Нужно найти программу или утилиту для выполнения конкретной задачи (скажем, для подсчета слов в файле), но вы не знаете, где искать.

Следует провести поиск по ключевым словам командой apropos или man -k.

Выбор команды неважен.

Например, для поиска команды подсчета слов в файле вводим следующую строку:

$ apropos count words
git-count-objects    (1)  - Count unpacked number of objects and their disk consumption
gnutls_pkcs12_bag_get_count (3)  - This function returns the bag`s elements count
gnutls_pkcs7_get_crl_count (3)  - This function returns the number of crls in a PKCS7 crl set
gnutls_pkcs7_get_crt_count (3)  - This function returns the number of certificates in a PKCS7 certificate set
gnutls_x509_crl_get_crt_count (3)  - This function returns the number of revoked certificates in a CRL
ieee1284_ref         (3)  - modify a port`s reference count
ieee1284_unref [ieee1284_ref] (3)  - modify a port`s reference count
ldap_count_entries [ldap_first_entry] (3)  - LDAP result entry parsing and counting routines
ldap_count_messages [ldap_first_message] (3)  - Stepping through messages in a result chain
ldap_count_references [ldap_first_reference] (3)  - Stepping through continuation references in a result chain
ldap_count_values [ldap_get_values] (3)  - LDAP attribute value handling routines
ldap_first_entry     (3)  - LDAP result entry parsing and counting routines
ldap_next_entry [ldap_first_entry] (3)  - LDAP result entry parsing and counting routines
llength              (n)  - Count the number of elements in a list
messages             (1)  - quick count of messages in mailbox or folder
ndb_select_count     (1)  - print row counts for NDB tables
nnacct               (1m)  - news accounting and access authorization (nn)
sa                   (8)  - summarizes accounting information
snmpdelta            (1)  - Monitor delta differences in SNMP Counter values
sum                  (1)  - checksum and count the blocks in a file
userdel              (8)  - Delete a user account and related files
usermod              (8)  - Modify a user account
wc                   (1)  - print newline, word, and byte counts for each file
Tcl_GetIndexFromObj  (3)  - lookup string in table of keywords
Tcl_GetIndexFromObjStruct [Tcl_GetIndexFromObj] (3)  - lookup string in table of keywords
Text::ParseWords     (3)  - parse text into an array of tokens or array of arrays
bn_add_words [bn_internal] (3)  - BIGNUM library internal functions
bn_cmp_words [bn_internal] (3)  - BIGNUM library internal functions
bn_div_words [bn_internal] (3)  - BIGNUM library internal functions
bn_mul_add_words [bn_internal] (3)  - BIGNUM library internal functions
bn_mul_words [bn_internal] (3)  - BIGNUM library internal functions
bn_sqr_words [bn_internal] (3)  - BIGNUM library internal functions
bn_sub_words [bn_internal] (3)  - BIGNUM library internal functions
des_read_2passwords [ui_compat] (3)  - Compatibility user interface functions
fontglide            (6)  - characters float onto the screen to form words
getw                 (3)  - input and output of words (ints)
grpconv [pwconv]     (8)  - convert to and from shadow passwords and groups
grpunconv [pwconv]   (8)  - convert to and from shadow passwords and groups
lppasswd             (1)  - add, change, or delete digest passwords
passwd.nntp [passwd] (5)  - passwords for connecting to remote NNTP servers
putw [getw]          (3)  - input and output of words (ints)
pwconv               (8)  - convert to and from shadow passwords and groups
pwunconv [pwconv]    (8)  - convert to and from shadow passwords and groups
sasl_getsecret_t     (3)  - The SASL callback for secrets (passwords)
shadowconfig         (8)  - toggle shadow passwords on and off
whatis               (1)  - search the whatis database for complete words

Из полученного обширного списка выбираем команду wc.

Упомянутые команды выполняют поиск по ключевым словам в секциях DESCRIPTION man-страниц. Количество искомых слов может быть произвольным, но чем больше слов, тем больше результатов можно получить, потому что команда поочередно ищет каждое слово.