====== Comando cat ======
O comando cat concatenar arquivos e lista o resultado na saída padrão, ou seja, mostra na tela o conteúdo de determinado arquivo ou da entrada padrão e ainda pode receber um redirecionamento. Apenas arquivos de texto puro podem ser lidos pelo comando //cat//.
Para listar o conteúdo do arquivo /etc/passwd com e sem numeração de linha:
<code bash>
# cat -n /etc/passwd
     1	root:x:0:0:root:/root:/bin/bash
     2	daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
     3	bin:x:2:2:bin:/bin:/usr/sbin/nologin
     4	sys:x:3:3:sys:/dev:/usr/sbin/nologin
     5	sync:x:4:65534:sync:/bin:/bin/sync
     6	games:x:5:60:games:/usr/games:/usr/sbin/nologin
     7	man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
     8	lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
     9	mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
    10	news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
    11	uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
    12	proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
    13	www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
    14	backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
    15	list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
    16	irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
    17	gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
    18	nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
    19	systemd-timesync:x:100:103:systemd Time Synchronization,,,:/run/systemd:/bin/false
    20	systemd-network:x:101:104:systemd Network Management,,,:/run/systemd/netif:/bin/false
    21	systemd-resolve:x:102:105:systemd Resolver,,,:/run/systemd/resolve:/bin/false
    22	systemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
    23	messagebus:x:104:109::/var/run/dbus:/bin/false
    24	avahi-autoipd:x:105:111:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/bin/false
    25	sshd:x:106:65534::/var/run/sshd:/usr/sbin/nologin
    26	gean:x:1000:1000:Gean Martins,,,:/home/gean:/bin/bash
    27	didiwiki:x:107:113:DidiWiki,,,:/var/lib/didiwiki:/bin/false
    28	mysql:x:108:115:MySQL Server,,,:/nonexistent:/bin/false
</code>

<code bash>
# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-timesync:x:100:103:systemd Time Synchronization,,,:/run/systemd:/bin/false
systemd-network:x:101:104:systemd Network Management,,,:/run/systemd/netif:/bin/false
systemd-resolve:x:102:105:systemd Resolver,,,:/run/systemd/resolve:/bin/false
systemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
messagebus:x:104:109::/var/run/dbus:/bin/false
avahi-autoipd:x:105:111:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/bin/false
sshd:x:106:65534::/var/run/sshd:/usr/sbin/nologin
gean:x:1000:1000:Gean Martins,,,:/home/gean:/bin/bash
didiwiki:x:107:113:DidiWiki,,,:/var/lib/didiwiki:/bin/false
mysql:x:108:115:MySQL Server,,,:/nonexistent:/bin/false
</code>

Para criar um arquivo ordenados.txt e listar por ordem acrescente somente os usuários que usam o bash:
<code bash>
# (cat /etc/passwd | grep '/bin/bash' | sort > ordenados.txt) | cat ordenados.txt
gean:x:1000:1000:Gean Martins,,,:/home/gean:/bin/bash
root:x:0:0:root:/root:/bin/bash
</code>

Para verificar se há espaço no final de cada linha:
<code bash>
# cat teste.txt 
exemplo 
de 
código 
oculto 
no 
final 
de 
cada 
linha
</code>

<code bash>
# cat -TE teste.txt 
exemplo $
de $
código $
oculto $
no $
final $
de $
cada $
linha$
</code>

Referência:
  - [[http://www.computerhope.com/unix/ucat.htm]]