Some Bash questions for a newbie

Question

I am studying Bash, but there are some things that are not explained in my book. First I'll post a script, and then I will ask questions going through scripts.

Bash Script:

$ cat sortmerg
#!/bin/bash
usage ()
{
if [ $# -ne 2 ]; then
echo "Usage: $0 file1 file2" 2>&1
exit 1
fi
}

# Default temporary directory
: ${TEMPDIR:=/tmp}

# Check argument count
usage "$@"

# Set up temporary files for sorting
file1=$TEMPDIR/$$.file1
file2=$TEMPDIR/$$.file2

# Sort
sort $1 > $file1
sort $2 > $file2

# Open $file1 and $file2 for reading. Use file descriptors 3 and 4.
exec 3<$file1
exec 4<$file2

# Read the first line from each file to figure out how to start.
read Line1 <&3
status1=$?
read Line2 <&4
status2=$?

# Strategy: while there is still input left in both files:
# Output the line that should come first.
# Read a new line from the file that line came from.
while [ $status1 -eq 0 -a $status2 -eq 0 ]
do
if [[ "$Line2" > "$Line1" ]]; then
echo -e "1.\t$Line1"
read -u3 Line1
status1=$?
else
echo -e "2.\t$Line2"
read -u4 Line2
status2=$?
fi
done

# Now one of the files is at end-of-file.
# Read from each file until the end.
# First file1:
while [ $status1 -eq 0 ]
do
echo -e "1.\t$Line1"
read Line1 <&3
status1=$?
done
# Next file2:
while [[ $status2 -eq 0 ]]
do
echo -e "2.\t$Line2"
read Line2 <&4
status2=$?
done
# Close and remove both input files
exec 3<&- 4<&-
rm -f $file1 $file2
exit 0

Questions:

1. First of all, how do I indent codes in Gedit? I copied and pasted codes from a book, and it doesn't indent the codes automatically. Do you normally use Gedit or any other editor popular for scripting bash?

2. : ${TEMPDIR:=/tmp}
   Can you explain what this is? I have programming knowledge of C# and other programming languages. So can you tell me, assuming I'm not total newbie at programming, what the : key and the curly bracket keys do?

3. if [[ "$Line2" > "$Line1" ]]; then
[] is same as test. But why does [[]] do any different?

4. status1=$?
   what is $? ?

Many thanks in advance.

Answer 1

1. I use vim for writing shell scripts.

2. ${TEMPDIR} will expand to the value of the variable named TEMPDIR. ${TEMPDIR:=/tmp} will do the same, but if it's empty (or unset), the value /tmp will be assigned to TEMPDIR, and also expanded.

   Having ${TEMPDIR:=/tmp} alone on a line, will cause it to be changed to e.g. /tmp which will try to execute /tmp as a command (which will obviously fail, since you can't execute a directory). That's why the : (null) command is used. The null command ignores all input, all arguments, and does absolutely nothing. Run help : to see the description of that builtin command.

   See http://mywiki.wooledge.org/BashFAQ/073 For the various things you can do with Parameter Expansion.

3. [[ "$line2" > "$Line1" ]] returns true if line2 sorts after line1 (like strcmp in C).

   [("test" command) and [[ ("new test" command) are used to evaluate expressions. [[ works only in Bash, Zsh and the Korn shell, and is more powerful; [ and test are available in POSIX shells.

   See http://mywiki.wooledge.org/BashFAQ/031 for the dfference between the [ command and the [[ keyword.

4. ? is a special parameter that holds the exit status of the last command executed. $? expands the value of that parameter.

On a side note, if that's an example from your book, I'd say that's a poor source for learning bash. I recommend reading http://mywiki.wooledge.org/BashGuide which also teaches good practices.

Answer 2

Yes you have indent in Gedit. Open gedit and at Edit-> Preferences you can have the automatic indent option.

About the second point , its saying like assume the TEMPDIR is at /tmp location.

Answer 3

1. To indent code in Gedit press the Tab key, and Shift-Tab to unindent. All the information you need and the hotkeys list is at Gnome Documentation section for gedit

2. The : command is a built-in Bash command that does nothing except return 0 (true); but in this case serves to permit the evaluation of the parameter expansion ${VAR:=VALUE} which assigns VALUE to $VAR only if it doesn't exist. If $VAR (in your case $TEMPDIR) already has a value it is not modified. It is equivalent to the construct:

   [[ -z "$TEMPDIR" ]] && TEMPDIR=/tmp
   

   i.e., "if $TEMPDIR is empty, evaluate command at the right (assign /tmp to TEMPDIR)".

3. The [[ command is a built-in Bash command (as also [ and test; they have executable equivalents too). The command [[ has several pros and cons versus [ or test.

   Pros: It is a built-in command, so is faster, and it has more functionality, like regular expression checking and parsing (remember: man bash is your friend) and doesn't need to escape some operands.

   Cons: It is not portable as it has different operators available in each of the shell interpreters where it exists.

4. The expression $? is, as the terminology in man bash states, a special parameter that resolves or "expands" to the exit status of the last command. If the last command was successful it should return true (0 for Bash), so $? will be a numeric zero; in other cases, $? will be the number returned by the command. For example this command line will show the error value returned by ls trying to find the file a-nonexistent-file.txt:

   ls a-nonexistent-file-txt ; echo $?
   

Bash as a programming language has a lot of quirks and peculiarities; studying the Bash manual is the route to success.

[Sorry for the apparent duplicate answer, but the others were unespecific and don't answered all the questions as they were asked (not meaning offense; I learned a couple of unrelated things there).]