp3 -b:a 192k mp3/{}.mp3

Conditional Execution

# If 'first' succeeds, 'second' will never be executed:
$ first || second

# Only run 'second' if 'first' is successful:
$ first && second

Using Libraries

Including/importing code from other files:

source functions.sh

# read_fps() is declared in functions.sh:
read_fps "$VIDEO_IN"

See: Import/Source Files in Bash (Dave Eddy)

Reviewing Existing Code

Codequiz

  • What are variables for?
  • What are functions for?
  • When to use a loop?
    • while vs for?
  • How to use parameters?
  • What is eval?
  • What are <, | and > for?
  • What do && and || mean?

Questions and Answers

It’s your turn!

- Fin -