Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD"
# Works fine.
old_tty_setting=$(stty −g) # Save old terminal setting.
echo "Hit a key "
stty −icanon −echo
# Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress.
stty "$old_tty_setting" # Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.
Thanks, S.C.
Using echo to output an unquoted variable set with command substitution removes trailing
newlines characters from the output of the reassigned command(s). This can cause unpleasant
surprises.
dir_listing=`ls −l`
echo $dir_listing
# unquoted
# Expecting a nicely ordered directory listing.
# However, what you get is:
# total 3 −rw−rw−r−− 1 bozo bozo 30 May 13 17:15 1.txt −rw−rw−r−− 1 bozo
# bozo 51 May 15 20:57 t2.sh −rwxr−xr−x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared.
echo "$dir_listing" # quoted
# −rw−rw−r−− 1 bozo 30 May 13 17:15 1.txt
# −rw−rw−r−− 1 bozo 51 May 15 20:57 t2.sh
# −rwxr−xr−x 1 bozo 217 Mar 5 21:13 wi.sh
Command substitution even permits setting a variable to the contents of a file, using either redirection or the
cat command.
variable1=`