Bash – Ignore escape characters when printing string in shell script

bashquotingshell-script

I just wrote a simple shell script to save a LaTeX expression as a PNG file. It works fine, except for the LaTeX-syntax for "next row", i.e. the double backslash \\.

When for instance my input expression is like this:

\left( \begin{array}{cc}1 & 2\\ 3 & 4\end{array} \right)

The double backslash is reduced to a single one. I know I can add more backslashes in my expression, but I want those expressions to be genuine LaTeX, not some weird LaTeX-bash combination.

My script:

#!/bin/bash

if [ $# -ne 2 ]; then
    echo -e "Er moeten twee argumenten worden opgegeven:\n(1) LaTeX code\n(2) Bestandsnaam zonder extensie"
else
    SaveDir="/home/pieter"
    echo "\documentclass{article}\usepackage[utf8x]{inputenc}\pagestyle{empty}\begin{document}\[ $1 \]\end{document}" > /tmp/$2.tex
    latex -output-directory /tmp -interaction=batchmode $2.tex
    dvips -o /tmp/$2.ps -E /tmp/$2.dvi
    convert -units PixelsPerInch -density 200 /tmp/$2.ps -trim $SaveDir/$2.png
fi

So, how can I ignore the escape characters and literally print my LaTeX expression? Preferably without using sed.

Best Answer

If the TeX you want to output is fixed in your script, the safest is to use something like this:

cat > $2.tex << 'ENDHEADER'
\documentclass{article}
 \usepackage[utf8x]{inputenc}
 \pagestyle{empty}
\begin{document}
\[
ENDHEADER
echo $1 >> $2.tex
cat >> $2.tex << 'ENDFOOTER'
\]
\end{document}
ENDFOOTER

The data between the cat and END markers will be output as is with no substitutions. (I'm assuming all the newlines added are ok.)

Make sure you use hard quotes (') when you pass your TeX as an argument:

./your_script '\left( \begin{array}{cc}1 & 2\\ 3 & 4\end{array} \right)'

Related Solutions

sed – Characters to Escape When Using sed in a sh Script

There are two levels of interpretation here: the shell, and sed.

In the shell, everything between single quotes is interpreted literally, except for single quotes themselves. You can effectively have a single quote between single quotes by writing '\'' (close single quote, one literal single quote, open single quote).

Sed uses basic regular expressions. In a BRE, in order to have them treated literally, the characters $.*[\^ need to be quoted by preceding them by a backslash, except inside character sets ([…]). Letters, digits and (){}+?| must not be quoted (you can get away with quoting some of these in some implementations). The sequences $, $, \n, and in some implementations \{, \}, \+, \?, \| and other backslash+alphanumerics have special meanings. You can get away with not quoting $^ in some positions in some implementations.

Furthermore, you need a backslash before / if it is to appear in the regex outside of bracket expressions. You can choose an alternative character as the delimiter by writing, e.g., s~/dir~/replacement~ or \~/dir~p; you'll need a backslash before the delimiter if you want to include it in the BRE. If you choose a character that has a special meaning in a BRE and you want to include it literally, you'll need three backslashes; I do not recommend this, as it may behave differently in some implementations.

In a nutshell, for sed 's/…/…/':

Write the regex between single quotes.
Use '\'' to end up with a single quote in the regex.
Put a backslash before $.*/[\]^ and only those characters (but not inside bracket expressions). (Technically you shouldn't put a backslash before ] but I don't know of an implementation that treats ] and \] differently outside of bracket expressions.)
Inside a bracket expression, for - to be treated literally, make sure it is first or last ([abc-] or [-abc], not ~~[a-bc]~~).
Inside a bracket expression, for ^ to be treated literally, make sure it is not first (use [abc^], not ~~[^abc]~~).
To include ] in the list of characters matched by a bracket expression, make it the first character (or first after ^ for a negated set): []abc] or [^]abc] (not ~~[abc]] nor [abc\]]~~).

In the replacement text:

& and \ need to be quoted by preceding them by a backslash, as do the delimiter (usually /) and newlines.
\ followed by a digit has a special meaning. \ followed by a letter has a special meaning (special characters) in some implementations, and \ followed by some other character means \c or c depending on the implementation.
With single quotes around the argument (sed 's/…/…/'), use '\'' to put a single quote in the replacement text.

If the regex or replacement text comes from a shell variable, remember that

The regex is a BRE, not a literal string.
In the regex, a newline needs to be expressed as \n (which will never match unless you have other sed code adding newline characters to the pattern space). But note that it won't work inside bracket expressions with some sed implementations.
In the replacement text, &, \ and newlines need to be quoted.
The delimiter needs to be quoted (but not inside bracket expressions).
Use double quotes for interpolation: sed -e "s/$BRE/$REPL/".

Shell Script – Why Escape Trivial Characters?

This is used for alias protection:

$ ls
.bashrc  a  b

$ alias ls
alias ls='ls $LS_OPTIONS'

$ \ls
a   b

Best Answer

Related Solutions

sed – Characters to Escape When Using sed in a sh Script

Shell Script – Why Escape Trivial Characters?

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