When "cat > xx.txt << EOF" is entered on the command line, further input from cmdline goes to file xx.txt until EOF is written. EOF is not a sacred word here, if instead the command was cat > xx.txt << BBB, then cmdline input goes to xx.txt until BBB is written. I don't know whats the rationale behind ( << end_of_input_sequence)this. Cat man page does not explain much.
I have only seen this in scripts etc.
It's a feature of the shell, not cat - that's why you won't find it in the cat manual.
It's known as a "Here document" - see this page of the Advanced Bash-Scripting Guide for some documentation.
This is called a here document. I believe it first appeared in shells, but some programming languages such as Perl, Ruby, and PHP also implement this style.
This syntax is called Here Document (scroll a bit to find it).
It's not specific to any command, not cat anymore than any other command ; and it can be find in the man of the shell ; for instance, man bash :
3.6.6 Here Documents
This type of redirection instructs the
shell to read input from the current
source until a line containing only
word (with no trailing blanks) is
seen. All of the lines read up to that
point are then used as the standard
input for a command.
(Not a full quote -- there is more to read in the man)
BTW, It's a syntax that has been re-used in some programming languages, like PHP ;-)
Related
I tried gedit, nano, vi, leafpad and other text editors , it won't open, I tried cat and other file looking commands, and I ensure you it's a file not a directory!
This type of approach has a lot of misunderstanding because using - as an argument refers to STDIN/STDOUT i.e dev/stdin or dev/stdout .So if you want to open this type of file you have to specify the full location of the file such as ./- .For eg. , if you want to see what is in that file use cat ./-
Both cat < - and ./- command will give you the output
you can use redirection
cat < -file_name
It looks like the rev command doesn't treat - as a special character.
From the man page
The rev utility copies the specified files to standard output, reversing the order of characters in every line.
so
rev - | rev
should show what's in the file in the correct order.
I tried with pico or vi command.pico readme which allowed me open in editor and read the contents.
if you want to open this type of file you have to specify the full location of the file such as ./- .For eg. , if you want to see what is in that file use cat ./-
cat ./- is the syntax that reveals the correct password for bandit the "rev -" reveals something else
please explain the line below used in shell scripts,
awk -F\| -v src=$storekey 'src==$41' $SRC_Path >> $DST_Path
Thanks!
Ok first ${variable} is a shell variable, so those would be defined higher in your script i.e.
storekey = "1234" or something
you can try this on your shell (linux or command line terminal)
type:
$ storekey="foo"
$ echo $storekey
So most of your question is pertaining to the variables and the command line which confuses how they are used, if you replaced the variables on a command line to test, you could work test it out to find out what they are doing.
In essence Awk is a stream parsing tool, so if you had a file of say 10 columns with a known delimiter such as "," or "|" you could ask awk for a specific column to be printed or output. This is what is happening below, but it is being confused by the presence of custom shell variables.
then to break down the command line awk is parsing a "|" delimited input (-F\| ) defined by $storekey variable, taking the column where src== $41 (this has some reference to the data being input), from $SRC_PATH (a directory) to $DST_PATH (another directory or path).
If you could share more of the shell script I could provide a more in depth answer.
btw, you could also find out more information, using the commands
man awk
info awk
from your command line, however these are a bit arcane for those not so familiar with *nix variants.
I'm currently working on a Unix box and came across this post which I found helpful, in order to learn about cat command in Unix. At the bottom of the page found this line saying: -A = Equivalent to -vET
As I'm new into Unix, I'm unaware of what does this mean actually? For example lets say I've created a file called new using cat and then apply this command to the file:
cat -A new, I tried this command but an error message comes up saying it's and illegal option.
To cut short, wanted to know what does cat -A really mean and how does it effect when I apply it to a file. Any help would be appreciated.
It means show ALL.
Basically its a combination of -vET
E : It will display '$' at the end of every line.
T : It will display tab character as ^I
v : It will use ^ and M-notation
^ and M-notation:
(Display control characters except for LFD(LineFeed or NewLine) and TAB using '^' notation and precede characters that have the high bit set with
'M-') M- notation is a way to display high-bit characters as low bit ones by preceding them with M-
You should read about little-endian and big-endian if you like to know more about M notation.
For example:
!http://i.imgur.com/0DGET5k.png?1
Check your manual page as below and it will list all options avaialable with your command and check is there -A present, if it is not present it is an illegal option.
man cat
It displays non-printing characters
In Mac OS you need to use -e flag and
-e Display non-printing characters (see the -v option), and display a dollar sign (`$') at the end of each line.
I have a trivial error that I cant seem to get around. Im trying to return the various section numbers of lets say "man" since it resides in all the sections. I am using the -s command but am having problems. Every time I use it I keep getting "what manual page do you want". Any help?
In the case of getting the section number of a command, you want something like man -k "page_name" | awk -F'-' "/^page_name \(/ {print $1}", replacing any occurrence of page_name with whatever command you're needing.
This won't work for all systems necessarily as the format for the "man" output is "implementation-defined". In other words, the format on FreeBSD, OS X, various flavours of Linux, etc. may not be the same. For example, mine is:
page_name (1) - description
If you want the section number only, I'm sure there is something you can do such as saving the result of that line in a shell variable and use parameter expansion to remove the parentheses around the section number:
man -k "page_name" | awk -F'-' "/^page_name \(/ {print $1}" | while IFS= read sect ; do
sect="${sect##*[(]}"
sect="${sect%[)]*}"
printf '%s\n' "$sect"
done
To get the number of sections a command appears in, add | wc -l at the end on the same line as the done keyword. For the mount command, I have 3:
2
2freebsd
8
You've misinterpreted the nature of -s. From man man:
-S list, -s list, --sections=list
List is a colon- or comma-separated list of `order specific' manual sections to search. This option overrides the
$MANSECT environment variable. (The -s
spelling is for compatibility with System V.)
So when man sees man -s man it thinks you want to look for a page in section "man" (which most likely doesn't exist, since it is not a normal section), but you didn't say what page, so it asks:
What manual page do you want?
BTW, wrt "man is just the test case cuz i believe its in all the sections" -- nope, it is probably only in one, and AFAIK there isn't any word with a page in all sections. More than 2 or 3 would be very unusual.
The various standard sections are described in man man too.
The correct syntax requires an argument. Typically you're looking for either
man -s 1 man
to read the documentation for the man(1) command, or
man -s 7 man
to read about the man(7) macro package.
If you want a list of standard sections, the former contains that. You may have additional sections installed locally, though. A directory listing of /usr/local/share/man might reveal additional sections, for example.
(Incidentally, -s is not a "command" in this context, it's an option.)
I want to create a bash script that is simular to a programming interpreter like mongo, node, redis-cli, mysql, etc.
I want to be able to use a command like test and it behave like the examples above.
thomas#workstation:~$ test
>
How do I make a command that behaves like this? What is this called?
I want to be able to take the content and turn it into a variable.
thomas#workstation:~$ test
> hello world
hello world
thomas#workstation:~$
I only want to take one "entry" after enter is pressed once I want to be able to process the string "hello world" in the code, like echo it.
What is this called? How do I make one using BASH?
I think "read" is what you are looking for, isn't it?
here is a link with some examples: http://bash.cyberciti.biz/guide/Getting_User_Input_Via_Keyboard
so you can do stuff like this:
read -p "Enter your name : " name
echo "Hi, $name. Let us be friends!"
I'm sorry this doesn't answer you directly, but it might be worth it to look into using a more fully capable programming language such as Python, Ruby, or Perl for a task like this. In Python you can use the raw_input() function.
user_command = raw_input('> ')
would yield your prompt.
First, do not name your script test. That generates too much confusion. Whatever you call it, you can do many things:
#!/bin/sh
printf '> '
read line
echo "$line"
If your shell supports it:
#!/bin/sh
read -p '> ' line
echo "$line"
or
#!/bin/sh
printf '> '
sed 1q # This will print the input. To store in in a variable: a=$( sed 1q )
[spatel#tux ~]$ read a
Hello World!!!!!
[spatel#tux ~]$ echo $a
Hello World!!!!!
Key word that might be useful here is REPL (Read–eval–print loop) used primarily for programming languages or coding environments. Your browsers console is a great example of a REPL.
Node allows you use their REPL to build interactive apps.