UNIX / Bash cheat/ LSF

unix_bash.md

0. Shortcuts.

CTRL+A # move to beginning of line CTRL+B # moves backward one character CTRL+C # halts the current command CTRL+D # deletes one character backward or logs out of current session, similar to exit CTRL+E # moves to end of line CTRL+F # moves forward one character CTRL+G # aborts the current editing command and ring the terminal bell CTRL+J # same as RETURN CTRL+K # deletes (kill) forward to end of line CTRL+L # clears screen and redisplay the line CTRL+M # same as RETURN CTRL+N # next line in command history CTRL+O # same as RETURN, then displays next line in history file CTRL+P # previous line in command history CTRL+R # searches backward CTRL+S # searches forward CTRL+T # transposes two characters CTRL+U # kills backward from point to the beginning of line CTRL+V # makes the next character typed verbatim CTRL+W # kills the word behind the cursor CTRL+X # lists the possible filename completefions of the current word CTRL+Y # retrieves (yank) last item killed CTRL+Z # stops the current command, resume with fg in the foreground or bg in the background

DELETE # deletes one character backward !! # repeats the last command exit # logs out of current session

LSF

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1. Bash Basics.

export # displays all environment variables

echo $SHELL # displays the shell you're using echo $BASH_VERSION # displays bash version

bash # if you want to use bash (type exit to go back to your normal shell) whereis bash # finds out where bash is on your system

clear # clears content on window (hide displayed lines)

1.1. File Commands.

ls # lists your files ls -l # lists your files in 'long format', which contains the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified ls -a # lists all files, including hidden files ln -s # creates symbolic link to file touch # creates or updates your file cat > # places standard input into file more # shows the first part of a file (move with space and type q to quit) head # outputs the first 10 lines of file tail # outputs the last 10 lines of file (useful with -f option) emacs # lets you create and edit a file mv # moves a file cp # copies a file rm # removes a file diff # compares files, and shows where they differ wc # tells you how many lines, words and characters there are in a file chmod -options # lets you change the read, write, and execute permissions on your files gzip # compresses files gunzip # uncompresses files compressed by gzip gzcat # lets you look at gzipped file without actually having to gunzip it lpr # print the file lpq # check out the printer queue lprm # remove something from the printer queue genscript # converts plain text files into postscript for printing and gives you some options for formatting dvips # print .dvi files (i.e. files produced by LaTeX) grep # looks for the string in the files grep -r

# search recursively for pattern in directory

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