Shell Scripting

Ref: LinuxCommand.org
Must write #!/bin/bash at the top of shell file
<<- instead of just << will make command line to exclude leading tabs but not spaces
while defining a variable, space around = is not allowed.
functions must be defined before use
you cannot leave a function body empty

#!/bin/bash

### Variables
var1="variable 1 value"
user_name_print="User name is $USER"


### Functions
func_name(){
    echo "function tasks"
    ls -la # some command execution
}

### Control Flow
if commands; then
    commands
elif commands; then
    commands
else
    commands
fi

# Multi line echo
cat <<- _EOF_  
    line 1 within cat
    line 2 within cat
    use of variable 1: $var1
    $user_name_print
    $(func_name) # function call
_EOF_

test [expression]

File Operations

Expression	Description
`-a FILE`	True if file exists.
`-b FILE`	True if file is block special.
`-c FILE`	True if file is character special.
`-d FILE`	True if file is a directory.
`-e FILE`	True if file exists.
`-f FILE`	True if file exists and is a regular file.
`-g FILE`	True if file is set-group-id.
`-h FILE`	True if file is a symbolic link.
`-L FILE`	True if file is a symbolic link.
`-k FILE`	True if file has its `sticky' bit set.
`-p FILE`	True if file is a named pipe.
`-r FILE`	True if file is readable by you.
`-s FILE`	True if file exists and is not empty.
`-S FILE`	True if file is a socket.
`-t FD`	True if FD is opened on a terminal.
`-u FILE`	True if the file is set-user-id.
`-w FILE`	True if the file is writable by you.
`-x FILE`	True if the file is executable by you.
`-O FILE`	True if the file is effectively owned by you.
`-G FILE`	True if the file is effectively owned by your group.
`-N FILE`	True if the file has been modified since it was last read.
_______________	__________________________________________________________
`FILE1 -nt FILE2`	True if file1 is newer than file2 (according to modification date).
`FILE1 -ot FILE2`	True if file1 is older than file2.
`FILE1 -ef FILE2`	True if file1 is a hard link to file2.

All file operators except -h and -L are acting on the target of a symbolic link, not on the symlink itself, if FILE is a symbolic link.

String operators

Expression	Description
`-z STRING`	True if string is empty.
`-n STRING`	True if string is not empty.
`STRING1 = STRING2`	True if the strings are equal.
`STRING1 != STRING2`	True if the strings are not equal.
`STRING1 < STRING2`	True if STRING1 sorts before STRING2 lexicographically.
`STRING1 > STRING2`	True if STRING1 sorts after STRING2 lexicographically.

Other operators

Expression	Description
`-o OPTION`	True if the shell option OPTION is enabled.
`-v VAR`	True if the shell variable VAR is set.
`-R VAR`	True if the shell variable VAR is set and is a name reference.
`! EXPR`	True if expr is false.
`EXPR1 -a EXPR2`	True if both expr1 AND expr2 are true.
`EXPR1 -o EXPR2`	True if either expr1 OR expr2 is true.
`arg1 OP arg2`	Arithmetic tests. OP is one of -eq, -ne, -lt, -le, -gt, or -ge.