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> Published at 2023-12-10T11:35:54+02:00
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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.
`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:
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!
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 $i; done \
| xargs -P10 -I{} bash -c 'some_expensive_operations "{}"'
... because `some_other_function` isn't exported! You will also need to add an `export -f some_other_function`!
You may know that `local` is how to declare local variables in a function. Most don't know that those variables actually have dynamic scope. Let's consider the following example:
#!/usr/bin/env bash
foo() {
local foo=bar # Declare local/dynamic variable
bar
echo "$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 "$foo"` echoes the global variable content.
Consider all variants here more or less equivalent:
#!/usr/bin/env bash
declare -r foo=foo
declare -r bar=bar
if [ "$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
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!
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:
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