Background
We often want to test the output with different values for a parameter. Here’s
an example: we have a parameter that
pandoc uses to / compile source code files to PDF / M$ Word / etc.
rm output.html; param=0; pandoc input$param.txt -o output.html; \
echo param = $param
In practice, a parameter can be a font, the font size, etc, and there can be
multiple parameters.
Problem
To change the value in the parameter (e.g. 0 in param=0) in the above
command, one has to move the cursor to the middle—that is a tedious thing to
do. It would be much more convenient to put the value for the parameter at
the end of the command, like the following line.
rm output.html; pandoc input$param.txt -o output.html; param=value
However, the $param isn’t taking the value at the end of the above command.
Is there a way to use param with a value to be specified at the end of the
line?
Analysis
My first idea was to invert the shell’s character parsing order. However, as an end user of any shell (e.g. Bash, Zsh, Sh), I couldn’t extract anything useful.
Then I thought about the idea of mathematical functions, which quickly
translated to shell scripts. Unluckily, the idea of
creating another file to be made executable (by chmod +x) might not satisfy
users seeking one-liners. Besides, that might not be a good idea for users
accessing remote shells because a remote user might lack the permissions to run
the command chmod +x.
Solution
Shells supporting function calls
I have tested function calls with Git Bash. The following works for many well-known shells like Bash, Zsh, Sh, etc.
my_func () { rm temp.txt; touch temp.txt && echo input$1.txt; }
my_func 1
# output: input1.txt
my_func 2
# output: input2.txt
Here I declared a shell function my_func, and $1 refers to the first
argument passed to my_func.
Shells supporting function calls
Some shells like (t)csh don’t support function calls. I tried rewriting the
above shell function with an alias.
alias helloWorld 'echo input$1.txt; echo do other stuff'
However, on tcsh, that’s not working.
hellowWorld first_argument
# outputs:
# input.txt
# do other stuff first_argument
Thanks to this Stack Overflow answer about tcsh alias, I know that
$1 is passed to the script containing the alias rather than the alias
itself. I should use \!:1 instead of $1.
alias helloWorld 'echo input\!:1.txt; echo do other stuff'
hellowWorld first_arg
# outputs:
# inputfirst_arg.txt
# do other stuff