Chapter 3 Cheat Sheet

Shells

• echo $SHELL
• Displays your current shell.
• cat /etc/shells
• Displays all available shells.
• chsh
• Changes your shell.
• chsh -s <shell> <user>
• As root, changes specified user's shell.
• Can also use usermod -s <shell> <user>

I/O Redirection

• > 
• redirect stdout (same as 1>).
• 2> 
• redirect stderr.
• &> 
• redirect both stdout and stderr (same as 2>&1).
• &> does not work when piping, must use 2>&1.
• >> 
• append to file; > overwrites contents.
• /dev/null 
• the garbage disposal.
•  < 
• redirect input from file.
• |
• Pipe output from command to another command.

File and Directory Concepts

• pwd
• Prints the current working directory. 
• cd ~/folder
• Changes directory to folder in your home directory.
• cd ~<user>
• Changes to user's home directory.
• cd -
• Changes to previous working directory.

File Lists and ls

• ls -l
• Long listing of the contents in a directory.
• ls -ld /directory
• Display long listing information about directory, not contents.
• ls -lh
• Display file sizes in human readable format.
• ls -lrt
• Reverse sort files by time modified (newest at bottom)
• ls -a
• Show hidden files and directories.
• ls -i
• Shows the inode number.
• ls -F
• Display a / after each directory, * after each executable file, and a @ after each symbolic link.

File Creation Commands

• touch file
• Creates the file. If file exists, updates last access date.
• cp file1 file2
• Makes a copy of file1 called file2.
• mv file1 file2 or mv directory1 directory2
• Renames file1 to file2.
• ln sourcefile linkfile
• Creates a hard link from sourcefile to linkfile.
• ln -s sourcefile linkfile
• Creates a soft link from sourcefile to linkfile.

Directory Creation and Deletion

• mkdir test
• mkdir -p /test1/test2/test3
• rmdir test
• deletes directory only if empty.
• rmdir -p /test1/test2/test3
• rm -rf test1
• Deletes directory and all subdirectories.

Alias

• Typing alias alone will show aliases.
• alias rm='mv -t ~/.Trash'
• Now when you type rm <file>, the file is move to trash rather than deleted.

Wildcards

• *
• matches 0 or more characters.
• ls ab* will return abcdefg, abc, abd, and ab.
• ?
• Matches one character.
• ls ab? will return abc and abd, but not abcdefg or ab.
• []
•  Matches a range.
• ls ab[123] will match ab1, ab2, and ab3, but not ab123.
• ls ab[A-C] will match abA, abB, and abC.

File Searches

• find <where> -name <what>
• finds what in where.
• -iname for case insensitive search.
• locate <what>
• Searches the locate database for what.
• Faster than find, but since database is only updated every 24 hours, not as accurate.
• Update database by typing /etc/cron.daily/mlocate.cron

Commands to Process Text Streams

• cat
• cat /etc/passwd
• less
• less /var/log/messages
• head and tail
• head -n20 /etc/passwd
• tail -f /var/log/secure
• sort
• sort /etc/passwd
• grep and egrep
• grep <"what"> <where>
• egrep supports +, ?, |, (, and ).
• diff
• diff file1 file2
• wc
• wc file
• wc -l file
• sed
•  sed 's/Windows/Linux' file > newfile
• awk
• awk '/mike/ {print $1}' /etc/passwd

vi

• A Great Cheat Sheet for vi 

Documentation

• command --help (or command by itself).
• man <command>
• whatis nfs (show all man pages associated with nfs).
• apropos nfs (show all man pages related to nfs).
• apropos and whatis both search the whatis database, which is updated every 24 hours. To update database manually, run /etc/cron.daily/makewhatis.cron.
• Type whatis smbpasswd and notice the output. A man 5 smbpasswd reads the man page related to the file, a man 8 smbpasswd reads the man page associated with the actual command.
• /usr/share/doc
• info bash
• For a full list of info manuals: ls /usr/share/info.

Network Daemons

• service network start/stop/status
• service NetworkManager start/stop/status
• chkconfig network off/on
• chkconfig NetworkManager off/on

Network Configuration Files

• /etc/sysconfig/network
• Turn networking on/off.
• Set hostname.
• Set the gateway. The device configuration file and dhclient both override this directive.
• /etc/sysconfig/ifcfg-<device>
• Configure network adapter.
• /etc/hosts
• Map IPs to hostnames on the local network.
• /etc/nsswitch.conf
• Tells what database to look at and in what order.
• /etc/resolv.conf
• Defines which name servers to use.

The ip command

• ip addr show <device> 
• shows devices configuration (e.g., ip address)
• ip link set <device> down 
• takes the device offline.
• ip address add <IP> dev <device> 
• configures the IP address for device.
• ip addr add <IP/mask> broadcast <IP> dev <device> 
• configures IP and broadcast.
• ip addr del <IP/bits> dev <device> 
• deletes IP address for device.
• ip addr <IP/mask> dev <device> label <device>:1 
• creates an alias for device.
• ip route show
• ip route add <IP/mask> dev<device>
• ip route add default via <IP>
• ip route delete <IP/mask> dev <device>
  
• ip neigh add <IP> lladdr <MAC> nud permanent dev <device> 
• adds an entry to the ARP table for device.
• ip link set dev <device> arp off 
• turns arp off for device.

Network Configuration Tools

• system-config-network (command line)
• nm-connection-editor (GUI)