Shell – Why file glob expansion is not suppressed

quotingshellwildcards

I have the following script my.sh:

echo $@

Now I want to see file glob expansion, so I do like this and it works OK:

$ bash my.sh *
$ f1 f2 my.sh

Now, I've tried suppress expansion using techniques described in this answer:

$ bash my.sh "*"
$ bash my.sh \*

But I still get listing of all files as the output. Why?

Best Answer

With the first invocation of your script

$ bash my.sh *

the file name globbing is performed by the shell before calling the script.

With the second invocation of your script

$ bash my.sh "*"

the file name globbing is performed in the script when running echo $@.

To avoid this, double quote $@ on that line:

echo "$@"

Further reading: Security implications of forgetting to quote a variable in bash/POSIX shells

Related Solutions

Bash – Recursive Glob

In order to do recursive globs in bash, you need the globstar feature from bash version 4 or higher.

From the bash manpage:

globstar
    If set, the pattern ** used in a pathname expansion context will
    match all files and zero or more directories and subdirectories.
    If the pattern is followed by a /, only directories and
    subdirectories match.

For your example pattern:

shopt -s globstar
ls -d -- **/*.py

Bash – Variable expansion in Bash

No, it shouldn't, because of the way bash operate the command.

When you type echo one|name, bash parse the command, treats | as a pipe token, so | perform pipeline.

When you type echo $variable, because token parsing occur before variable expansion, bash parsing the command into two parts, echo and $variable. After that, it performs variable expansion, expand $variable to one|name. In this case, one|name is a string, | is a part of string and can not be treated as a pipe token (of course, the token recognition phrase was done). The only thing it can be special if IFS variable contains |, bash will use | as delimiter to perform field spliting:

$ variable='one|name'
$ IFS='|'
$ echo $variable
one name

Best Answer

Related Solutions

Bash – Recursive Glob

Bash – Variable expansion in Bash

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