Bash Golf Part 3

Published at 2023-12-10T11:35:54+02:00

    '\       '\        '\                   .  .          |>18>>
      \        \         \              .         ' .     |
     O>>      O>>       O>>         .                 'o  |
      \       .\. ..    .\. ..   .                        |
      /\    .  /\     .  /\    . .                        |
     / /   .  / /  .'.  / /  .'    .                      |
jgs^^^^^^^`^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        Art by Joan Stark, mod. by Paul Buetow

This is the third blog post about my Bash Golf series. This series is random Bash tips, tricks, and weirdnesses I have encountered over time.

2021-11-29 Bash Golf Part 1
2022-01-01 Bash Golf Part 2
2023-12-10 Bash Golf Part 3 (You are currently reading this)

FUNCNAME

FUNCNAME is an array you are looking for a way to dynamically determine the name of the current function (which could be considered the callee in the context of its own execution), you can use the special variable FUNCNAME. This is an array variable that contains the names of all shell functions currently in the execution call stack. The element FUNCNAME[0] holds the name of the currently executing function, FUNCNAME[1] the name of the function that called that, and so on.

This is particularly useful for logging when you want to include the callee function in the log output. E.g. look at this log helper:

#!/usr/bin/env bash

log () {
    local -r level="$1"; shift
    local -r message="$1"; shift
    local -i pid="$$"

    local -r callee=${FUNCNAME[1]}
    local -r stamp=$(date +%Y%m%d-%H%M%S)

    echo "$level|$stamp|$pid|$callee|$message" >&2
}

at_home_friday_evening () {
    log INFO 'One Peperoni Pizza, please'
}

at_home_friday_evening

The output is as follows:

❯ ./logexample.sh
INFO|20231210-082732|123002|at_home_friday_evening|One Peperoni Pizza, please

:(){ :|:& };:

This one may be widely known already, but I am including it here as I found a cute image illustrating it. But to break :(){ :|:& };: down:

:(){ } is really a declaration of the function :
The ; is ending the current statement
The : at the end is calling the function :
:|:& is the function body

Let's break down the function body :|:&:

The first : is calling the function recursively
The |: is piping the output to the function : again (parallel recursion)
The & lets it run in the background.

So, it's a fork bomb. If you run it, your computer will run out of resources eventually. (Modern Linux distributions could have reasonable limits configured for your login session, so it won't bring down your whole system anymore unless you run it as root!)

And here is the cute illustration:

Inner functions

Bash defines variables as it is interpreting the code. The same applies to function declarations. Let's consider this code:

#!/usr/bin/env bash

outer() {
  inner() {
    echo 'Intel inside!'
  }
  inner
}

inner
outer
inner

And let's execute it:

❯ ./inner.sh
/tmp/inner.sh: line 10: inner: command not found
Intel inside!
Intel inside!

What happened? The first time inner was called, it wasn't defined yet. That only happens after the outer run. Note that inner will still be globally defined. But functions can be declared multiple times (the last version wins):

#!/usr/bin/env bash

outer1() {
  inner() {
    echo 'Intel inside!'
  }
  inner
}

outer2() {
  inner() {
    echo 'Wintel inside!'
  }
  inner
}

outer1
inner
outer2
inner

And let's run it:

❯ ./inner2.sh
Intel inside!
Intel inside!
Wintel inside!
Wintel inside!

Exporting functions

Have you ever wondered how to execute a shell function in parallel through xargs? The problem is that this won't work:

#!/usr/bin/env bash

some_expensive_operations() {
  echo "Doing expensive operations with '$1' from pid $$"
}

for i in {0..9}; do echo $i; done \
  | xargs -P10 -I{} bash -c 'some_expensive_operations "{}"'

We try here to run ten parallel processes; each of them should run the some_expensive_operations function with a different argument. The arguments are provided to xargs through STDIN one per line. When executed, we get this:

❯ ./xargs.sh
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found
bash: line 1: some_expensive_operations: command not found

There's an easy solution for this. Just export the function! It will then be magically available in any sub-shell!

#!/usr/bin/env bash

some_expensive_operations() {
  echo "Doing expensive operations with '$1' from pid $$"
}
export -f some_expensive_operations

for i in {0..9}; do echo $i; done \
  | xargs -P10 -I{} bash -c 'some_expensive_operations "{}"'

When we run this now, we get:

❯ ./xargs.sh
Doing expensive operations with '0' from pid 132831
Doing expensive operations with '1' from pid 132832
Doing expensive operations with '2' from pid 132833
Doing expensive operations with '3' from pid 132834
Doing expensive operations with '4' from pid 132835
Doing expensive operations with '5' from pid 132836
Doing expensive operations with '6' from pid 132837
Doing expensive operations with '7' from pid 132838
Doing expensive operations with '8' from pid 132839
Doing expensive operations with '9' from pid 132840

If some_expensive_function would call another function, the other function must also be exported. Otherwise, there will be a runtime error again. E.g., this won't work:

#!/usr/bin/env bash

some_other_function() {
  echo "$1"
}

some_expensive_operations() {
  some_other_function "Doing expensive operations with '$1' from pid $$"
}
export -f some_expensive_operations

for i in {0..9}; do echo $i; done \
  | xargs -P10 -I{} bash -c 'some_expensive_operations "{}"'

... because some_other_function isn't exported! You will also need to add an export -f some_other_function!

Dynamic variables with local

You may know that local is how to declare local variables in a function. Most don't know that those variables actually have dynamic scope. Let's consider the following example:

#!/usr/bin/env bash

foo() {
  local foo=bar # Declare local/dynamic variable
  bar
  echo "$foo"
}

bar() {
  echo "$foo"
  foo=baz
}

foo=foo # Declare global variable
foo # Call function foo
echo "$foo"

Let's pause a minute. What do you think the output would be?

Let's run it:

❯ ./dynamic.sh
bar
baz
foo

What happened? The variable foo (declared with local) is available in the function it was declared in and in all other functions down the call stack! We can even modify the value of foo, and the change will be visible up the call stack. It's not a global variable; on the last line, echo "$foo" echoes the global variable content.

if conditionals

Consider all variants here more or less equivalent:

#!/usr/bin/env bash

declare -r foo=foo
declare -r bar=bar

if [ "$foo" = foo ]; then
  if [ "$bar" = bar ]; then
    echo ok1
  fi
fi

if [ "$foo" = foo ] && [ "$bar" == bar ]; then
  echo ok2a
fi

[ "$foo" = foo ] && [ "$bar" == bar ] && echo ok2b

if [[ "$foo" = foo && "$bar" == bar ]]; then
  echo ok3a
fi

 [[ "$foo" = foo && "$bar" == bar ]] && echo ok3b

if test "$foo" = foo && test "$bar" = bar; then
  echo ok4a
fi

test "$foo" = foo && test "$bar" = bar && echo ok4b

The output we get is:

❯ ./if.sh
ok1
ok2a
ok2b
ok3a
ok3b
ok4a
ok4b

Multi-line comments

You all know how to comment. Put a # in front of it. You could use multiple single-line comments or abuse heredocs and redirect it to the : no-op command to emulate multi-line comments.

#!/usr/bin/env bash

# Single line comment

# These are two single line
# comments one after another

: <<COMMENT
This is another way a
multi line comment
could be written!
COMMENT

I will not demonstrate the execution of this script, as it won't print anything! It's obviously not the most pretty way of commenting on your code, but it could sometimes be handy!

Don't change it while it's executed

Consider this script:

#!/usr/bin/env bash

echo foo
echo echo baz >> $0
echo bar

When it is run, it will do:

❯ ./if.sh
foo
bar
baz
❯ cat if.sh
#!/usr/bin/env bash

echo foo
echo echo baz >> $0
echo bar
echo baz

So what happened? The echo baz line was appended to the script while it was still executed! And the interpreter also picked it up! It tells us that Bash evaluates each line as it encounters it. This can lead to nasty side effects when editing the script while it is still being executed! You should always keep this in mind!

E-Mail your comments to paul@nospam.buetow.org :-)

Other related posts are:

2021-05-16 Personal Bash coding style guide
2021-06-05 Gemtexter - One Bash script to rule it all
2021-11-29 Bash Golf Part 1
2022-01-01 Bash Golf Part 2
2023-12-10 Bash Golf Part 3 (You are currently reading this)

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