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Missing Semester Lecture 2 - Shell Tools and Scripting

Posted at 2024-08-27 Missing Semester 

MIT The Missing semester Lecture of Your CS Education Lecture 2 - Shell Tools and Scripting

Shell Scripting

To assign variables in bash, use the syntax foo=bar and access the value of the variable with $foo. Note that foo = bar will not work since it it interpreted as calling the foo program with arguments = and bar. In general, in shell scripts the space character will perform argument splitting.

Strings in bash can be defined with ' and " delimiters, but they are not equivalent. Strings delimited with ' are literal strings and will not substitute variable values whereas " delimited strings will.

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foo=bar
echo "$foo"
# prints bar
echo '$foo'
# prints $foo

Here is an example of a function that creates a directory and cds into it:

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mcd () {
    mkdir -p "$1"
    cd "$1"
}

Here $1 is the first argument to the script/function. Unlike other scripting languages, bash uses a variety of special variables to refer to arguments, error codes and other relevant variables. Below is a list of some of them.

• $0 - Name of the script
• $1 to $9 - Arguments to the script. $1 is the first argument and so on.
• $@ - All the arguments
• $# - Number of arguments
• $? - Return code of the previous command (A value of 0 usually means everything went OK; anything different from 0 means an error occurred.)
• $$ - PID for the current script
• !! - Entire last command, including arguments.
• $_ - Last argument from the last command.

  A more comprehensive list can be found here.

Exit codes can be used to conditionally execute commands using && and ||, both of which are short-circuiting operators. Commands can also be separated within the same line using a semicolon ;.

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false || echo "Oops, fail"
# Oops, fail

true || echo "Will not be printed"
#

true && echo "Things went well"
# Things went well

false && echo "Will not be printed"
#

true ; echo "This will always run"
# This will always run

false ; echo "This will always run"
# This will always run

Another common pattern is wanting to get the output of a command as a variable. This can be done with command substitution. Whenever you place $( CMD ) it will execute CMD, get the output of the command and substitute it in place. For example, if you do for file in $(ls), the shell will first call ls and then iterate over those values.
A lesser known similar feature is process substitution, <( CMD ) will execute CMD and place the output in a temporary file and substitute the <() with that file’s name. This is useful when commands expect values to be passed by file instead of by STDIN. For example, diff <(ls foo) <(ls bar) will show differences between files in dirs foo and bar.

Let’s see an example that showcases some of these features. It will iterate through the arguments we provide, grep for the string foobar, and append it to the file as a comment if it’s not found.

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#!/bin/bash

echo "Starting program at $(date)" # Date will be substituted

echo "Running program $0 with $# arguments with pid $$"

for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null
    # When pattern is not found, grep has exit status 1
    # We redirect STDOUT and STDERR to a null register since we do not care about them
    if [[ $? -ne 0 ]]; then
        echo "File $file does not have any foobar, adding one"
        echo "# foobar" >> "$file"
    fi
done

Here grep foobar "$file" > /dev/null means throw away the output of grep. 2 is a file descriptor in bash means stderr, so 2> /dev/null means rewire the stderr to null.

Shell globbing

• Wildcards - Whenever you want to perform some sort of wildcard matching, you can use ? and * to match one or any amount of characters respectively. For instance, given files foo, foo1, foo2, foo10 and bar, the command rm foo? will delete foo1 and foo2 whereas rm foo* will delete all but bar.
• Curly braces {} - Whenever you have a common substring in a series of commands, you can use curly braces for bash to expand this automatically. This comes in very handy when moving or converting files.

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convert image.{png,jpg}
# Will expand to
convert image.png image.jpg

cp /path/to/project/{foo,bar,baz}.sh /newpath
# Will expand to
cp /path/to/project/foo.sh /path/to/project/bar.sh /path/to/project/baz.sh /newpath

# Globbing techniques can also be combined
mv *{.py,.sh} folder
# Will move all *.py and *.sh files


mkdir foo bar
# This creates files foo/a, foo/b, ... foo/h, bar/a, bar/b, ... bar/h
touch {foo,bar}/{a..h}
touch foo/x bar/y
# Show differences between files in foo and bar
diff <(ls foo) <(ls bar)
# Outputs
# < x
# ---
# > y

Exercises

1. Read man ls and write an ls command that lists files in the following manner

   • Includes all files, including hidden files
   • Sizes are listed in human readable format (e.g. 454M instead of 454279954)
   • Files are ordered by recency
   • Output is colorized

   A sample output would look like this

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   -rw-r--r-- 1 user group 1.1M Jan 14 09:53 baz drwxr-xr-x 5 user group 160 Jan 14 09:53 . -rw-r--r-- 1 user group 514 Jan 14 06:42 bar -rw-r--r-- 1 user group 106M Jan 13 12:12 foo drwx------+ 47 user group 1.5K Jan 12 18:08 ..

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ls -a -t -h -l --color

-a, --all
             do not ignore entries starting with .
-t                         sort by modification time, newest first
--color[=WHEN]
             colorize  the  output;  WHEN can be 'never', 'auto', or 'always' (the
             default); more info below
-h, --human-readable
             with -l, print sizes in human readable format (e.g., 1K 234M 2G)

2. Write bash functions marco and polo that do the following. Whenever you execute marco the current working directory should be saved in some manner, then when you execute polo, no matter what directory you are in, polo should cd you back to the directory where you executed marco. For ease of debugging you can write the code in a file marco.sh and (re)load the definitions to your shell by executing source marco.sh.

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marco() {
    touch /tmp/missing/current_path
    echo $(pwd) > /tmp/missing/current_path
}

polo() {
    cd $(cat /tmp/missing/current_path)
}

3. Say you have a command that fails rarely. In order to debug it you need to capture its output but it can be time consuming to get a failure run. Write a bash script that runs the following script until it fails and captures its standard output and error streams to files and prints everything at the end. Bonus points if you can also report how many runs it took for the script to fail.

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#!/usr/bin/env bash
    
n=$(( RANDOM % 100 ))
    
if [[ n -eq 42 ]]; then
   echo "Something went wrong"
   >&2 echo "The error was using magic numbers"
   exit 1
fi
    
echo "Everything went according to plan"

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#!/usr/bin/env bash

touch random_stdout
touch random_stderr

echo "" > random_stdout
echo "" > random_stderr

cnt=0
until [[ $? -ne 0 ]];
do
(( cnt += 1 ))
./random.sh 1>>random_stdout 2>>random_stderr
done

echo "STDOUT:"
cat random_stdout

echo "STDERR:"
cat random_stderr

echo "Iterates $cnt times until failed."

4. As we covered in the lecture find’s -exec can be very powerful for performing operations over the files we are searching for. However, what if we want to do something with all the files, like creating a zip file? As you have seen so far commands will take input from both arguments and STDIN. When piping commands, we are connecting STDOUT to STDIN, but some commands like tar take inputs from arguments. To bridge this disconnect there’s the xargs command which will execute a command using STDIN as arguments. For example ls | xargs rm will delete the files in the current directory. Your task is to write a command that recursively finds all HTML files in the folder and makes a zip with them. Note that your command should work even if the files have spaces (hint: check -d flag for xargs).

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[root@localhost missing]# find . -name "*.html" | xargs -d "\n" tar -cf htmls.tar

5. (Advanced) Write a command or script to recursively find the most recently modified file in a directory. More generally, can you list all files by recency?

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find . -type f | ls -t | head -1

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find . -type f | ls -t