I have a shell script in my home directory called "echo". I added my home directory to my path, so that this echo would replace the other one.
To do this, I used: export PATH=/home/me:$PATH
When I do which echo, it shows the one I want. /home/me/echo
But when I actually do something like echo asdf it uses the system echo.
Am I doing something wrong?
which is an external command, so it doesn't have access to your current shell's built-in commands, functions, or aliases. In fact, at least on my system, /usr/bin/which is a shell script, so you can examine it and see how it works.
If you want to know how your shell will interpret a command, use type rather than which. If you're using bash, type -a will print all possible meanings in order of precedence. Consult your shell's documentation for details.
For most shells, built-in commands take precedence over commands in your $PATH. The whole point of having a built-in echo, for example, is that it's faster than loading /bin/echo into memory.
If you want your own echo command to override the shell's built-in echo, you can define it as a shell function.
On the other hand, overriding the built-in echo command doesn't strike me as a good idea in the first place. If it behaves the same as the built-in echo, there's not much point. If it doesn't, then it could break scripts that use echo expecting it to work a certain way. If possible, I suggest giving your command a different way. If it's an enhanced version of echo, you could even call it Echo.
It is likely using the shell's builtin.
If you want the one in your path you can do
`which echo` asdf
From this little article that explains the rules, here's a list in descending order of precedence:
Aliases
Shell functions
Shell builtin commands
Hash tables
PATH variable
echo is a shell builtin command (al least in bash) and PATH has the lowest priority. I guess you'll need to create a function or an alias.
Related
How to correctly pass to the script and substitute a variable that is already defined there?
My script test.sh:
#!/bin/bash
TARGETARCH=amd64
echo $1
When I enter:
bash test.sh https://example/$TARGETARCH
I want to see
https://example/amd64
but I actually see
https://example/
What am I doing wrong?
The first problem with the original approach is that the $TARGETARCH is removed by your calling shell before your script is ever invoked. To prevent that, you need to use quotes:
./yourscript 'https://example.com/$TARGETARCH'
The second problem is that parameter expansions only happen in code, not in data. This is, from a security perspective, a Very Good Thing -- if data were silently treated as code it would be impossible to write secure scripts handling untrusted data -- but it does mean you need to do some more work. The easy thing, in this case, is to export your variable and use GNU envsubst, as long as your operating system provides it:
#!/bin/bash
export TARGETARCH=amd64
substitutedValue=$(envsubst <<<"$1")
echo "Original value was: $1"
echo "Substituted value is: $substitutedValue"
See the above running in an online sandbox at https://replit.com/#CharlesDuffy2/EcstaticAfraidComputeranimation#replit.nix
Note the use of yourscript instead of test.sh here -- using .sh file extensions, especially for bash scripts as opposed to sh scripts, is an antipattern; the essay at https://www.talisman.org/~erlkonig/documents/commandname-extensions-considered-harmful/ has been linked by the #bash IRC channel on this topic for over a decade.
For similar reasons, changing bash yourscript to ./yourscript lets the #!/usr/bin/env bash line select an interpreter, so you aren't repeating the "bash" name in multiple places, leading to the risk of those places getting out of sync with each other.
Summary
How can I guarantee that my shell scripts will do what I expect, regardless of the environment?
(Let's assume that people have alias'd and function'd everything they can, but that they haven't touched any system binaries eg. /bin/ls)
Explanation
I am distributing shell scripts as part of an app. These shell scripts are executed in the user's environment - this cannot be changed.
This means users may have aliases for anything and functions redefining "standard" behavior. There have already been a few cases when normal shell keywords have been redefined (eg. local), causing unexpected side effects and crashes.
The only tokens that cannot be defined as functions are as follows:
Bash:
! [[ ]] case coproc do done elif else esac fi for function if in select then time until while { }
ZSH:
! [[ case coproc do done elif else end esac fi for foreach function if nocorrect repeat select then time until while { }
I am aware that:
You can escape a word to skip alias lookup
You can use builtin to always run a builtin
You can use command to always run a command
However, builtin and command can be redefined, so \builtin <command> may not always do what I expect.
Aliases are not expanded in bash scripts (unless you explicitly request this), and functions are usually not inherited by child processes. The caller of your script just has to avoid sourcing it. Problems could be environment variables and file handles.
It is difficult to make a script completely self-containing. For instance, I have seen cases where even standard programs (ls, cat,....) are stored in different locations, which means that if you set up your own PATH and don't know anything about the target platform, you have to apply some heuristics (searching a list of "commonly known directories") and hope that your search is correct.
A more reliable way would be to require from the user of the script to provide a certain minimal configuration (typically containing the basic definition for a PATH) and pass this configuration as parameter to your script.
There is one problem pointed out in the comment by Renaud Pacalet, in that bash allows functions to be exported (using export -f), and in bash, you would have to find out which functions exist, and explicitly remove their definitions (similarily as you would do it with environment variables). However, I see that you have tagged your question by bash and zsh, and if you don't mind, which script language you are using, writing the script in zsh would be perhaps better, because zsh does not have exported functions.
One point to keep in mind is, that every shell, bash and zsh, processes on startup certain files, before the commands in your script have any chance to run. For instance, no matter how you start your zsh, it will always process /etc/zshenv. For instance, if your script at one point invokes a zsh child script too, it would again run /etc/zshenv.
Of course, those startup files could set up functions, and in zsh, aliases are (AFIK) even expanded inside scripts. The strategy would be therefore to initially loop over your environment variables, the currently defined functions, the currently defined aliases (in zsh), and remove those definitions. Then you set up your own definitions (functions, variables).
Original Title: Indirect parameter substitution breaks when the script is sourced (zsh)
zsh 5.7.1 (x86_64-apple-darwin19.0)
GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)
I’m developing a shell script on a Mac and I’m trying to keep it portable between bash & zsh, so array indexing is a consideration. I know that I can set KSH_ARRAYS to get indexing to start at 0, but I decided to query the OS for the shell that’s in use and set the start index accordingly, which led to the issue described below.
It made sense (to me anyway!) to use indirect expansion, which is what led to the problem. Consider the script indirect.sh:
#! /bin/bash
declare -r ARRAY_START_BASH=0
declare -r ARRAY_START_ZSH=1
declare -r SHELL_BASH=0
declare -r SHELL_ZSH=1
# Indirect expansion is used to reference the values of the variables declared
# in this case statement e.g. ${!ARRAY_START}
case $(basename $SHELL) in
"bash" )
declare -r SHELL_ID=SHELL_BASH
declare -r ARRAY_START=ARRAY_START_BASH
;;
"zsh" )
declare -r SHELL_ID=SHELL_ZSH
declare -r ARRAY_START=ARRAY_START_ZSH
;;
* )
return 1
;;
esac
echo "Shell ID: ${!SHELL_ID} Index arrays from: ${!ARRAY_START}"
It works fine when run from the command line while in the same directory:
<my home> ~ % echo "$(./indirect.sh)"
Shell ID: 1 Index arrays from: 1
Problems arise when I source the script:
<my home> ~ % echo "$(. ~/indirect.sh)"
/Users/<me>/indirect.sh:28: bad substitution
I don’t understand why sourcing the script changes the behavior of the parameter expansion.
Is this expected behavior? If so, I’d be grateful if someone could explain it and hopefully, offer a work around.
The problem described in the original post has nothing to do with indirect expansion. The difference in behavior is a result of different shells being invoked depending on whether the script is “executed” or “sourced”. These differences reveal the basic flaw in deriving the shell from the $SHELL variable that underpins the script's design. If the shell defined in $SHELL does not match the shebang, the script will fail either when sourced or executed. An explanation follows.
Indirect expansion doesn’t offer value in the given scenario because values could just as easily be assigned directly. They’ll have to be assigned that way regardless given the different syntax used for indirect expansion between shells. In fact, other syntax differences between shells makes the entire premise for detecting the shell moot! However, putting that aside, the difference in behavior is a result of different shells being invoked based on whether the script is “executed” or “sourced”. The behavior of sourcing is well documented with numerous explanations on the web, but for context here’s how it works:
Executing a Script
Use the “./“ syntax to execute a script.
When run this way, the script executes in a sub-shell. Any changes the
script makes to it’s shell are applied to the sub-shell, not the shell
in which the script was launched, so those changes are lost when the
shell exits because the sub-shell in which it executed is destroyed as
well. For example, if the script changes the working directory, it
does so in the sub-shell. The working directory of the main shell that
launched the script is unchanged when the script terminates. If you
want to make changes to the shell in which the script was launched, it
must be sourced.
Sourcing a Script
Use the “source “ syntax to source a
script. When run this way, the script essentially becomes an argument
for the source command, which handles invoking the appropriate
execution. Some shells (e.g. ksh) use a single period “.” instead of
“source”.
When a script is executed with the “./“ syntax, the shebang at the top of the file is used to determine which shell to use. When a script is sourced, the shebang is ignored and the shell in which the script is launched is used instead. Also note that the period that appears in the “./“ command syntax used to execute a script, is not related to the period that’s occasionally used as an alias for the source command.
The script in the post uses bash in the shebang statement, so it works when executed because it’s run using bash. When it’s sourced from zsh, it encounters the incorrect indirect expansion syntax:
“${!A_VAR}"
The correct syntax is:
"${(P)A_VAR}"
However, correcting the syntax won’t help because it will then fail when executed. The shebang will invoke bash and the syntax will be wrong again. That renders indirection useless for accessing a variable designed to indicate the shell in use. More importantly, a design based on querying an environment variable for the shell is flawed due to differences in the shell that’s ultimately used depending on whether the script is executed or sourced.
To add to your answer (what I'm going to say is too long for a comment), I can not think of any application, why your script could be useful if not sourced. Actually, I came accross the need of such a script by myself in exactly one occasion:
Since I use as interactive shell not only zsh, but also sometimes bash, so I have written my .zshrc and .bashrc to set up everything (including defining variables and shell functions for interactive use). In order to safe work,
I try to put code which works under both bash and zsh into a single file (say: .commonrc), and my .zshrc and .bashrc have inside them a
source .commonrc
While many things are so different in bash and zsh, that I can't put them into .commonrc, some can, provided I do some tweaking. One reason for headache is obviously the different indexing of arrays, which you seemingly try to solve. So I have also a similar feature. However, I don't nee ca case construct for this. Instead, my .bashrc looks like this (using your naming of the variables):
...
declare -r ARRAY_START=0
source .commonrc
...
and my .zshrc looks like this:
...
declare -r ARRAY_START=1
source .commonrc
...
Since it does not happen that the .bashrc is run from a zsh and vice versa, I don't need to query what kind of shell I have.
Let's say a script is called with /bin/sh. Is it possible to source another script from that script and to have it be interpreted with #!/bin/bash?
It would appear that the #!/bin/bash doesn't do anything...
And by source, at this point I am meaning the functionality of manipulating the parent environment.
No. The whole point of sourcing a script is that the script is interpreted by the shell doing the sourcing. If, as is often the case, /bin/sh is bash, then you will get the desired behavior. Otherwise, you are out of luck.
Try the source command, or dot operator. You might also try the env command. Note, make sure you export if you're using source (or dot).
I switched from bash to zsh and I was wondering if there was a way to put arguments after file name like in bash
Example:
cp dir1 dir2 -r
Thank you
This depends only on the command, not on the shell. The shell passes the arguments in the order they're given, and makes no special treatment for arguments beginning with a -.
zsh has some expanded/different features in the area of globbing and tab completion (two of the primary reasons folks may switch to zsh). Both of these provide you interesting way to add command line parameters. Is that what you are asking about?
Note also that most commands are not impacted by the shell you choose: ls, awk, grep, vim, etc. Obviously things like alias and function that are shell commands are potentially different.