-Detail HELP (http://mywiki.wooledge.org/BashFAQ)
type -t command # this will give you what kind of command is this
type -a command # this will give you all types of the given command
$ ./ScriptName arg1 arg2 arg3
echo $0 # this will print the script name with the path (argument zero)
echo $(basename $0) # this will print only the file name
echo $1 # this will print the first argument given when executing the shell script
echo $* # this will print all the arguments given when executing the script
echo $@ # this will print all the argument given when executing the script
echo $# # this will print the number of argument that given to the script when it executed
echo $? # display Exit numerical value of the last command ran on the shell
echo $$ # print the pid of the program
myVar = "this is a variable"
echo $myVar # this will print the variable. this is the informal way to use the variable
echo ${myVar} # this is the formal way to access a variable value
echo ${varname:- default value} # if var name is not defined it will print the default value instead null
echo ${varname:= default value} # if var name is not defined it will define the varname to default value
echo ${varname:? message} # if the varname not defined, print the message and exit the script with error
echo ${varname:+ message} # if varname defined it will print the message (not the varname), else it will print a null
echo ${varname:offset:length} # only print the part of the varname, ex: $(varname:3:4)
echo ${@:2:3} # it will print the second, third args
echo ${#varname} # Number of chars in the variable
varname="home/dirname/something/dirname/Documents"
echo ${varname#pattern} # start from the beginning, delete till the fist occurrence
echo ${varname#*/dirname} # print /something/dirname/Documents
echo ${varname##pattern} # start from the beginning, delete till the last occurrence
echo ${varname##*/dirname} # print /Documents
echo ${varname%pattern} # start from the end, delete till the first occurrence
echo ${varname%%pattern} # start from the end, delete till the last occurrence
myvar="Prabath"
echo 'my name is $myvar' # this will output 'my name is $myvar'
echo "my name is $myvar" # this will output 'my name is Prabath'
myVar = 'this is a global variable'
function myfunc() {
local mylocalvar = 'this is a local variable to the function'
local myVar = 'this will overwrite the global variable within the function'
echo $1 # this will be the first function argument NOT the script argument
}
IFS=,
echo "$*" # this will print arg1,arg2,arg3
echo "$@" # this will print arg1 arg2 arg3
echo "$#" # this will print 3
IFS=$'\n'
files=$(ls -l) # assign the output of ls -l to files variable
file_contents=$(<filename.txt) # this will load the file contents to file_content variable
if condition; then
echo 'ok'
else
echo 'no'
fi
[ -n "test" ]
[ var1 == var2 ] # if this true exit status is 0, else it will 1
[ -n $var ] # check if the var is not null
if [[ -n "Hello" ]]; then
echo "Not Null"
fi
[ -z "test" ]
# [ -z $var ] check if the var is null
if [[ -z "Hello" ]]; then
echo "Not Null"
fi
[ -e $filename ] # return 0 if file exists and 1 if does not
if ! [[ -n "test"]]; then
echo "ok"
fi
[ -s $filename ]
[ -s $filename ] # filename exists and its not empty
exits with a non-zero status
set -e
-e Exit immediately if a command exits with a non-zero status
- http://mywiki.wooledge.org/BashFAQ/105
- http://stackoverflow.com/questions/19622198/what-does-set-e-mean-in-a-bash-script
kill -0 3234
- exit 0 if the process is running do not kill the process
- exit with error if the process is not running. "bash: kill: (3234) - No such process"
https://www.youtube.com/watch?v=0btsvoSt76M
Trap a command that can be executed at predefined state of the script
trap 'rm -f $FILE;' EXIT # file remove at exit of the script
# INT - interrupt
# HUP - Hangup
# QUIT - quit
# TERM - Terminate
# EXIT - exit
trap 'rm -f $FILE;' INT HUP QUIT TERM EXIT
- Enable accidental overrides to files. When this option is set, you can not write to the file and replace the contents. If you want intentionally wirte to the file you can use >| command
set -o noclobber
- Write to the protected file
echo "write forcefully" >| file.txt
- disable the override protection
set +o noclobber
https://www.cyberciti.biz/tips/howto-keep-file-safe-from-overwriting.html
http://www.bahmanm.com/blogs/command-line-options-how-to-parse-in-bash-using-getopt http://floppsie.comp.glam.ac.uk/Glamorgan/gaius/scripting/4.html
Command line input viewed in 3 categories by getopt
- Short options (like -a)
- use a single character without argument (example
-aor-A) - use with arguments
- with required arguments (example
-a arg1or-H arg2) - with optional arguments (example
-aarg1, note the sapces between the option-aand the argumentarg1)
- with required arguments (example
- use a single character without argument (example
- Long options (Options start with
--like--some-option)- without argument (
--options-aor--more-2-go) - with arguments
- with required orgumnets
--file-to-process reportsor--package-name-prefix='com.bahmanm' - without optional arguments
--package-name-prefix='com.bahmanm'(note the=)
- with required orgumnets
- without argument (
- Non-option parameters
getopt takes 2 options strings normally (one for short options and one for long options).
- Option string for short options
- each character stands for an option
- A: (colon character) tells that the option has a required argument
- A:: (two consequent colon character) tells that the option has an optional argument
Example
- f:gh::i: (short options)
- 4 options
- f - has required argument (:)
- g - has a no args
- h - has optional arg (::)
- i - has a required arg (:)
- foo,bar::,baz:,foobar (long options)
- 3 options
- foo - has no args
- bar - has optional arg (::)
- baz - has required arg (:)
- foobar - has no args
Shift http://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_09_07.html
ls -1 | cut -f1 -d\- | sort | uniq
ls | xargs -0 | cut -d'-' -f-1 | uniq
grep -rl subsestion . | xargs sed -i 's/subsestion/subsection/g'