$$ \newcommand{\problemdivider}{\begin{center}\large \bf\ldots\ldots\ldots\ldots\ldots\ldots\end{center}} \newcommand{\subproblemdivider}{\begin{center}\large \bf\ldots\ldots\end{center}} \newcommand{\pdiv}{\problemdivider} \newcommand{\spdiv}{\subproblemdivider} \newcommand{\ba}{\begin{align*}} \newcommand{\ea}{\end{align*}} \newcommand{\rt}{\right} \newcommand{\lt}{\left} \newcommand{\bp}{\begin{problem}} \newcommand{\ep}{\end{problem}} \newcommand{\bsp}{\begin{subproblem}} \newcommand{\esp}{\end{subproblem}} \newcommand{\bssp}{\begin{subsubproblem}} \newcommand{\essp}{\end{subsubproblem}} \newcommand{\atag}[1]{\addtocounter{equation}{1}\label{#1}\tag{\arabic{section}.\alph{subsection}.\alph{equation}}} \newcommand{\btag}[1]{\addtocounter{equation}{1}\label{#1}\tag{\arabic{section}.\alph{equation}}} \newcommand{\ctag}[1]{\addtocounter{equation}{1}\label{#1}\tag{\arabic{equation}}} \newcommand{\dtag}[1]{\addtocounter{equation}{1}\label{#1}\tag{\Alph{chapter}.\arabic{section}.\arabic{equation}}} \newcommand{\unts}[1]{\ \text{#1}} \newcommand{\textop}[1]{\operatorname{#1}} \newcommand{\textopl}[1]{\operatornamewithlimits{#1}} \newcommand{\prt}{\partial} \newcommand{\pderi}[3]{\frac{\prt^{#3}#1}{\prt #2^{#3}}} \newcommand{\deri}[3]{\frac{d^{#3}#1}{d #2^{#3}}} \newcommand{\del}{\vec\nabla} \newcommand{\exval}[1]{\langle #1\rangle} \newcommand{\bra}[1]{\langle #1|} \newcommand{\ket}[1]{|#1\rangle} \newcommand{\ham}{\mathcal{H}} \newcommand{\arr}{\mathfrak{r}} \newcommand{\conv}{\mathop{\scalebox{2}{\raisebox{-0.2ex}{$\ast$}}}} \newcommand{\bsm}{\lt(\begin{smallmatrix}} \newcommand{\esm}{\end{smallmatrix}\rt)} \newcommand{\bpm}{\begin{pmatrix}} \newcommand{\epm}{\end{pmatrix}} \newcommand{\bdet}{\lt|\begin{smallmatrix}} \newcommand{\edet}{\end{smallmatrix}\rt|} \newcommand{\bs}[1]{\boldsymbol{#1}} \newcommand{\uvec}[1]{\bs{\hat{#1}}} \newcommand{\qed}{\hfill$\Box$} $$
Tags:
  • linux
  • shell

    • Variable tricks

    Indirect expansion

    ${!VAR} expands to the value of the variable whose name is $VAR. For example,

    $ VAR=foo
$ X=VAR
$ echo ${!X}
foo

    Weird shell tricks

    Inline comments

    Abuse command substitution to add inline comments, e.g.

    command --foo bar `#comment goes here` --baz -ABC

    Note that this spawns a subshell for each comment, so don’t use in performance critical areas.

    File descriptors

    Valuable blog post on file descriptors and their associated device files: http://www.sichong.site/linux/2021/11/06/dev-stdout-vs-2-a-dive-into-linux-stdio.html

    Miscellany

    • Nonexistent globs return string literals, e.g.
    for y in x*; do echo $y; done
# will print "x1", "x2", if "x1", "x2" exist, otherwise will print "x*"
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