Would love for someone to help educate me on this line of code.
using diff command to compare two files which somehow allows for other commands such as message:0:1 and message:23:17 to access it's results.
How does this work?
message=$(diff previousscan.txt scan.txt | grep 192)
#get first char which indicates if the host came up or went away
iostring="${message:0:1}"
#get first ip-number from the list
computer="${message:23:17}"
#show ip-number in notify if host came up
if [ "$iostring" = \> ]; then
notify-send "$computer online"
fi
#show ip-number in notify if host went away
if [ "$iostring" = \< ]; then
notify-send "$computer offline"
fi
$message is not a command; it is a variable which holds the output of the diff command. The later lines reference substrings; ${message:0:1} is the first character (1 character starting at offset 0) of whatever is stored in $message.
A simple example to show the substring mechanism:
$ message="abcdefghijklmnop"
$ echo ${message:0:1}
a
$ echo ${message:7:3}
hij
The construction foo=$(bar) runs the command bar in a subshell, and places the output you would normally see in your terminal if you simply ran the command bar in the variable $foo.
So I read the easiest way to use .conf files for bash scripts is to use source to load such files. Now, what if I want to edit this file ?
Some code I found does a really good job :
function set_config(){
sed -i "s/^\($1\s*=\s*\).*\$/\1$2/" $conf_file
}
But, if the variable is not yet defined, it doesn't define it, nor does it check if the parameters are passed well, isn't secure, doesn't handle default values etc...
Does reliable tools/code already exists to edit .conf file which contain key="value" pairs ? For instance, I would like to be able to do things like this :
$conf_file="my_script.conf"
conf_load $conf_file #should create the file if it doesn't exist !
read=$(conf_get_value "data" "default_value") #should read the value with key "data", defaulting to "default_value"
if [[ $? = 0 ]] #we should be able to know if the read was successful
then
echo "Successfully read value for field \"data\" : $read"
else
echo "Default value for field \"data\" : $read"
fi
conf_set "something_new" "a great value!" #should add the key "something_new" as it doesn't exist
conf_set "data" "new_value" #should edit the value with key "data"
if [[ $? = 0 ]]
then
echo "Edit successful !"
else #something went wrong :-/
echo "Edit failed !"
fi
before running this code, the conf file would contain
data="some_value"
and after it would be
data="new_value"
something_new="a great value!"
and the code should output
Successfully read value for field "data" : some_value
Edit successful !
I am using bash version 4.3.30 .
Thanks for your help.
I'd to that with awk since it's rather good at tokenizing:
# overwrite config's entries for KEY with VALUE or else appends the definition
# Usage: set_config KEY VALUE
set_config() {
[ -n "$1" ] && awk -F= -v key="$1" -v new="$1=\"$2\"" '
$1 == key { $0 = new; key_found = 1; }
{ print }
END { if (!key_found) { print new; }
' "$conf_file" > "$conf_file.new" \
&& cat "$conf_file.new" > "$conf_file" && rm "$conf_file.new"
}
If run without arguments, set_config() will do nothing and return false. If run with only one argument, it will create an empty value (outputting KEY="").
The awk command parses the .conf file line by line, looking for each definition of the given key and altering it to the new value. All lines are then printed (with or without modification), preserving the original order. If the key hasn't yet been found by the end of the file, this appends the new definition.
Because you can't pipe a file atop itself, this gets saved with a ".new" extension and then copied atop the original in a manner that preserves permissions. The ".new" copy is then removed. I used && to ensure that these never happen if an error occurred earlier in the function.
Also note that the type of ".conf file" you're referring to (the type you source with a POSIX shell) will never have spaces around its equals signs, so the \s* parts of your sed command aren't needed.
In Windows Command Line I normally write empty line in a file with
echo; >> file
However, what I have now is a variable
$param1%
If I want echo to write it in the file I have to do
echo %param1% >> file
HERE IS WHERE THE PROBLEM START :
If I'd like an empty like I'd make
set param1=;
However since the ; is not in contact with the echo word the command is
echo ; >> file
which write the ; in the file...
I need the variable to sometime contains text, and sometime nothing. How can I do it?
if "%param1%"=="" echo;>>file else echo %param1%>>file
If a param1 variable does not exist (the same as set "param1="), then %param1% results to:
In a .bat script: %param1% results to an empty string (a string of zero length);
In a CLI window: %param1% results to the %param1% string.
In a .bat script use (note no spaces surrounding %param1%)
>> file (echo;%param1%)
In a CLI window use
>>file (if not defined param1 (echo;) else echo;%param1%)
Note proper using of parentheses in if-else! For instance, check interesting result of next command:
if ""=="" echo;"THEN branch">>file else echo;"ELSE branch">>file
Output:
==>if ""=="" echo;"THEN branch">>file else echo;"ELSE branch">>file
==>type file
"THEN branch" else echo;"ELSE branch"
Consider this PS1
PS1='\n${_:+$? }$ '
Here is the result of a few commands
$ [ 2 = 2 ]
0 $ [ 2 = 3 ]
1 $
1 $
The first line shows no status as expected, and the next two lines show the
correct exit code. However on line 3 only Enter was pressed, so I would like the
status to go away, like line 1. How can I do this?
Here's a funny, very simple possibility: it uses the \# escape sequence of PS1 together with parameter expansions (and the way Bash expands its prompt).
The escape sequence \# expands to the command number of the command to be executed. This is incremented each time a command has actually been executed. Try it:
$ PS1='\# $ '
2 $ echo hello
hello
3 $ # this is a comment
3 $
3 $ echo hello
hello
4 $
Now, each time a prompt is to be displayed, Bash first expands the escape sequences found in PS1, then (provided the shell option promptvars is set, which is the default), this string is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal.
The trick is then to have an array that will have the k-th field set (to the empty string) whenever the (k-1)-th command is executed. Then, using appropriate parameter expansions, we'll be able to detect when these fields are set and to display the return code of the previous command if the field isn't set. If you want to call this array __cmdnbary, just do:
PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
Look:
$ PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
0 $ [ 2 = 3 ]
1 $
$ # it seems that it works
$ echo "it works"
it works
0 $
To qualify for the shortest answer challenge:
PS1='\n${a[\#]-$? }${a[\#]=}$ '
that's 31 characters.
Don't use this, of course, as a is a too trivial name; also, \$ might be better than $.
Seems you don't like that the initial prompt is 0 $; you can very easily modify this by initializing the array __cmdnbary appropriately: you'll put this somewhere in your configuration file:
__cmdnbary=( '' '' ) # Initialize the field 1!
PS1='\n${__cmdnbary[\#]-$? }${__cmdnbary[\#]=}\$ '
Got some time to play around this weekend. Looking at my earlier answer (not-good) and other answers I think this may be probably the smallest answer.
Place these lines at the end of your ~/.bash_profile:
PS1='$_ret$ '
trapDbg() {
local c="$BASH_COMMAND"
[[ "$c" != "pc" ]] && export _cmd="$c"
}
pc() {
local r=$?
trap "" DEBUG
[[ -n "$_cmd" ]] && _ret="$r " || _ret=""
export _ret
export _cmd=
trap 'trapDbg' DEBUG
}
export PROMPT_COMMAND=pc
trap 'trapDbg' DEBUG
Then open a new terminal and note this desired behavior on BASH prompt:
$ uname
Darwin
0 $
$
$
$ date
Sun Dec 14 05:59:03 EST 2014
0 $
$
$ [ 1 = 2 ]
1 $
$
$ ls 123
ls: cannot access 123: No such file or directory
2 $
$
Explanation:
This is based on trap 'handler' DEBUG and PROMPT_COMMAND hooks.
PS1 is using a variable _ret i.e. PS1='$_ret$ '.
trap command runs only when a command is executed but PROMPT_COMMAND is run even when an empty enter is pressed.
trap command sets a variable _cmd to the actually executed command using BASH internal var BASH_COMMAND.
PROMPT_COMMAND hook sets _ret to "$? " if _cmd is non-empty otherwise sets _ret to "". Finally it resets _cmd var to empty state.
The variable HISTCMD is updated every time a new command is executed. Unfortunately, the value is masked during the execution of PROMPT_COMMAND (I suppose for reasons related to not having history messed up with things which happen in the prompt command). The workaround I came up with is kind of messy, but it seems to work in my limited testing.
# This only works if the prompt has a prefix
# which is displayed before the status code field.
# Fortunately, in this case, there is one.
# Maybe use a no-op prefix in the worst case (!)
PS1_base=$'\n'
# Functions for PROMPT_COMMAND
PS1_update_HISTCMD () {
# If HISTCONTROL contains "ignoredups" or "ignoreboth", this breaks.
# We should not change it programmatically
# (think principle of least astonishment etc)
# but we can always gripe.
case :$HISTCONTROL: in
*:ignoredups:* | *:ignoreboth:* )
echo "PS1_update_HISTCMD(): HISTCONTROL contains 'ignoredups' or 'ignoreboth'" >&2
echo "PS1_update_HISTCMD(): Warning: Please remove this setting." >&2 ;;
esac
# PS1_HISTCMD needs to contain the old value of PS1_HISTCMD2 (a copy of HISTCMD)
PS1_HISTCMD=${PS1_HISTCMD2:-$PS1_HISTCMD}
# PS1_HISTCMD2 needs to be unset for the next prompt to trigger properly
unset PS1_HISTCMD2
}
PROMPT_COMMAND=PS1_update_HISTCMD
# Finally, the actual prompt:
PS1='${PS1_base#foo${PS1_HISTCMD2:=${HISTCMD%$PS1_HISTCMD}}}${_:+${PS1_HISTCMD2:+$? }}$ '
The logic in the prompt is roughly as follows:
${PS1_base#foo...}
This displays the prefix. The stuff in #... is useful only for its side effects. We want to do some variable manipulation without having the values of the variables display, so we hide them in a string substitution. (This will display odd and possibly spectacular things if the value of PS1_base ever happens to begin with foo followed by the current command history index.)
${PS1_HISTCMD2:=...}
This assigns a value to PS1_HISTCMD2 (if it is unset, which we have made sure it is). The substitution would nominally also expand to the new value, but we have hidden it in a ${var#subst} as explained above.
${HISTCMD%$PS1_HISTCMD}
We assign either the value of HISTCMD (when a new entry in the command history is being made, i.e. we are executing a new command) or an empty string (when the command is empty) to PS1_HISTCMD2. This works by trimming off the value HISTCMD any match on PS1_HISTCMD (using the ${var%subst} suffix replacement syntax).
${_:+...}
This is from the question. It will expand to ... something if the value of $_ is set and nonempty (which it is when a command is being executed, but not e.g. if we are performing a variable assignment). The "something" should be the status code (and a space, for legibility) if PS1_HISTCMD2 is nonempty.
${PS1_HISTCMD2:+$? }
There.
'$ '
This is just the actual prompt suffix, as in the original question.
So the key parts are the variables PS1_HISTCMD which remembers the previous value of HISTCMD, and the variable PS1_HISTCMD2 which captures the value of HISTCMD so it can be accessed from within PROMPT_COMMAND, but needs to be unset in the PROMPT_COMMAND so that the ${PS1_HISTCMD2:=...} assignment will fire again the next time the prompt is displayed.
I fiddled for a bit with trying to hide the output from ${PS1_HISTCMD2:=...} but then realized that there is in fact something we want to display anyhow, so just piggyback on that. You can't have a completely empty PS1_base because the shell apparently notices, and does not even attempt to perform a substitution when there is no value; but perhaps you can come up with a dummy value (a no-op escape sequence, perhaps?) if you have nothing else you want to display. Or maybe this could be refactored to run with a suffix instead; but that is probably going to be trickier still.
In response to Anubhava's "smallest answer" challenge, here is the code without comments or error checking.
PS1_base=$'\n'
PS1_update_HISTCMD () { PS1_HISTCMD=${PS1_HISTCMD2:-$PS1_HISTCMD}; unset PS1_HISTCMD2; }
PROMPT_COMMAND=PS1_update_HISTCMD
PS1='${PS1_base#foo${PS1_HISTCMD2:=${HISTCMD%$PS1_HISTCMD}}}${_:+${PS1_HISTCMD2:+$? }}$ '
This is probably not the best way to do this, but it seems to be working
function pc {
foo=$_
fc -l > /tmp/new
if cmp -s /tmp/{new,old} || test -z "$foo"
then
PS1='\n$ '
else
PS1='\n$? $ '
fi
cp /tmp/{new,old}
}
PROMPT_COMMAND=pc
Result
$ [ 2 = 2 ]
0 $ [ 2 = 3 ]
1 $
$
I need to use great script bash-preexec.sh.
Although I don't like external dependencies, this was the only thing to help me avoid to have 1 in $? after just pressing enter without running any command.
This goes to your ~/.bashrc:
__prompt_command() {
local exit="$?"
PS1='\u#\h: \w \$ '
[ -n "$LASTCMD" -a "$exit" != "0" ] && PS1='['${red}$exit$clear"] $PS1"
}
PROMPT_COMMAND=__prompt_command
[-f ~/.bash-preexec.sh ] && . ~/.bash-preexec.sh
preexec() { LASTCMD="$1"; }
UPDATE: later I was able to find a solution without dependency on .bash-preexec.sh.
I want to add the following line:
%optflags "-O2"
In a file a.txt using shell.
When I write:
{ echo "%optflags "-O2""
} >> a.txt
It prints:
%optflags -O2
How can I get this right?
Escape the double quotes:
echo "%optflags \"-O2\""
or
Use single quotes:
echo '%optflags "-O2"'
Note: you can partly see what was going on with your original code if you check it in bash -x:
$ echo "%optflags "-O2""
+ echo '%optflags -O2'
It passed a single argument to echo.. Why? Well, it concatenated "%optflags ", -O2, and "" (the empty string) to a single argument.