Escape Sequences And Locale Settings: How They Affect Shell Parsing

Understanding Escape Sequences

An escape sequence is a combination of keyboard characters that does not represent a printable character but has a special meaning to be interpreted by the computer program that receives it. Escape sequences are used in many computing contexts such as programming languages and command shells to encode information in a compact yet meaningful way. Some common examples in Linux shells and terminal emulators are the newline sequence ‘\n’ and tab sequence ‘\t’. An escape character, usually the backslash \ signals that the next character has a special meaning and should not be interpreted literally.

In shells like bash and zsh, escape sequences allow inserting special characters into the command lines and strings processed by the shell. As the shell parses each line of input, escape sequences are substituted with their encoded meaning to transform the literal characters into control characters and values. This allows compactly expressing concepts like newlines, tabs, backspaces, alert sounds and more which would otherwise require multiple key presses. Shells have defined rules on which escape sequences they understand, what the sequences represent, and how they are handled in different contexts like double quotes and single quotes.

Some common escape sequences used in bash and other Linux shells include:

Table of Contents

\a – Alert or bell
\b – Backspace
\n – Newline
\r – Carriage return
\t – Horizontal tab
\\ – Backslash
\’ – Single quote
\” – Double quote

Locale Settings and Character Encoding

A locale defines language and cultural settings regarding how users in a geographical region expect dates, times, currencies, sorting and other formatting to work in a computer system or software application. Locales encompass details like date formats being MM/DD/YYYY or DD/MM/YYYY, currency symbols showing $, € or ¥, and so on. Underlying the formatting is the character encoding which determines how text and characters map to underlying integer codes that computers process.

Some examples of common locale environmental settings on Linux systems include:

en_US.UTF-8 – English language, United States region, UTF-8 character encoding
fr_FR.UTF-8 – French language, France region, UTF-8 encoding
zh_CN.GB18030 – Chinese language, China region, GB18030 encoding

A special encoding is UTF-8 which can represent virtually any character used in modern writing systems. It is a variable-width encoding where English letters take up 1 byte while more complex Asian script characters can take up to 4 bytes. UTF-8 is compatible with ASCII characters so it works well for encoding text in programming and on web pages.

Escaped Characters in Double Quotes vs Single Quotes

In most shells, the handling rules for escape sequences vary depending on whether they appear within double quotes (“) or single quotes (‘). Within double quotes, escape sequences like \n and \t are interpreted and substituted with their encoded meanings. So “\n” in double quotes would become an actual newline in the parsed command string. In contrast, within single quotes the escape sequences are not interpreted and get passed literally as “\n” without changing.

This difference allows either parsing escapes when desired inside double quotes or disabling parsing to pass sequences verbatim with single quotes. Some examples in bash are:

echo “\nTwo Lines” – Prints two lines
echo ‘\nJust \n’ – Prints \n verbatim

One exception is that single quotes still allow for escaping the quote itself to embed literally with \’. Double quotes need escaping too but also have alternate forms like \” that work inside double quoted substrings.

Locale Affects on Sorting and Character Ranges

The locale settings greatly impact the order in which characters sort for ranging and collation operations. Sort order also called collation order differs across languages. Locales define information like which letters and symbols should be treated as base letters vs variants, how accents affect order, and precedence of uppercase vs lowercase.

As an example, while in English lowercase comes before uppercase, in languages like French and German dictionary order has uppercase letters before lowercase. This changes sorting output. Locale collation rules also affect character ranges expressed with square brackets [ ] which have locale specific orderings applied.

Some unsorted English output with range expressions:

[A-M] Apple Grape Orange
[a-m] apple Grape orange

And the same output sorted in French locale order:

[A-M] Grape Orange Apple
[a-m] apple Grape orange

Setting Locales in Shell Scripts

Shell scripts can query the locale settings with the locale command. It prints details on language, region, character encoding and other active locale information. Scripts can also temporarily set the locale for the duration of their execution to ensure consistent sorting, formatting or language rules. After the script work finishes, resetting the original locale is good practice.

Below is an example script snippet that sets the locale, does some string work, then resets to the original locale:

ORIG_LOCALE=$(locale)
export LC_ALL=fr_FR.UTF-8

# Script work here, e.g. sorting
myFrenchSortingOperations

# Reset locale 
export LC_ALL=$ORIG_LOCALE

Now any operations in myFrenchSortingOperations will use French dictionary sort order and language rules consistently regardless of the environment locale settings.

Troubleshooting Unexpected Parsing and Encoding

When scripts contain escape sequences and locale aware operations, there are sometimes unexpected output and parsing behavior if locales and encodings mismatch or get inconsistent. Debugging odd output from escape sequences being shown literally instead of interpreted control codes can indicate gaps in understanding between the encoding emitted and encoding parsed.

Similar trouble comes from sorting, formatting, and language inconsistencies caused by unmatched locales between inputs and environment settings during intermediate processing steps. Tools like iconv can help debug multi-byte character encoding and locale issues by inspecting and converting documents to reveal how their codes align to expectations.

With vigilance on escape handling, character encodings, and setting appropriate parsing locales, shell scripts can parse escape sequences and handle text robustly across operating environments.