sh command

George2 · 09-03-2007, 03:45 AM

Hello everyone,

I am wondering what is the differences between executing a shell script itself and using sh command to execute?

Example (supposing foo.sh is a Linux shell script),

1. foo.sh
2. sh foo.sh

thanks in advance,
George

druuna · 09-03-2007, 04:02 AM

Hi,

When executing a script by itself, it must have execute permissions. (example 1 foo.sh)

You can use sh to execute a script that does not have execute permissions (example 2 sh foo.sh).

Hope this helps.

George2 · 09-03-2007, 04:45 AM

Thanks druuna,

Quote:

Originally Posted by druuna

Hi,

When executing a script by itself, it must have execute permissions. (example 1 foo.sh)

You can use sh to execute a script that does not have execute permissions (example 2 sh foo.sh).

Hope this helps.

If I grant execution permission, for example, chmod a+x, then using sh to execute or using the script itself to execute directly are the same effect?

regards,
George

samwise17 · 09-03-2007, 05:24 AM

The character '#!' at the beginning of a file lets the system know what program executes it,
e.g. #!/bin/bash or #!/bin/perl.
If you run 'sh foo.sh' you probably don't need this, however if you just type './foo.sh' you do.
I generally just put the #!/bin/<whatever> at the top and run scripts by running 'chmod u+x <script> and then './<script>'

cfaj · 09-03-2007, 04:20 PM

Quote:

Originally Posted by samwise17

The character '#!' at the beginning of a file lets the system know what program executes it,
e.g. #!/bin/bash or #!/bin/perl.
If you run 'sh foo.sh' you probably don't need this, however if you just type './foo.sh' you do.

No, the shebang is not necessary unless you are using an interpreter other than the default shell, or in special circumstances such as a CGI script.

Quote:

I generally just put the #!/bin/<whatever> at the top

I generally don't use a shebang unless it is for special use. I often move scripts between different systems. If the script requires bash, #!/bin/bash will not work on most of them (on one it's /usr/local/bin/bash, on another it's /usr/pkg/bin/bash, etc.).

Quote:

and run scripts by running 'chmod u+x <script> and then './<script>'

I never put scripts in the current directory. I always use a central directory ($HOME/bin or /usr/local/bin) which is in my PATH.

George2 · 09-04-2007, 12:12 AM

Thanks samwise17,

Quote:

Originally Posted by samwise17

The character '#!' at the beginning of a file lets the system know what program executes it,
e.g. #!/bin/bash or #!/bin/perl.
If you run 'sh foo.sh' you probably don't need this, however if you just type './foo.sh' you do.
I generally just put the #!/bin/<whatever> at the top and run scripts by running 'chmod u+x <script> and then './<script>'

Your reply is great! I am wondering the only differences between executing shell script directly (method 1) and using sh to execute shell script (method 2) is, method 2 will use another shell to start the shell script, no other differences?

regards,
George

George2 · 09-04-2007, 12:15 AM

Thanks cfaj,

Quote:

Originally Posted by cfaj

[INDENT]No, the shebang is not necessary unless you are using an interpreter other than the default shell, or in special circumstances such as a CGI script.

Two more questions,

1. What is *shebang*?

2. Why *the shebang is not necessary unless you are using an interpreter*? Show an example please?

regards,
George

chrism01 · 09-04-2007, 01:45 AM

The character sequence '#!' is colloquially called 'shebang',
e.g.
#!/bin/bash
or
#!/usr/bin/perl

If you are writing a shell script, it's usually advised (in prod systems at least) to specify (via the #! invocation) which shell is required to run this script properly.
Otherwise, what can happen is that eg you write it in bash, but the default shell on the target system turns out to be csh.
Not all bash cmds are understood by the csh shell.

Using the shebang line as described above means that you simplify the cmd line by not having to specify the actual shell by name externally to the script.
Note that eg cron has a minimal/almost non-existent default shell env, which means you should always specify the complete path for a cmd eg /usr/bin/perl and not the short form eg perl.
HTH

cfaj · 09-04-2007, 02:53 AM

Quote:

Originally Posted by chrism01

The character sequence '#!' is colloquially called 'shebang',
e.g.
#!/bin/bash
or
#!/usr/bin/perl

If you are writing a shell script, it's usually advised (in prod systems at least) to specify (via the #! invocation) which shell is required to run this script properly.
Otherwise, what can happen is that eg you write it in bash, but the default shell on the target system turns out to be csh.

It is just as likely that bash may be in a different location on the target system; a shebang makes the script less portable.

If you are writing a script to be used on more than one system, it is safest to use POSIX compliant code that can be run by /bin/sh on most systems.

Quote:

Not all bash cmds are understood by the csh shell.

"Not all" is an understatement. A Bourne-type shell (such as bash) is a completely different language from csh; it's not just the commands, but the syntax is very different.

Quote:

Using the shebang line as described above means that you simplify the cmd line by not having to specify the actual shell by name externally to the script.

You do not have to specify the shell on the command line if there is no shebang. If the script is written for /bin/sh, that will be used to execute the script even if you call it from csh.

Quote:

Note that eg cron has a minimal/almost non-existent default shell env, which means you should always specify the complete path for a cmd eg /usr/bin/perl and not the short form eg perl.

George2 · 09-04-2007, 02:58 AM

Thanks chrism01,

I want to confirm with you that there is no functonal differences between using sh to execute and execute the shell script directly, right?

In my shell script, I am using #!/bin/bash at the first line.

regards,
George

Quote:

Originally Posted by chrism01

The character sequence '#!' is colloquially called 'shebang',
e.g.
#!/bin/bash
or
#!/usr/bin/perl

If you are writing a shell script, it's usually advised (in prod systems at least) to specify (via the #! invocation) which shell is required to run this script properly.
Otherwise, what can happen is that eg you write it in bash, but the default shell on the target system turns out to be csh.
Not all bash cmds are understood by the csh shell.

Using the shebang line as described above means that you simplify the cmd line by not having to specify the actual shell by name externally to the script.
Note that eg cron has a minimal/almost non-existent default shell env, which means you should always specify the complete path for a cmd eg /usr/bin/perl and not the short form eg perl.
HTH

George2 · 09-04-2007, 03:19 AM

Thanks man,

If I am using #!/bin/bash at the first line, there are any functional differences between executing the shell script directly and using sh to execute?

regards,
George

Quote:

Originally Posted by cfaj

It is just as likely that bash may be in a different location on the target system; a shebang makes the script less portable.

If you are writing a script to be used on more than one system, it is safest to use POSIX compliant code that can be run by /bin/sh on most systems.

"Not all" is an understatement. A Bourne-type shell (such as bash) is a completely different language from csh; it's not just the commands, but the syntax is very different.

You do not have to specify the shell on the command line if there is no shebang. If the script is written for /bin/sh, that will be used to execute the script even if you call it from csh.

samwise17 · 09-04-2007, 06:00 AM

AFAIK, nope. But what cfaj says is new to me. I like the idea of putting stuff in /usr/local/bin but most of my stuff has a specific purpose and not really general use, so I keep them in the directory related to the project.

cfaj · 09-04-2007, 11:39 AM

Quote:

Originally Posted by George2

If I am using #!/bin/bash at the first line, there are any functional differences between executing the shell script directly and using sh to execute?

If you are calling a script as the argument to a shell, the shebang makes no difference; it is just another comment. The shebang is only used when the script is executed as a command.

makyo · 09-04-2007, 01:12 PM

Hi.

The portability of scripts is decreased if you specify an absolute path.

I once used Solaris often and ran into such problems going back and forth to and from Linux.

I use a perl utility, fixin (fix interpreter), that adjusts the shebang line of scripts for the correct path. It works not only for perl shebangs, but for anything you have in a shebang -- awk, sh, etc.

The fixin program was published in one of the first Programming perl books. An on-line copy of the code is http://search.cpan.org/src/JOHNL/DBD...mples/fixin.pl

Best wishes ... cheers, makyo

PS The origin of the word shebang can be found at http://en.wikipedia.org/wiki/Shebang_%28Unix%29

chrism01 · 09-04-2007, 08:45 PM

The point of specifying the shell your code requires (on production systems) is so that you don't end up with a script that partially runs, then screws up/crashes because it's running under the wrong intepreter.
It's much safer for it not to run at all if it can't find the correct interpreter.
It can cost a company a lot of money if it goes wrong...
It also tells the sysadmin/maintenance prog which shell it will work with, rather than them having to guess.
You might not be avail to be asked when it goes pear-shaped ...