linux - Why does /bin/sh behave differently to /bin/bash even if one points to the other? -


while playing around in shell investigating answer this question, noticed that, though /bin/sh pointing /bin/bash on system, 2 commands behave differently. first of all, output of

ls -lh /bin/sh

is:

lrwxrwxrwx 1 root root 4 apr 22  2013 /bin/sh -> bash*

however, invoking following command through /bin/sh:

/bin/sh -c "script.sh 2> >( grep -v filter 2>&1 )"

returns error:

/bin/sh: -c: line 0: syntax error near unexpected token '>' /bin/sh: -c: line 0: 'script.sh 2> >( grep -v filter 2>&1 )'

while running same command through /bin/bash:

/bin/bash -c "script.sh 2> >( grep -v filter 2>&1 )"

executes successfully, here output:

this should on stderr

for reference, here contents of script.sh:

#!/bin/sh echo "filter: should filtered out" 1>&2 echo "this should on stderr" 1>&2 echo "filter: should filtered out" 1>&2

why 2 invocations behave differently?

bash looks @ value of $argv[0] (bash implemented in c) determine how invoked.

its behavior when invoked sh documented in manual:

if bash invoked name sh, tries mimic startup behavior of historical versions of sh closely possible, while conforming posix standard well.

when invoked interactive login shell, or non-interactive shell -login option, first attempts read , execute commands /etc/profile , ~/.profile, in order. --noprofile option may used inhibit behavior. when invoked interactive shell name sh, bash looks variable env, expands value if defined, , uses expanded value name of file read , execute. since shell invoked sh not attempt read , execute commands other startup files, --rcfile option has no effect. non-interactive shell invoked name sh not attempt read other startup files.

when invoked sh, bash enters posix mode after startup files read

there's long list (currently 46 items) of things change when bash in posix mode, documented here.

(posix mode useful way test scripts portability non-bash shells.)

incidentally, programs change behavior depending on name under invoked common. versions of grep, fgrep, , egrep implemented single executable (though gnu grep doesn't this). view typically symbolic link vi or vim; invoking view causes open in read-only mode. busybox system includes number of individual commands symlinks master busybox executable.


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