SHLVL blog post
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@ -1,2 +1,3 @@
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* [SHLVL PS1](./shlvl.md)
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* [Backing up my ZFS NAS to an external drive](./zfs-nas-backup.md)
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* [The traditional first software engineer blog post](./blog-start.md)
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---
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title: SHLVL PS1
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pubdate: 2023-12-18T09:53:17-08:00
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feed: blog
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---
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I often find myself in a subshell, usually because of `nix-shell` or `nix develop`, but also sometimes when I run `bash` from `bash` instead of using `pushd` and `popd`. The prompting in Nix shells can be inconsistent betwen tools or versions: `nix-shell` overrides the prompt to `[nix-shell:cwd]$`, but `nix shell`, the `nix` command's replacement for `nix-shell -p`, doesn't change `PS1` at all.
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I want a way to tell when I'm in a subshell so I can quit it without quitting my terminal window entirely. There is not, to my knowledge as of writing, an environment variable that distinguishes `nix shell`. However, there is a `bash` feature that helps here: the [`SHLVL`](https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-SHLVL) environment variable.
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> **SHLVL**
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> Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.
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The default `PS1` on NixOS 23.11 GNOME is this[^1]:
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PS1="\n\[\033[1;32m\][\[\e]0;\u@\h: \w\a\]\u@\h:\w]\$\[\033[0m\] "
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# |------------|||-----+-----------+||------||\/|---------|
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# ANSI color | xterm text to prompt || ANSI color
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# code to make | escape set the text || reset
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# the prompt | to set title to ||
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# green | title |+- $ for user, # for root
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# literal [ literal ]
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[^1]: [Stack Overflow explanation of xterm title escape](https://unix.stackexchange.com/questions/306716/meaning-of-e0-in-ps1-in-bashrc)
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This is what I added to my `.bashrc`. I decomposed it a bit to be more legible to my future self:
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_TITLE_BAR="\u@\h: \w"
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_SET_TITLE_BAR="\[\e]0;$_TITLE_BAR\a\]"
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_PROMPT="\A \u\h:\w"
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_SHLVL=$(printf '\$%.0s' $(seq 1 $SHLVL))
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_PS1="$_SET_TITLE_BAR[$_PROMPT]$_SHLVL "
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export PS1="\[\033[1;32m\]$_PS1\[\033[0m\]"
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The `printf` expression here is taken from [this Stack Overflow post](https://stackoverflow.com/questions/5349718/how-can-i-repeat-a-character-in-bash); essentially, it prints the string we want to repeat (`\$`), then formats the input to 0 characters wide. Repeating inputs lets us repeat the format, which `seq` does. Now the prompt shows how many subshells we're in:
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[09:04 tvbimperium:~]$ bash
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[09:04 tvbimperium:~]$$ bash
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[09:04 tvbimperium:~]$$$ bash
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[09:04 tvbimperium:~]$$$$ bash
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[09:04 tvbimperium:~]$$$$$
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exit
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[09:05 tvbimperium:~]$$$$
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exit
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[09:05 tvbimperium:~]$$$
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exit
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[09:05 tvbimperium:~]$$
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exit
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[09:05 tvbimperium:~]$
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@ -4,5 +4,6 @@ title: Blog
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[RSS](./feed.xml)
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* [SHLVL PS1](./2023/shlvl.md)
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* [Backing up my ZFS NAS to an external drive](./2023/zfs-nas-backup.md)
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* [The traditional first software engineer blog post](./2023/blog-start.md)
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