I tried to implement a bash command system("cd /home/user") in perl , but I get an error saying
Can't exec "cd": No such file or directory at temp.pl
Is there a way to change the current working directory to the specified one , and the change remains after the perl script has exited also.
No. A process can't change its parent process's current working directory. Shells implement commands like cd as "builtins", meaning they're a function in the shell itself, and not a separate process that gets run.
You can change the current directory in perl using chdir($dir), and that change will be inherited by child processes — but it won't be passed along to the parent process.
When you want to change the directory inside your Script, you can use the Perl command chdir('dir')
Example:
chdir($dir);
You actually cant modify the directory of the parents process, but you can of the current process
You cannot modify the current working directory of a different process in UNIX, at least not without some serious hackery.
This is why cd is a built-in in all shells. It is not an external program (nor can it be implemented as an external program).
cd is not a process, it is a shell builtin command that changes the current working directory for that shell process.
So use system("sh -c 'cd /my/dir'"). but here system command itself invoke another shall so still it not change directory.
use chdir for that.
cd is not a process, it is a shell builtin command that changes the current working directory for that shell process. So system("sh -c 'cd /cat/bat'") would "succeed", but still wouldn't change the working directory of your perl process; use chdir for that.
Related
I am new in Perl. I am using the following command to remove a folder in Perl, under Windows:
system "del trash_folder";
But I intend to run the same script under Unix as well. I could get the OS name in the code and run a different command based on the OS. But is there no better way in Perl? I am thinking of possibly an API or so that is OS-ignorant.
The del command is never going to create a new directory, is it? :-)
If you want to create directories, use the mkdir command.
If you want to remove directories, use the rmdir command.
Update: In general, if you have a choice between using a Perl built-in function or an external command, then the Perl function will be the better choice. Firstly, your code will be more portable and, secondly, opening a sub-shell to run the external command will slow your program down.
Running Lubuntu -
Beginner Perl programmer
Script is XXX.pl located at ~/projects/XXX/XXX.pl
First line is the shebang
#!/usr/bin/perl
Permission to run is set to Anyone.
In directory ~/projects/XXX, the command
~/projects/XXX$ perl XXX.pl
works as desired, but the command
~/projects/XXX$ XXX.pl
Fails with XXX.pl: command not found
What am I missing ?
The two usual options to execute your Perl script are:
perl XXX.pl
or
./XXX.pl
Both ways assume that your current working directory contains the script XXX.pl, otherwise it won't work.
As already pointed out by jm666 in the comments, you can usually not execute a program or script from your current working directory without prepending ./, primarily because of security reasons. Now, you may wonder why it's necessary.
Explanation:
Your shell uses the contents of an environment variable called $PATH to find out about where external commands (non-builtin programs) are located in your filesystem. If you want to see what's in $PATH, just type the following in your shell:
echo $PATH
Now you can see that the $PATH variable does NOT contain your current working directory. The consequence is that your shell is not able to find the program XXX.pl. By prepending ./ you instruct the shell to execute the program which comes after.
But there are two requirements if you want to execute your Perl script with ./script.pl:
The script has to be executable (check with ls -l)
The first line (shebang line) has to be #!/path/to/your/perl because your shell needs that information to find the perl interpreter in order to run your script
However, #1 and #2 are NOT required when you execute your script with
perl XXX.pl
because it invokes the perl interpreter directly with your script.
See how to make Perl scripts executable on Linux and make the script itself directly executable with chmod for some more details.
Can the script be found?
Is . in your path? If it's not, add it to your path, or use ./XXX.pl instead of XXX.pl.
Can the script be executed?
Do you have execute permission to the file? Fix using chmod u+x XXX.pl.
Is the interpreter correct?
which perl will tell you which interpreter is used when you use perl XXX.pl. That's the path that should be on your shebang (#!) line.
I am trying to run a perl script from within a *.csh script, but I get the following error:
Can't open perl script "checkLength.perl": No such file or directory
I think it has something to do with the path of the perl script. I can run a perl script fine when it's not being called within the shell script, but a perl script won't run when I have it being called in the cshell script.
The shebang I use for the perl script is : #!/usr/bin/perl. I checked, and perl is located there (not in /usr/bin/env perl). I made sure it's executable.
Do I need to edit the $PATH to get this to work? I'm confused because if I run a perl script without calling it within another csh script, it runs just fine.
Of course one obvious solution to your problem would be to simply use the full path to the script. But this is often not preferable, as the script would brake on installations that do not mirror exactly the original structure.
So there are two possibilities:
Either you put your perl script into a well defined location that is contained in your PATH variable so any program that runs under the respective users environment finds it.
Or you have some well defined location relative to the location of your calling script. Say your bash script is located in somedir and you perl script is in somedir/subdir, then your call would be subdir/perlscript.pl. Then, to become independent of possible changes in the working directory of the caller, you could determine the current full path of the perl script to be called on start of the bash script.
A template for this would be:
#!/bin/bash
FULLPATH=$(pwd)/subdir/somescript.pl
# do something else, cd ...
$FULLPATH
I am assuming your Perl script is kept in a fixed path relative to your bash script. Assuming your directory structure to be like so:
(BASEDIR)/bin/example.sh
(BASEDIR)/perl/example.pl
To allow you to run your bash script from anywhere in your system you must specify the relative path to your Perl script by getting the BASEDIR.
#!/bin/bash
BASEDIR=$(dirname $0)
perl $BASEDIR/../perl/example.pl
What we are doing above is finding the location of your bash script(BASEDIR/bin) that you call and then finding the relative location of the Perl script using the location of the bash script as reference. Now you will be able to call the bash script from anywhere and run your Perl script normally.
I have a perl script exist in the follwoing path (/home/Leen/Desktop/Tools/bin/tool.pl)
Every time I want to run this tool I go to the terminal
>
and then change the directory to
..../bin>
Then I run the perl by writing
..../bin> perl tool.pl file= whatever config= whatever
The problem is that I want to run this perl script without the need to go to the bin folder where it exist . so I can run perl script from any directory and as soon as I enter shell
I went to the etc/environment and I wrote the follwoing
export PERL5LIB=$PERL5LIB:/home/Leen/Desktop/Tools/bin
But when I go to terminal and write the follwoing straight ahead without going to bin folder where tool.pl exist
>perl tool.pl file=... config=...
it says the file "tool.pl" does not exist???
The first argument to the perl program is the path to an executable file. These calls are equivalent:
:~$ perl /home/Leen/Desktop/Tools/bin/tool.pl
:~$ perl ~/Desktop/Tools/bin/tool.pl
:~$ perl ./Desktop/Tools/bin/tool.pl
:~/Desktop/Tools/bin$ perl tool.pl
:~/Desktop/Tools/bin$ perl ./tool.pl
etc.
In the shell the tilde ~ expands to your home directory, and ./ symbolizes the current directory. On *nix shells (including the various terminal emulators on ubuntu), the command prompt ususally is $ in nomal mode, # as root user and seldom %. > Is a secondary command prompt, e.g. when continuing a multiline argument, unlike cmd.exe on Windows.
The PERL5LIB variable determines where Perl looks for modules, not for executable files.
You can set a script as executable via chmod +x FILENAME. You can then call the script without specifying the perl program:
:~/Desktop/Tools/bin$ ./tool.pl
You can modify the PATH variable to change where the shell looks for executables. The PATH usually contains /usr/bin/ and other directories. You can add a directory of your own via
PATH=$PATH:/home/Leen/Desktop/Tools/bin
Add your directory at the end of the PATHes, so you don't overrule other programs.
If you want to set this permanently, you can add this line to the file ~/.bashrc (only for your user and only for the bash shell).
Then you can call your script from anywhere, without a full path name:
:~/foo/bar$ tool.pl
You should consider using a more specific command name in this case, to prevent name clashes.
I have a Korn shell script at a location like /opt/apps/abc/folder/properties.env. I can execute it from Unix bash using the dot command:
. /opt/apps/abc/folder/properties.env
This works.
I have a Perl script abc.pl from which I am calling the script properties.env. I tried the following different:
system('/usr/bin/ksh','-c', '. /opt/apps/abc/folder/properties.env');
/usr/bin/ksh -c /opt/apps/abc/folder/properties.env;
system('. /opt/apps/abc/folder/properties.env');
None of the above work. I don't want to use exec because I want to return to the Perl script. What am I doing wrong?
The environment changes will only last as long as the life of the ksh session spawned by the system command. If you want the environment changes to affect the Perl script, then you have to source that file before you launch the Perl program.
If you need those environment variables in your perl code, (not in the environment where you called perl), you can also read and parse that properties.env and set the environment in the %ENV variable.
e.g
$ENV{'ENV_VAR1'}=VALUE_OF_ENV_VAR1
using system() spawn another process, as the other poster said. changing environment in the child does not affect the parent.