I want to execute some scripts on dash shell compared to standard default bash. This is an example (test.sh)
#!/bin/dash
echo $SHELL
echo $0
This execution gives me
/bin/bash
./test.sh
as output. I was expecting '/bin/dash' as output.
If this is wrong, can someone let me know how do I actually work on dash.
Thanks
SHELL environment variable picks up the value from /etc/passwd. (It denotes the path to user's preferred command language interpreter.)
This value wouldn't change if you change the shell in your session or your script.
You can validate that you are running dash by adding the command
ps | grep $$
The $$ variable contains the PID of the process of the running shell.
This one would show the exact command.
ps o command --no-header --pid "$$"
Related
In bash (GNU bash, version 3.2.57), I would like to substitute the exact content of a variable (unescaped) into a following command.
To illustrate what I mean, given the following string variable:
s="2>&1 > /dev/null"
If I try to insert that exact string into a command:
bash --version $s || echo "will install bash"
(this command is just a simple example for the sake of the question)
The command actually executed looks like this:
bash --version '2>&1' '>' /dev/null
The inserted strings are escaped, which I don't want.
What I would like instead is to somehow insert the content of s, unescaped, into the executed command, so that the executed command is this one:
bash --version 2>&1 > /dev/null
How could I achieve that ?
How could I achieve that ?
Instead of a variable, use a function.
run_this_silent() {
"$#" 2>&1 >/dev/null
}
run_this_silent bash --version
It is not possible to store redirections in a variable without using eval (or the equivalent bash -c COMMAND), and eval is a bad solution in pretty much every case imaginable. If you want to unconditionally silence a command (or a hundred commands) it's better to explicitly add the redirects to each of them.
In my program I need to know the maximum number of process I can run. So I write a script. It works when I run it in shell but but when in program using system("./limit.sh"). I work in bash.
Here is my code:
#/bin/bash
LIMIT=\`ulimit -u\`
ACTIVE=\`ps -u | wc -l \`
echo $LIMIT > limit.txt
echo $ACTIVE >> limit.txt
Anyone can help?
Why The Original Fails
Command substitution syntax doesn't work if escaped. When you run:
LIMIT=\`ulimit -u\`
...what you're doing is running a command named
-u`
...with the environment variable named LIMIT containing the value
`ulimit
...and unless you actually have a command that starts with -u and contains a backtick in its name, this can be expected to fail.
This is because using backticks makes characters which would otherwise be syntax into literals, and running a command with one or more var=value pairs preceding it treats those pairs as variables to export in the environment for the duration of that single command.
Doing It Better
#!/bin/bash
limit=$(ulimit -u)
active=$(ps -u | wc -l)
printf '%s\n' "$limit" "$active" >limit.txt
Leave off the backticks.
Use modern $() command substitution syntax.
Avoid multiple redirections.
Avoid all-caps names for your own variables (these names are used for variables with meaning to the OS or system; lowercase names are reserved for application use).
Doing It Right
#!/bin/bash
exec >limit.txt # open limit.txt as output for the rest of the script
ulimit -u # run ulimit -u, inheriting that FD for output
ps -u | wc -l # run your pipeline, likewise with output to the existing FD
You have a typo on the very first line: #/bin/bash should be #!/bin/bash - this is often known as a "shebang" line, for "hash" (#) + "bang" (!)
Without that syntax written correctly, the script is run through the system's default shell, which will see that line as just a comment.
As pointed out in comments, that also means only the standardised options available to the builtin ulimit command, which doesn't include -u.
I am trying to source a script from a Perl script (script.pl).
system ("source /some/generic/script");
Please note that this generic script could be a shell, python or any other script. Also, I cannot replicate the logic present inside this generic script into my Perl script. I tried replacing system with ``, exec, and qx//. Each time I got the following error:
Can't exec "source": No such file or directory at script.pl line 18.
I came across many forums on the internet, which discussed various reasons for this problem. But none of them provided a solution. Is there any way to run/execute source command from a Perl script?
In bash, etc, source is a builtin that means read this file, and interpret it locally (a little like a #include).
In this context that makes no sense - you either need to remove source from the command and have a shebang (#!) line at the start of the shell script that tells the system which shell to use to execute that script, or you need to explicitly tell system which shell to use, e.g.
system "/bin/sh", "/some/generic/script";
[with no comment about whether it's actually appropriate to use system in this case].
There are a few things going on here. First, a child process can't change the environment of its parent. That source would only last as long as its process is around.
Here's a short program that set and export an environment variable.
#!/bin/sh
echo "PID" $$
export HERE_I_AM="JH";
Running the file does not export the variable. The file runs in its own proces. The process IDs ($$) are different in set_stuff.sh and the shell:
$ chmod 755 set_stuff.sh
$ ./set_stuff.sh
PID 92799
$ echo $$
92077
$ echo $HERE_I_AM # empty
source is different. It reads the file and evaluates it in the shell. The process IDs are the same in set_stuff.sh and the shell, so the file is actually affecting its own process:
$ unset HERE_I_AM # start over
$ source set_stuff.sh
PID 92077
$ echo $$
92077
$ echo $HERE_I_AM
JH
Now on to Perl. Calling system creates a child process (there's an exec in there somewhere) so that's not going to affect the Perl process.
$ perl -lwe 'system( "source set_stuff.sh; echo \$HERE_I_AM" );
print "From Perl ($$): $ENV{HERE_I_AM}"'
PID 92989
JH
Use of uninitialized value in concatenation (.) or string at -e line 1.
From Perl (92988):
Curiously, this works even though your version doesn't. I think the different is that in this there are no special shell metacharacters here, so it tries to exec the program directory, skipping the shell it just used for my more complicated string:
$ perl -lwe 'system( "source set_stuff.sh" ); print $ENV{HERE_I_AM}'
Can't exec "source": No such file or directory at -e line 1.
Use of uninitialized value in print at -e line 1.
But, you don't want a single string in that case. The list form is more secure, but source isn't a file that anything can execute:
$ which source # nothing
$ perl -lwe 'system( "source", "set_stuff.sh" ); print "From Perl ($$): $ENV{HERE_I_AM}"'
Can't exec "source": No such file or directory at -e line 1.
Use of uninitialized value in concatenation (.) or string at -e line 1.
From Perl (93766):
That is, you can call source, but as something that invokes the shell.
Back to your problem. There are various ways to tackle this, but we need to get the output of the program. Instead of system, use backticks. That's a double-quoted context so I need to protect some literal $s that I want to pass as part of the shell commans
$ perl -lwe 'my $o = `echo \$\$ && source set_stuff.sh && echo \$HERE_I_AM`; print "$o\nFrom Perl ($$): $ENV{HERE_I_AM}"'
Use of uninitialized value in concatenation (.) or string at -e line 1.
93919
From Shell PID 93919
JH
From Perl (93918):
Inside the backticks, you get what you like. The shell program can see the variable. Once back in Perl, it can't. But, I have the output now. Let's get more fancy. Get rid of the PID stuff because I don't need to see that now:
#!/bin/sh
export HERE_I_AM="JH";
And the shell command creates some output that has the name and value:
$ perl -lwe 'my $o = `source set_stuff.sh && echo HERE_I_AM=\$HERE_I_AM`; print $o'
HERE_I_AM=JH
I can parse that output and set variables in Perl. Now Perl has imported part of the environment of the shell program:
$ perl -lwe 'my $o = `source set_stuff.sh && echo HERE_I_AM=\$HERE_I_AM`; for(split/\R/,$o){ my($k,$v)=split/=/; $ENV{$k}=$v }; print "From Perl: $ENV{HERE_I_AM}"'
From Perl: JH
Let's get the entire environment, though. env outputs every value in the way I just processed it:
$ perl -lwe 'my $o = `source set_stuff.sh && env | sort`; print $o'
...
DISPLAY=:0
EC2_PATH=/usr/local/ec2/ec2-api-tools
EDITOR=/usr/bin/vi
...
I have a few hundred varaibles set in the shell, and I don't want to expose most of them. Those are all set by the Perl process, so I can temporarily clear out %ENV:
$ perl -lwe 'local %ENV=(); my $o = `source set_stuff.sh && env | sort`; print $o'
HERE_I_AM=JH
PWD=/Users/brian/Desktop/test
SHLVL=1
_=/usr/bin/env
Put that together with the post processing code and you have a way to pass that information back up to the parent.
This is, by the way, similar to how you'd pass variables back up to a parent shell process. Since that output is already something the shell understands, you use the shell's eval instead of parsing it.
You can't. source is a shell function that 'imports' the contents of that script into your current environment. It's not an executable.
You can replicate some of it's functionality by rolling your own - run or parse whatever you're 'sourcing' and capture the result:
print `. file_to_source; echo $somevar`;
or similar.
I wrote a small script and for some reason I need to escape any spaces passed in parameters to get it to work.
I read numerous other articles about people with this issue and it is typically due to not quoting $#, but all of my variables are quoted within the script and the parameters quoted on the command line as well. Also, if I run the script in debug mode the line that is returned can be run successfully via copy paste but fails when executed from within the script.
CODE:
connections ()
{
args="$#"
pid="$(pgrep -nf "$args")"
echo $pid
# code that shows TCP and UDP connections for $pid
}
connections "$#"
EXAMPLE:
bash test.sh "blah blah"
fails and instead returns the pid of the currently running shell
bash test.sh "blah\ blah"
succeeds and returns the pid of the process you are searching for via pgrep
Your problem has nothing to do with "$#".
If you add a -l option to pgrep, you can see why it's matching the current process.
The script you're running also includes what you're trying to search for in its own arguments.
It's like doing this, and seeing grep:
$ ps -U $USER -o pid,cmd | grep gnome-terminal
12410 grep gnome-terminal
26622 gnome-terminal --geometry=180x65+135+0
The reason the backslash makes a difference? pgrep thinks backslash+space just means space. It doesn't find your script, because that contains blah\ blah, not blah blah.
I want to inject a transparent wrappering command on each shell command in a make file. Something like the time shell command. ( However, not the time command. This is a completely different command.)
Is there a way to specify some sort of wrapper or decorator for each shell command that gmake will issue?
Kind of. You can tell make to use a different shell.
SHELL = myshell
where myshell is a wrapper like
#!/bin/sh
time /bin/sh "$0" "$#"
However, the usual way to do that is to prefix a variable to all command calls. While I can't see any show-stopper for the SHELL approach, the prefix approach has the advantage that it's more flexible (you can specify different prefixes for different commands, and override prefix values on the command line), and could be visibly faster.
# Set Q=# to not display command names
TIME = time
foo:
$(Q)$(TIME) foo_compiler
And here's a complete, working example of a shell wrapper:
#!/bin/bash
RESULTZ=/home/rbroger1/repos/knl/results
if [ "$1" == "-c" ] ; then
shift
fi
strace -f -o `mktemp $RESULTZ/result_XXXXXXX` -e trace=open,stat64,execve,exit_group,chdir /bin/sh -c "$#" | awk '{if (match("Process PID=\d+ runs in (64|32) bit",$0) == 0) {print $0}}'
# EOF
I don't think there is a way to do what you want within GNUMake itself.
I have done things like modify the PATH env variable in the Makefile so a directory with my script linked to all name the bins I wanted wrapped was executed rather than the actual bin. The script would then look at how it was called and exec the actual bin with the wrapped command.
ie. exec time "$0" "$#"
These days I usually just update the targets in the Makefile itself. Keeping all your modifications to one file is usually better IMO than managing a directory of links.
Update
I defer to Gilles answer. It's a better answer than mine.
The program that GNU make(1) uses to run commands is specified by the SHELL make variable. It will run each command as
$SHELL -c <command>
You cannot get make to not put the -c in, since that is required for most shells. -c is passed as the first argument ($1) and <command> is passed as a single argument string as the second argument ($2).
You can write your own shell wrapper that prepends the command that you want, taking into account the -c:
#!/bin/sh
eval time "$2"
That will cause time to be run in front of each command. You need eval since $2 will often not be a single command and can contain all sorts of shell metacharacters that need to be expanded or processed.