CODEWITHSUNDEEP
bash
user1929290
user1929290

Reputation: 295

Difference between /bin/bash and /bin/sh and calling the script directly

This is the content of my script

#!/bin/bash
# test.sh
# Note: Set permissions on this script to 555 or 755,
# then call it with ./test.sh or sh ./test.sh.
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
# Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl-C to exit ===================="
sleep 1
exec $0 # Spawns another instance of this same script
#+ that replaces the previous one.
echo "This line will never echo!" # Why not?
exit 99 # Will not exit here!
# Exit code will not be 99!

And this is the output when I run the script using /bin/bash 

[user@localhost ~]$ /bin/bash test.sh 

This line appears ONCE in the script, yet it keeps echoing.
The PID of this instance of the script is still 4872.
==================== Hit Ctl-C to exit ====================
test.sh: line 11: exec: test.sh: not found

This is the output when I run the script using /bin/sh

[user@localhost ~]$ /bin/sh ./test.sh 

This line appears ONCE in the script, yet it keeps echoing.
The PID of this instance of the script is still 4934.
==================== Hit Ctl-C to exit ====================

This line appears ONCE in the script, yet it keeps echoing.
The PID of this instance of the script is still 4934.

==================== Hit Ctl-C to exit ====================

I had to use Ctl-C to stop this.

Why does the same script behave differently based on different mode of execution. FYI: I had to use Ctl-C which I executed the script like this: ./test.sh

Upvotes: 0

Views: 1182

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 753455

You get the error test.sh: not found because you don't have the current directory in your PATH.

When you run it with ./test.sh, the pathname is relative and PATH is not consulted, so the command is found.

Note that /bin/bash ./test.sh would iterate, and /bin/sh test.sh would fail; it is not an issue of the difference between /bin/bash and /bin/sh.

Upvotes: 1

simont
simont

Reputation: 72527

The first time you run this script using /bin/bash test.sh. The second time, you run it with /bin/sh ./test.sh. Note the difference: test.sh vs ./test.sh.

For whatever reason, the script is unable to spawn a second process running the script, because it's unable to find test.sh. (I'm not sure why this is. I'd guess a PATH issue, perhaps?) See the below error message:

[user@localhost ~]$ /bin/bash test.sh 

This line appears ONCE in the script, yet it keeps echoing.
The PID of this instance of the script is still 4872.
==================== Hit Ctl-C to exit ====================
test.sh: line 11: exec: test.sh: not found

The script can't execute test.sh because it doesn't know where test.sh is; ./test.sh is a full path to the script, which is findable.

The second time you run it, the child processes spawn happily, because ./test.sh is found.

Try running: /bin/bash ./test.sh and /bin/sh ./test.sh and see if there's any difference in the output (shouldn't be).

Upvotes: 1

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