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About bash : Cant-get-expandaliases-to-take-effect

Question Detail

I can’t get expand_aliases to take effect in bash. I’ve tried a lot of different things, and nothing works.

Here’s the simple test case:

/bin/bash -c 'shopt -s expand_aliases; alias cdtmp="cd /tmp"; alias; cdtmp; pwd;'

And the output:

$ /bin/bash -c 'shopt -s expand_aliases; alias cdtmp="cd /tmp"; alias; cdtmp; pwd;'
alias cdtmp='cd /tmp'
/bin/bash: cdtmp: command not found

$ /bin/bash --version
GNU bash, version 3.2.25(1)-release (i686-redhat-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

(Yes, I’m using shopt instead of the -O option to bash, just to prove it’s being done.)

Any ideas?

Question Answer

Aliases aren’t available on the same line or in the same function where they are defined.

From the Bash man page:

       The rules concerning the definition and use  of  aliases  are  somewhat
       confusing.   Bash  always  reads  at  least  one complete line of input
       before executing any  of  the  commands  on  that  line.   Aliases  are
       expanded  when  a command is read, not when it is executed.  Therefore,
       an alias definition appearing on the same line as another command  does
       not  take  effect  until  the next line of input is read.  The commands
       following the alias definition on that line are not affected by the new
       alias.   This  behavior  is  also an issue when functions are executed.
       Aliases are expanded when a function definition is read, not  when  the
       function  is  executed,  because a function definition is itself a com‐
       pound command.  As a consequence, aliases defined in a function are not
       available  until  after  that function is executed.  To be safe, always
       put alias definitions on a separate line, and do not use alias in  com‐
       pound commands.

       For almost every purpose, aliases are superseded by shell functions.

The Bash Reference Manual says

For almost every purpose, shell functions are preferred over aliases.

instead of the last sentence above [emphasis mine]. I consider aliases to be a command-line convenience rather than something that should be used in scripts (including those which consist solely of bash -c one-liners).

I came late for 10 years.
you can use the eval to make the alias cdtmp expand again.

/bin/bash -c 'shopt -s expand_aliases; alias cdtmp="cd /tmp"; alias; eval cdtmp; pwd;'

will give you output

alias cdtmp='cd /tmp'

try /bin/bash -O expand_aliases -c 'xx'

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