Advanced Bash−Scripting Guide

command1 2> − | command2 # Trying to redirect error output of command1 into a pipe...
# ...will not work.
command1 2>& − | command2 # Also futile.
Thanks, S.C.
Advanced Bash−Scripting Guide
Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines may
have version 1.XX of Bash as the default installation.
#!/bin/bash
minimum_version=2
# Since Chet Ramey is constantly adding features to Bash,
# you may set $minimum_version to 2.XX, or whatever is appropriate.
E_BAD_VERSION=80
if [ "$BASH_VERSION" \< "$minimum_version" ]
then
echo "This script works only with Bash, version $minimum or greater."
echo "Upgrade strongly recommended."
exit $E_BAD_VERSION
fi
...
Using Bash−specific functionality in a Bourne shell script (#!/bin/sh) on a non−Linux machine may cause
unexpected behavior. A Linux system usually aliases sh to bash, but this does not necessarily hold true for a
generic UNIX machine.
A script with DOS−type newlines (\r\n) will fail to execute, since #!/bin/bash\r\n is not recognized,
not the same as the expected #!/bin/bash\n. The fix is to convert the script to UNIX−style newlines.
#!/bin/bash
echo "Here"
unix2dos $0
chmod 755 $0
# Script changes itself to DOS format.
# Change back to execute permission.
# The 'unix2dos' command removes execute permission.
./$0 # Script tries to run itself again.
# But it won't work as a DOS file.
echo "There"
exit 0
A shell script headed by #!/bin/sh may not run in full Bash−compatibility mode. Some Bash−specific
functions might be disabled. Scripts that need complete access to all the Bash−specific extensions should start
with #!/bin/bash.
A script may not export variables back to its parent process, the shell, or to the environment. Just as we
learned in biology, a child process can inherit from a parent, but not vice versa.
WHATEVER=/home/bozo
export WHATEVER
Chapter 32. Gotchas 329