EBoard 05: Shell Scripting

This class may be recorded! Its use will be limited to members of the class. Please do not share with others.

Approximate overview

• Administrivia

Administrivia

• I have a plane to catch. With the snow, I’m leaving at 3:20 instead of 3:50.
• Enjoy next Tuesday’s “Work Differently” day.
• Please fill out the Data Buddies survey if you haven’t already.
• Consider volunteering to be a CPU.
• Say hello to the little moppets (or something like that).

Upcoming work

• To be determined. Ned is teaching next week and I forgot to check with him.

Shell Scripting Basics

Just because I don’t know how much of this you learned on your own.

To write an algorithm, you need …

• Naming
• Conditionals
• Repetition
• Sequencing
• Subroutines
• Basic values and operations on those values

Some core concepts

VAR=VALUE

Assign to a variable. You access the variable with $VAR.

$ X="Hello World"
$ echo $X
Hello World

VAR=`COMMAND`

Evaluate a command and assign to a variable. You can also use the backtick form when you want to insert the result of a command on the command line.

$ GITFILES=`grep -l git *.md`
$ vi `grep -l git *.md`   # Sam edits all the files with git

$((EXP_)

Evaluate an expression

  $ echo $((3 + 4))
  7

if [ TEST ]; then STATEMENTS; fi

Conduct the test. If it succeeds, evaluate the statement. There’s also an optional else clause. And you can separate things with newlines rather than semicolons. ``` $ X=10 $ if [ $X -gt 0 ]; then echo “$X is positive.”; fi 10 is positive

$ if [ $X -lt 100 ]

then echo “$X is smallish” fi 10 is smallish

$ if [ -f ~/.bashrc ]; then

echo “You have a bashrc file” else echo “Your Unix life is incomplete” fi

while [ TEST ]; do STATEMENTS; done

Conduct the test. If it holds, run the statements and do it all over again.

$ X=10
while [ $X -gt 0 ]
do
  echo -n "$X "
  X=$(($X - 1))     # I may have gotten this wrong; sometimes bash is annoying
done

for VAR in LIST; do STATEMENT; done

Repeat the statement for each variable in a list.

for X in *.txt
do
  echo "Backing up $X"
  cp $X $X.bak
done
for X in `grep -l "Sam" *.txt`
do
  echo "Backing up $X"
  cp $X $X.bak
done

function NAME { BODY }

Define a function. Within the function, parameters are given by $1, $2, etc.

$ function add {
>   RESULT=$(($1 + $2))
>   echo $RESULT
> }
$ add 2 3
5

Your shell scripts

… and what we learn from them

Note: I appreciate the hard work I see in many of the scripts. However, I will still suggest improvements (i.e., make fun of them).

Some comments as we go …

• alias wordle="python wordle.cpython.pyc"
• If you’re going to share with another human being, comment.
• If you might forget how to use it and bad use can corrupt things, add error checking.
• It’s good to check before clobbering a file, even if you’re Ben Grimm.
• Try not to clobber files.
  • Option 1: Have something that tries for file.txt then file1.txt then file2.txt then file3.txt …
  • Traditional option: use /tmp/file.$$
  • Option 3: Embrace Ben Grimm
• Multiple ways to get counts
  • wc -w verbs.txt | cut -d' ' -f 1
  • wc -w < verbs.txt
• Be careful about violating trademark.