#!/bin/bash# Redirecting stdin using 'exec'.exec 6<&0  # Link file descriptor #6 with stdin.   # Saves stdin.exec < data-file   # stdin replaced by file "data-file" read a1 # Reads first line of file "data-file".read a2 # Reads second line of file "data-file." echoecho "Following lines read from file." echo "-------------------------------" echo $a1echo $a2echo; echo; echoexec 0<&6 6<&-#  Now restore stdin from fd #6, where it had been saved,#+ and close fd #6 ( 6<&- ) to free it for other processes to use.## <&6 6<&- also works.echo -n "Enter data  " read b1  # Now "read" functions as expected, reading from normal stdin.echo "Input read from stdin." echo "----------------------" echo "b1 = $b1" echoexit 0

#!/bin/bash# reassign-stdout.shLOGFILE=logfile.txtexec 6>&1   # Link file descriptor #6 with stdout. # Saves stdout.exec > $LOGFILE # stdout replaced with file "logfile.txt".# ----------------------------------------------------------- ## All output from commands in this block sent to file $LOGFILE.echo -n "Logfile: " dateecho "-------------------------------------" echoecho "Output of "ls -al" command" echols -alecho; echoecho "Output of "df" command" echodf# ----------------------------------------------------------- #exec 1>&6 6>&-  # Restore stdout and close file descriptor #6.echoecho "== stdout now restored to default == " echols -alechoexit 0

#!/bin/bash# upperconv.sh# Converts a specified input file to uppercase.E_FILE_ACCESS=70E_WRONG_ARGS=71if [ ! -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_ACCESSfi   #  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_ARGSfiexec 4<&0exec < $1 # Will read from input file.exec 7>&1exec > $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

#!/bin/bash# avoid-subshell.sh# Suggested by Matthew Walker.Lines=0echocat myfile.txt | while read line; do {   echo $line   (( Lines++ ));  #  Incremented values of this variable   #+ inaccessible outside loop.   #  Subshell problem. } doneecho "Number of lines read = $Lines" # 0 # Wrong!echo "------------------------" exec 3<> myfile.txtwhile read line <&3do {  echo "$line"   (( Lines++ ));   #  Incremented values of this variable   #+ accessible outside loop.   #  No subshell, no problem.}doneexec 3>&-echo "Number of lines read = $Lines" # 8echoexit 0# Lines below not seen by script.$ cat myfile.txtLine 1.Line 2.Line 3.Line 4.Line 5.Line 6.Line 7.Line 8.

#!/bin/bash# redir2.shif [ -z "$1" ]then  Filename=names.data   # Default, if no filename specified.else  Filename=$1fi  #+ Filename=${1:-names.data}#  can replace the above test (parameter substitution).count=0echowhile [ "$name" != Smith ]  # Why is variable $name in quotes?do  read name # Reads from $Filename, rather than stdin.  echo $name  let "count += 1" done <"$Filename"   # Redirects stdin to file $Filename. # ^^^^^^^^^^^^echo; echo "$count names read"; echoexit 0#  Note that in some older shell scripting languages,#+ the redirected loop would run as a subshell.#  Therefore, $count would return 0, the initialized value outside the loop.#  Bash and ksh avoid starting a subshell *whenever possible*,#+ so that this script, for example, runs correctly.#  (Thanks to Heiner Steven for pointing this out.)#  However . . .#  Bash *can* sometimes start a subshell in a PIPED "while-read" loop,#+ as distinct from a REDIRECTED "while" loop.abc=hiecho -e "123" | while read l do abc="$l" echo $abc doneecho $abc#  Thanks, Bruno de Oliveira Schneider, for demonstrating this#+ with the above snippet of code.#  And, thanks, Brian Onn, for correcting an annotation error.

#!/bin/bash# This is an alternate form of the preceding script.#  Suggested by Heiner Steven#+ as a workaround in those situations when a redirect loop#+ runs as a subshell, and therefore variables inside the loop# +do not keep their values upon loop termination.if [ -z "$1" ]then  Filename=names.data # Default, if no filename specified.else  Filename=$1fi  exec 3<&0 # Save stdin to file descriptor 3.exec 0<"$Filename" # Redirect standard input.count=0echowhile [ "$name" != Smith ]do  read name   # Reads from redirected stdin ($Filename).  echo $name  let "count += 1" done  #  Loop reads from file $Filename  #+ because of line 20.#  The original version of this script terminated the "while" loop with#+  done <"$Filename" #  Exercise:#  Why is this unnecessary?exec 0<&3 # Restore old stdin.exec 3<&- # Close temporary fd 3.echo; echo "$count names read"; echoexit 0

#!/bin/bash# Same as previous example, but with "until" loop.if [ -z "$1" ]then  Filename=names.data # Default, if no filename specified.else  Filename=$1fi  # while [ "$name" != Smith ]until [ "$name" = Smith ] # Change  !=  to =.do  read name   # Reads from $Filename, rather than stdin.  echo $namedone <"$Filename" # Redirects stdin to file $Filename. # ^^^^^^^^^^^^# Same results as with "while" loop in previous example.exit 0

#!/bin/bashif [ -z "$1" ]then  Filename=names.data  # Default, if no filename specified.else  Filename=$1fi  line_count=`wc $Filename | awk '{ print $1 }'`#   Number of lines in target file.##  Very contrived and kludgy, nevertheless shows that#+ it's possible to redirect stdin within a "for" loop...#+ if you're clever enough.## More concise is line_count=$(wc -l < "$Filename")for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers.# while [ "$name" != Smith ]   --   more complicated than a "while" loop   --do  read name # Reads from $Filename, rather than stdin.  echo $name  if [ "$name" = Smith ]   # Need all this extra baggage here.  then break  fi  done <"$Filename"  # Redirects stdin to file $Filename. # ^^^^^^^^^^^^exit 0

#!/bin/bashif [ -z "$1" ]then  Filename=names.data  # Default, if no filename specified.else  Filename=$1fi  Savefile=$Filename.new # Filename to save results in.FinalName=Jonah # Name to terminate "read" on.line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.for name in `seq $line_count`do  read name  echo "$name"   if [ "$name" = "$FinalName" ]  then break  fi  done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename,# ^^^^^^^^^^^^^^^^^^^^^^^^^^^   and saves it to backup file.exit 0

#!/bin/bashif [ -z "$1" ]then  Filename=names.data   # Default, if no filename specified.else  Filename=$1fi  TRUE=1if [ "$TRUE" ]  # if true and   if :   also work.then read name echo $namefi <"$Filename" #  ^^^^^^^^^^^^# Reads only first line of file.# An "if/then" test has no way of iterating unless embedded in a loop.exit 0

AristotleArrheniusBelisariusCapablancaDickensEulerGoetheHegelJonahLaplaceMaroczyPurcellSchmidtSchopenhauerSemmelweissSmithSteinmetzTukhashevskyTuringVennWarshawskiZnosko-Borowski#  This is a data file for#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".