genpw

Generate random passwords or strings with configurable length, charset, and character exclusions.

Especially useful for sites with restrictive password requirements -- "must be exactly 12 characters, lowercase and numbers only, no special characters" becomes genpw -c '[:lower:][:digit:]' 12.

Quick start

$ genpw
eOC3OCKtXbLGQ=}q?GC)p&8MqhO&e|2/

Default: 32 characters from alphanumeric + punctuation (equivalent to [:alnum:][:punct:]).

Common examples

Generate a 12-character password:

$ genpw 12
il6ZtXx2%/NC

Numeric PIN (8 digits):

$ genpw -c '[:digit:]' 8
50743836

Lowercase letters and numbers only (common restriction):

$ genpw -c '[:lower:][:digit:]' -l 10
j27n9o641w

Exclude ambiguous characters (0/O, 1/l, I) for readability:

$ genpw -e '0O1lI' 16
v'k8p=39('~{cEfL

Exclude special characters that break certain parsers:

$ genpw -l 16 -e '!@#$%^&*()'
lW?mj";.xO]ZPA5w

Uppercase only (rare but occasionally required):

$ genpw -c '[:upper:]' 6
FZSAQS

Charsets

Charsets are composed of literal characters, ranges, POSIX classes, and/or bracket expressions.

POSIX classes

Expression	Expands to
`[:digit:]`	`0123456789`
`[:upper:]`	`ABCDEFGHIJKLMNOPQRSTUVWXYZ`
`[:lower:]`	`abcdefghijklmnopqrstuvwxyz`
`[:alpha:]`	`[:upper:][:lower:]`
`[:alnum:]`	`[:alpha:][:digit:]`
`[:punct:]`	`` !"#$%&'()*+,-./:;<=>?@[]^_`{

Mix and match: [:lower:][:digit:]_- gives you lowercase, digits, underscore, and hyphen.

Character ranges

Hyphenated ranges like a-z or 0-9 expand in place at the top level or inside bracket expressions:

genpw -c 'a-z0-9' 10         # lowercase + digits
genpw -c 'A-Fa-f0-9' 16      # hex characters

Bracket expressions

Group characters, ranges, and POSIX classes together with [...]:

genpw -c '[0-9ab]' 12                # digits plus a and b
genpw -c '[[:lower:][:digit:]]' 12   # same as [:lower:][:digit:]
genpw -c '[A-Za-z0-9_]' 12           # identifier-safe chars

Unclosed brackets produce a clear error (exit 5).

Default charset: [:alnum:][:punct:] (all printable ASCII except space).

Excluding characters

The -e / --exclude flag removes characters from the final charset. Useful for sites that disallow certain symbols or to avoid ambiguous characters:

# Avoid shell-sensitive characters
genpw -e '\$`"' 20

# Classic ambiguous set
genpw -e '0O1lI' 16

Exclusion applies after charset expansion, so genpw -c '[:alnum:]' -e '0O1lI' works as expected.

Reference

All options

Flag	Description
`-l, --length LENGTH`	Generate LENGTH characters (default: 32)
`-c, --charset CHARSET`	Define charset (default: `[:alnum:][:punct:]`)
`-e, --exclude CHARSET`	Exclude characters from the charset (default: none)
`-h, --help`	Display help

Positional argument: If a single number is provided without flags, it's treated as the length (e.g. genpw 16).

Exit codes

Code	Meaning
0	Success
2	Usage error (unknown flag, missing value)
4	Charset is empty after exclusions
5	Invalid charset (unclosed bracket, unrecognized POSIX class)

Dependencies

/dev/random -- entropy source