ASCII Table, Character Codes, And Linux Text Tools

ASCII stands for American Standard Code for Information Interchange. It’s a 7-bit character encoding standard mapping numeric values (0-127) to printable characters and control codes. Since computers work with numbers at the lowest level, ASCII provides a universal way to represent text—each character has a unique numeric value.

Standardized in 1963, ASCII remains foundational to modern computing. Most character-encoding schemes today (UTF-8, UTF-16, ISO-8859-1) maintain backward compatibility with ASCII for the 0-127 range, meaning the first 128 characters are identical across these encodings.

ASCII Ranges

The 128 ASCII codes divide into three categories:

Control codes (0-31): Non-printing characters originally designed for device control, text formatting, and transmission protocols. Most original purposes are obsolete, but several remain essential: LF (line feed), CR (carriage return), TAB, and ESC (escape sequences).
Printable characters (32-126): Space, punctuation, digits 0-9, uppercase letters A-Z, lowercase letters a-z, and symbols.
DEL (127): The delete character, technically classified as a control code.

Converting Characters and ASCII Codes

Using printf

Convert a character to its ASCII code:

printf '%d\n' "'A"
# Output: 65

Convert an ASCII code to a character:

printf '\x41\n'
# Output: A

Or using octal:

printf "\\$(printf '%03o' 65)\n"
# Output: A

Multiple conversions at once:

printf 'Char: A\n'
printf '  Decimal: %d\n' "'A"
printf '  Hex: %x\n' "'A"
printf '  Octal: %o\n' "'A"
# Output:
# Char: A
#   Decimal: 65
#   Hex: 41
#   Octal: 101

Using od (octal dump)

Inspect ASCII values in a file or string:

echo "Hello" | od -An -td1
#  72  101  108  108  111

Display in hexadecimal:

echo "Hello" | od -An -tx1
#  48  65  6c  6c  6f

Display with character names:

echo "test" | od -c
# 0000000 t e s t \n

Using xxd for hex dumps

echo "test" | xxd
# 00000000: 7465 7374 0a                             test.

The 0a at the end is the newline character (LF, decimal 10).

Using Python

Convert individual characters:

ord('A')        # Returns: 65
chr(65)         # Returns: 'A'

Process entire strings:

[ord(c) for c in "Hello"]  # Returns: [72, 101, 108, 108, 111]
{ord(c): c for c in "ASCII"}  # Returns: {65: 'A', 83: 'S', 67: 'C', 73: 'I'}

Using Bash parameter expansion

Extract the ASCII value of the first character:

var="Hello"
printf '%d\n' "'${var:0:1}"
# Output: 72

Key Control Characters

Most control codes are obsolete for their original purposes, but a few remain relevant in system administration and text processing:

Code	Decimal	Hex	Name	Purpose
NUL	0	0x00	Null	String terminator in C
BEL	7	0x07	Bell	Alert signal (rarely used)
BS	8	0x08	Backspace	Cursor control
TAB	9	0x09	Horizontal Tab	Text formatting, field separation
LF	10	0x0A	Line Feed	Newline (Unix/Linux)
CR	13	0x0D	Carriage Return	Legacy newline (Windows/old Mac)
ESC	27	0x1B	Escape	ANSI escape sequences, terminal control
DEL	127	0x7F	Delete	Erase character

Complete ASCII Table (0-127)

Dec	Oct	Hex	Char	Dec	Oct	Hex	Char	Dec	Oct	Hex	Char	Dec	Oct	Hex	Char
0	000	0x00	NUL	32	040	0x20	SP	64	100	0x40	@	96	140	0x60	`
1	001	0x01	SOH	33	041	0x21	!	65	101	0x41	A	97	141	0x61	a
2	002	0x02	STX	34	042	0x22	“	66	102	0x42	B	98	142	0x62	b
3	003	0x03	ETX	35	043	0x23	#	67	103	0x43	C	99	143	0x63	c
4	004	0x04	EOT	36	044	0x24	$	68	104	0x44	D	100	144	0x64	d
5	005	0x05	ENQ	37	045	0x25	%	69	105	0x45	E	101	145	0x65	e
6	006	0x06	ACK	38	046	0x26	&	70	106	0x46	F	102	146	0x66	f
7	007	0x07	BEL	39	047	0x27	‘	71	107	0x47	G	103	147	0x67	g
8	010	0x08	BS	40	050	0x28	(	72	110	0x48	H	104	150	0x68	h
9	011	0x09	TAB	41	051	0x29	)	73	111	0x49	I	105	151	0x69	i
10	012	0x0A	LF	42	052	0x2A	*	74	112	0x4A	J	106	152	0x6A	j
11	013	0x0B	VT	43	053	0x2B	+	75	113	0x4B	K	107	153	0x6B	k
12	014	0x0C	FF	44	054	0x2C	,	76	114	0x4C	L	108	154	0x6C	l
13	015	0x0D	CR	45	055	0x2D	–	77	115	0x4D	M	109	155	0x6D	m
14	016	0x0E	SO	46	056	0x2E	.	78	116	0x4E	N	110	156	0x6E	n
15	017	0x0F	SI	47	057	0x2F	/	79	117	0x4F	O	111	157	0x6F	o
16	020	0x10	DLE	48	060	0x30	0	80	120	0x50	P	112	160	0x70	p
17	021	0x11	DC1	49	061	0x31	1	81	121	0x51	Q	113	161	0x71	q
18	022	0x12	DC2	50	062	0x32	2	82	122	0x52	R	114	162	0x72	r
19	023	0x13	DC3	51	063	0x33	3	83	123	0x53	S	115	163	0x73	s
20	024	0x14	DC4	52	064	0x34	4	84	124	0x54	T	116	164	0x74	t
21	025	0x15	NAK	53	065	0x35	5	85	125	0x55	U	117	165	0x75	u
22	026	0x16	SYN	54	066	0x36	6	86	126	0x56	V	118	166	0x76	v
23	027	0x17	ETB	55	067	0x37	7	87	127	0x57	W	119	167	0x77	w
24	030	0x18	CAN	56	070	0x38	8	88	130	0x58	X	120	170	0x78	x
25	031	0x19	EM	57	071	0x39	9	89	131	0x59	Y	121	171	0x79	y
26	032	0x1A	SUB	58	072	0x3A	:	90	132	0x5A	Z	122	172	0x7A	z
27	033	0x1B	ESC	59	073	0x3B	;	91	133	0x5B	[	123	173	0x7B	{
28	034	0x1C	FS	60	074	0x3C	<	92	134	0x5C	\	124	174	0x7C	\|
29	035	0x1D	GS	61	075	0x3D	=	93	135	0x5D	]	125	175	0x7D	}
30	036	0x1E	RS	62	076	0x3E	>	94	136	0x5E	^	126	176	0x7E	~
31	037	0x1F	US	63	077	0x3F	?	95	137	0x5F	_	127	177	0x7F	DEL

Practical Use Cases

Finding Non-ASCII or Control Characters

Identify lines containing bytes outside the standard ASCII range:

grep -P -n "[\x80-\xFF]" filename

View control characters in context:

od -c filename | head -20

Check for embedded null bytes (often problematic in text files):

grep -c $'\x00' filename

Byte-Level File Analysis

Examine raw bytes with ASCII equivalents side-by-side:

xxd -g 1 filename | head -20

This format shows hex values and printable ASCII characters, making it straightforward to spot text embedded in binary data.

Add the -l flag to limit output by byte count:

xxd -g 1 -l 512 filename

Extracting Text from Binary Files

Extract all printable ASCII strings from a binary file:

strings filename

Filter by minimum string length:

strings -n 8 filename    # Only strings 8+ characters long

Combine with grep to find specific patterns:

strings binary_file | grep -i "version"

Text Processing and Line Endings

Check file line ending types:

file filename  # Shows line ending type (CRLF, LF, etc.)

Find lines with non-standard line endings (mixed LF/CR):

od -c filename | grep -E "(\\\\n|\\\\r)"

Convert DOS line endings (CR+LF) to Unix (LF):

dos2unix filename

Or using sed (works on systems without dos2unix):

sed -i 's/\r$//' filename

Convert Unix to DOS line endings:

sed -i 's/$/\r/' filename

Log Analysis

Check for control characters that might corrupt log parsing:

cat logfile | od -c | grep -E '\\[0-7][0-7][0-7]'

Display any character outside printable ASCII in escape notation:

cat logfile | sed 's/[^[:print:]\n]/./g'

Generate a summary of non-ASCII characters found:

od -An -tx1 logfile | tr -s ' ' '\n' | sort | uniq -c | grep -v '^[[:space:]]*[0-9]\+ 2[0-7]' | grep -v '^[[:space:]]*[0-9]\+ [0-4][0-9a-f]'

Working with Special Characters in Scripts

Safely handle strings containing ASCII codes:

# Store a tab character
tab=$'\t'
# Store a newline
newline=$'\n'
# Use in commands
awk -v tab="$tab" '{print $1 tab $2}' input.txt

Generate test data with specific ASCII codes:


# Create a file with null-terminated strings
printf 'string1\x00string

ASCII Table, Character Codes, and Linux Text Tools