upperconv.sh

Shall高级编程

#!/bin/bash
# upperconv.sh
# Converts a specified input file to uppercase.

E_FILE_ACCESS=70
E_WRONG_ARGS=71

if [ ! -r "$1" ]     # Is specified input file readable?
then
  echo "Can't read from input file!"
  echo "Usage: $0 input-file output-file"
  exit $E_FILE_ACCESS
fi                   #  Will exit with same error
                     #+ even if input file ($1) not specified (why?).

if [ -z "$2" ]
then
  echo "Need to specify output file."
  echo "Usage: $0 input-file output-file"
  exit $E_WRONG_ARGS
fi


exec 4<&0
exec < $1            # Will read from input file.

exec 7>&1
exec > $2            # Will write to output file.
                     # Assumes output file writable (add check?).

# -----------------------------------------------
    cat - | tr a-z A-Z   # Uppercase conversion.
#   ^^^^^                # Reads from stdin.
#           ^^^^^^^^^^   # Writes to stdout.
# However, both stdin and stdout were redirected.
# Note that the 'cat' can be omitted.
# -----------------------------------------------

exec 1>&7 7>&-       # Restore stout.
exec 0<&4 4<&-       # Restore stdin.

# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."

exit 0