EXECLINE-BLOCK(7)

💅 EXECLINE-BLOCK(7) 💅

okay so bestie, this is the execline-block manual page — officially a Miscellaneous Information Manual page — and it’s literally all about blocks in execline. Buckle up, it’s giving command-line geometry. ✨

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🎀 NAME 🎀

execline-block — overview of execline blocks. That’s the name, that’s the vibe. 🫶

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🌈 DESCRIPTION 🌈

A command line (and thus an execline script) is one-dimensional. But bestie, a Unix execution flow can be two-dimensional: when two instructions are sequenced, for instance. In that case, we need a way to extract two command lines from one argv. That is precisely what blocks are made for. 💅

execline commands that need more than one linear set of arguments use blocks. For instance, the foreground(1) command needs to spawn a first process, then execute into a second one. It reads the command line for the first process from a block, and the command line for the second process from the rest of the argv. In the following script:

#!/command/execlineb
foreground { echo 1 } echo 2

echo 1 is read from a block and spawned; then echo 2 is executed. iconic behavior, no cap. 🔥

🌀 execlineb syntax 🌀

In execlineb(1) scripts, blocks are delimited by braces. They can be nested. slay. 🦋

🦋 argv syntax 🦋

execlineb(1) reads and parses the script, and converts it into an argv (a simple Unix command line) with a different syntax for blocks. In an argv, blocks are not delimited by braces; they are made of quoted arguments and terminated by an empty word (""). A quoted argument begins with a space. Nested blocks are represented by arguments being quoted several times, i.e. having several spaces in front of them; an empty word inside a block gets quoted too, i.e. it will be represented as a series of spaces. 💅

Actually, the block-reading commands know nothing about braces; they only understand the “quoted arguments + empty word” syntax. So if you want to use foreground(1) from your shell to sequence echo 1 and echo 2, you will have to write

$ foreground ' echo' ' 1' '' echo 2

You do not really need to quote every argument inside a block in that simple case. The following command works as well:

$ foreground echo 1 '' echo 2

However, this is bad practice, because it leads to a security hole: commands that perform substitution (cf. execline-substitute(7)) inside a block may produce empty words, which may modify your script’s execution flow. 🚨

$ define FOO '' foreground ' echo' ' ${FOO}' ' rm' ' -rf' ' /' '' echo blah

is safe, whereas

$ define FOO '' foreground echo '${FOO}' rm -rf / '' echo blah

has very much unwanted results. (Don’t try this at home.) 😭

You can use the EXECLINE_STRICT environment variable to check proper block quoting. If that variable contains 1, commands that read blocks will print a warning message every time they find an unquoted argument inside a block. If that variable contains 2 or a bigger integer, commands will print an error message and die on unquoted arguments. 💀

You can use execlineb(1)’s -w or -W switch to set EXECLINE_STRICT to 1 or 2. periodt. 👑

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👀 SEE ALSO 👀

• execlineb(1)
• foreground(1)
• execline-substitute(7)

This man page is ported from the authoritative documentation at: https://skarnet.org/software/execline/el_semicolon.html.html

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👑 AUTHORS 👑

• Laurent Bercot
• Alexis <flexibeast@gmail.com> (man page port)

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mwah 💖 this EXECLINE-BLOCK(7) moment is brought to you by Debian, dated February 16, 2021 — bye bestie, go quote those blocks properly! 🎀