Why "alias" does not work from script file

rng · 12-20-2017, 07:33 AM

I am working on Debian Stable Linux which is otherwise working very well. I find that 'alias' command works on terminal but does not work when it is kept in a script file:

Code:

auser@debian:~$ cat findalias.sh 
#! /bin/bash
alias

auser@debian:~$ ./findalias.sh 
auser@debian:~$

Where is the problem and how can it be solved? Thanks for your help.

keefaz · 12-20-2017, 08:36 AM

You need to remove space character between '#!' and '/bin/bash'

rng · 12-20-2017, 10:01 AM

Quote:

Originally Posted by keefaz

You need to remove space character between '#!' and '/bin/bash'

Corrected. It still does not work.

Adding or removing the space does not matter. In following version, "echo" command works but not "alias" command whether or not there is a space between "#!" and "/bin/bash":

Code:

$ cat ./findalias.sh 
#! /bin/bash
echo "testing"
alias

auser@debian:~$ ./findalias.sh 
testing
auser@debian:~$ cat ./findalias.sh 
#!/bin/bash
echo "testing"
alias

auser@debian:~$ ./findalias.sh 
testing
auser@debian:~$

MadeInGermany · 12-20-2017, 10:27 AM

Run your script with

Code:

bash -i /path/to/script

Or change the shebang to

Code:

#!/bin/bash -i

If you want to see how it initializes, run it with

Code:

bash -ix /path/to/script

rtmistler · 12-20-2017, 10:28 AM

Few things to consider:

I always forget if you have a default environment or your same environment when you enter a script
Use set -xv as the second line of your script to enable verbose output and debugging to see more details
Check to verify you actually have any aliases. And then this circulates back to the question about what your environment is when you enter a script (I'm web searching that now)

rtmistler · 12-20-2017, 10:30 AM

My read from brief research is that environment variables do not get imported to a script, they have to be exported specifically for the script to be able to see them. Meanwhile aliases are environment variables. I think within your aliases file you can choose to put "export" prior to each line defining the alias.

rng · 12-20-2017, 11:17 AM

Quote:

MadeInGermany wrote:

Or change the shebang to

Code:

#!/bin/bash -i

Above works perfectly. Thanks. It will be useful if you can explain when and why -i should be used. I checked with "man bash" command and it is mentioned to be for 'interactive' mode. Could not really understand.

rng · 12-20-2017, 12:00 PM

Quote:

rtmistler wrote:

you can choose to put "export" prior to each line defining the alias.

Writing "export" at start of alias line in .bashrc file actually makes alias not available. That particular alias does not work while alias lines where export is not mentioned works. I checked from a new terminal after editing .bashrc file.

MadeInGermany · 12-20-2017, 12:14 PM

For safety reason bash in script mode does not read the .bashrc in the homedirectory of the user who starts it.
But -i, interactive mode, enforces that.
You can consider bash -i unsafe but convenient in certain cases.

The export of aliases and functions (to environment), as mentioned by rtmistler, is specific to bash, does not work on some non-Linux platforms, and bares risks. This environment looks strange and is inherited by all programs that the script runs.
And it is not so simple to use.
I consider this bash feature unnecessary and generally do not recommend it.

keefaz · 12-20-2017, 12:48 PM

Quote:

Originally Posted by rng

Writing "export" at start of alias line in .bashrc file actually makes alias not available. That particular alias does not work while alias lines where export is not mentioned works. I checked from a new terminal after editing .bashrc file.

Maybe source bashrc file in your script?

scasey · 12-20-2017, 01:25 PM

sourcing .bashrc within the script would have the same effect as using the -i switch, with the same reservations as expressed by MadeInGermany.

Just curious, rng, what are you trying to accomplish by executing the alias command in a script?

rng · 12-20-2017, 08:02 PM

Quote:

scasey wrote:

Just curious, rng, what are you trying to accomplish by executing the alias command in a script?

I have created many aliases (and forget them) and I use this script to find them by pattern:

Code:

alias | grep $1

So if I had created an alias for "grep -i --color='always'", I can search for it by giving command "findalias grep". It is really more for experimenting with terminal.

I am still not clear why all other bash commands work in scripts but not alias. I never had to use '-i' switch over last many years that I have been using Linux regularly.

rknichols · 12-20-2017, 08:44 PM

Quote:

Originally Posted by rng

I am still not clear why all other bash commands work in scripts but not alias. I never had to use '-i' switch over last many years that I have been using Linux regularly.

The alias command works fine in scripts, and that's with or without "-i". It's just that aliases are not exported, so your script starts out with no aliases defined. Unless your script sources your .bashrc or other file containing alias definitions there won't be any output.

Aliases are not exported because, by default, aliases are not expanded in non-interactive shells. You would have to set the expand_aliases shell option ("shopt -s expand_aliases") in such a shell in order to make use of aliases.

scasey · 12-21-2017, 12:50 PM

Quote:

Originally Posted by rng

I have created many aliases (and forget them) and I use this script to find them by pattern:

Code:

alias | grep $1

So if I had created an alias for "grep -i --color='always'", I can search for it by giving command "findalias grep". It is really more for experimenting with terminal.

OK. Makes sense, except that 1.

Code:

alias | grep grep

would always work, and is only four more keystrokes than using the script and you don't have to maintain and remember that the script is there.

Of course, then we wouldn't have had the opportunity for this most interesting discussion and larnin'

MadeInGermany · 12-21-2017, 01:12 PM

Where you define your aliases you can define a function

Code:

findalias(){
alias | grep "$1"
}

A function has the same scope as an alias.