1.2. Directory Commands.

mkdir # makes a new directory cd # changes to home cd # changes directory pwd # tells you where you currently are

1.3. SSH, System Info & Network Commands.

ssh user@host # connects to host as user ssh -p user@host # connects to host on specified port as user ssh-copy-id user@host # adds your ssh key to host for user to enable a keyed or passwordless login

whoami # returns your username passwd # lets you change your password quota -v # shows what your disk quota is date # shows the current date and time cal # shows the month's calendar uptime # shows current uptime w # displays whois online finger # displays information about user uname -a # shows kernel information man # shows the manual for specified command df # shows disk usage du # shows the disk usage of the files and directories in filename (du -s give only a total) last # lists your last logins ps -u yourusername # lists your processes kill # kills (ends) the processes with the ID you gave killall # kill all processes with the name top # displays your currently active processes bg # lists stopped or background jobs ; resume a stopped job in the background fg # brings the most recent job in the foreground fg # brings job to the foreground

ping # pings host and outputs results whois # gets whois information for domain dig # gets DNS information for domain dig -x # reverses lookup host wget # downloads file

2. Basic Shell Programming.

2.1. Variables.

varname=value # defines a variable varname=value command # defines a variable to be in the environment of a particular subprocess echo varname # checks a variable's value echo $$ # prints process ID of the current shell echo ! # prints process ID of the most recently invoked background job echo $? # displays the exit status of the last command export VARNAME=value # defines an environment variable (will be available in subprocesses)

array[0] = val # several ways to define an array array[1] = val array[2] = val array=([2]=val [0]=val [1]=val) array(val val val)

{array[i]} # displays array's value for this index. If no index is supplied, array element 0 is assumed {#array[i]} # to find out the length of any element in the array ${#array[@]} # to find out how many values there are in the array

declare -a # the variables are treaded as arrays declare -f # uses funtion names only declare -F # displays function names without definitions declare -i # the variables are treaded as integers declare -r # makes the variables read-only declare -x # marks the variables for export via the environment

{varname:-word} # if varname exists and isn't null, return its value; otherwise return word {varname:=word} # if varname exists and isn't null, return its value; otherwise set it word and then return its value {varname:?message} # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script {varname:+word} # if varname exists and isn't null, return word; otherwise return null {varname:offset:length} # performs substring expansion. It returns the substring of varname starting at offset and up to length characters

{variable#pattern} # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest {variable##pattern} # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest {variable%pattern} # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest {variable%%pattern} # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest {variable/pattern/string} # the longest match to pattern in variable is replaced by string. Only the first match is replaced {variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced

${#varname} # returns the length of the value of the variable as a character string

*(patternlist) # matches zero or more occurences of the given patterns +(patternlist) # matches one or more occurences of the given patterns ?(patternlist) # matches zero or one occurence of the given patterns @(patternlist) # matches exactly one of the given patterns !(patternlist) # matches anything except one of the given patterns

$(UNIX command) # command substitution: runs the command and returns standard output

2.2. Functions.

The function refers to passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.

@ is equal to "\1" "$2"... "$N", where N is the number of positional parameters.$# holds the number of positional parameters.

functname() { shell commands }

unset -f functname # deletes a function definition declare -f # displays all defined functions in your login session

2.3. Flow Control.

statement1 && statement2 # and operator statement1 || statement2 # or operator

-a # and operator inside a test conditional expression -o # or operator inside a test conditional expression

str1=str2 # str1 matches str2 str1!=str2 # str1 does not match str2 str1<str2 # str1 is less than str2 str1>str2 # str1 is greater than str2 -n str1 # str1 is not null (has length greater than 0) -z str1 # str1 is null (has length 0)

-a file # file exists -d file # file exists and is a directory -e file # file exists; same -a -f file # file exists and is a regular file (i.e., not a directory or other special type of file) -r file # you have read permission -r file # file exists and is not empty -w file # your have write permission -x file # you have execute permission on file, or directory search permission if it is a directory -N file # file was modified since it was last read -O file # you own file -G file # file's group ID matches yours (or one of yours, if you are in multiple groups) file1 -nt file2 # file1 is newer than file2 file1 -ot file2 # file1 is older than file2

-lt # less than -le # less than or equal -eq # equal -ge # greater than or equal -gt # greater than -ne # not equal

if condition then statements [elif condition then statements...] [else statements] fi

for x := 1 to 10 do begin statements end

for name [in list] do statements that can use $name done

for (( initialisation ; ending condition ; update )) do statements... done

case expression in pattern1 ) statements ;; pattern2 ) statements ;; ... esac

select name [in list] do statements that can use $name done

while condition; do statements done

until condition; do statements done

3. Command-Line Processing Cycle.

The default order for command lookup is functions, followed by built-ins, with scripts and executables last.

There are three built-ins that you can use to override this order: command, builtin and enable.

command # removes alias and function lookup. Only built-ins and commands found in the search path are executed builtin # looks up only built-in commands, ignoring functions and commands found in PATH enable # enables and disables shell built-ins

eval # takes arguments and run them through the command-line processing steps all over again

4. Input/Output Redirectors.

cmd1|cmd2 # pipe; takes standard output of cmd1 as standard input to cmd2

file # directs standard output to file < file # takes standard input from file

file # directs standard output to file; append to file if it already exists |file # forces standard output to file even if noclobber is set n>|file # forces output to file from file descriptor n even if noclobber is set <> file # uses file as both standard input and standard output n<>file # uses file as both input and output for file descriptor n <<label # here-document n>file # directs file descriptor n to file n<file # takes file descriptor n from file n>>file # directs file description n to file; append to file if it already exists n>& # duplicates standard output to file descriptor n n<& # duplicates standard input from file descriptor n n>&m # file descriptor n is made to be a copy of the output file descriptor n<&m # file descriptor n is made to be a copy of the input file descriptor &>file # directs standard output and standard error to file <&- # closes the standard input &- # closes the standard output n>&- # closes the ouput from file descriptor n n<&- # closes the input from file descripor n

5. Process Handling.

To suspend a job, type CTRL+Z while it is running. You can also suspend a job with CTRL+Y.

This is slightly different from CTRL+Z in that the process is only stopped when it attempts to read input from terminal.

Of course, to interupt a job, type CTRL+C.

myCommand & # runs job in the background and prompts back the shell

jobs # lists all jobs (use with -l to see associated PID)

fg # brings a background job into the foreground fg %+ # brings most recently invoked background job fg %- # brings second most recently invoked background job fg %N # brings job number N fg %string # brings job whose command begins with string fg %?string # brings job whose command contains string

kill -l # returns a list of all signals on the system, by name and number kill PID # terminates process with specified PID

ps # prints a line of information about the current running login shell and any processes running under it ps -a # selects all processes with a tty except session leaders

trap cmd sig1 sig2 # executes a command when a signal is received by the script trap "" sig1 sig2 # ignores that signals trap - sig1 sig2 # resets the action taken when the signal is received to the default

disown <PID|JID> # removes the process from the list of jobs

wait # waits until all background jobs have finished

6. Tips and Tricks.

set an alias

cd; nano .bash_profile

alias gentlenode='ssh admin@gentlenode.com -p 3404' # add your alias in .bash_profile

to quickly go to a specific directory

cd; nano .bashrc

shopt -s cdable_vars export websites="/Users/mac/Documents/websites"

source .bashrc cd websites

7. Debugging Shell Programs.

bash -n scriptname # don't run commands; check for syntax errors only set -o noexec # alternative (set option in script)

bash -v scriptname # echo commands before running them set -o verbose # alternative (set option in script)

bash -x scriptname # echo commands after command-line processing set -o xtrace # alternative (set option in script)

trap 'echo $varname' EXIT # useful when you want to print out the values of variables at the point that your script exits

function errtrap { es=? echo "ERROR line \1: Command exited with status $es." }

trap 'errtrap $LINENO' ERR # is run whenever a command in the surrounding script or function exists with non-zero status

function dbgtrap { echo "badvar is $badvar" }

trap dbgtrap DEBUG # causes the trap code to be executed before every statement in a function or script

...section of code in which the problem occurs...

trap - DEBUG # turn off the DEBUG trap

function returntrap { echo "A return occured" }

trap returntrap RETURN # is executed each time a shell function or a script executed with the . or source commands finishes executing