3.2. Special characters used in shell scripts

# This line is a comment.

Comments may also occur at the end of a command.

echo "A comment will follow." # Comment here.

Comments may also follow white space at the beginning of a line.

# A tab precedes this comment.

echo hello; echo there

Note that the ; sometimes needs to be escaped (\).

Endless loop:

while : do operation-1 operation-2 ... operation-n done

Placeholder in if/then test:

if condition then : # Do nothing and branch ahead else take-some-action fi

Provides a placeholder where a binary operation is expected, see Section 3.3.1.

: ${username=`whoami`} # ${username=`whoami`} without the leading : gives an error

Evaluate string of variables using "parameter substitution", see Example 3-6:

: ${HOSTNAME?} ${USER?} ${MAIL?}

Prints error message if one or more of essential environmental variables not set.

See Section 3.3 for more details.

The code block enclosed in braces may have I/O redirected to and from it.

#!/bin/bash

{
read fstab
} < /etc/fstab

echo "First line in /etc/fstab is:"
echo "$fstab"

exit 0

#!/bin/bash

#                rpm-check
#                ---------
# Queries an rpm file for description, listing, and whether it can be installed.
# Saves output to a file.
# 
# This script illustrates using a code block.

NOARGS=1

if [ -z $1 ]
then
  echo "Usage: `basename $0` rpm-file"
  exit $NOARGS
fi  

{ 
  echo
  echo "Archive Description:"
  rpm -qpi $1  #Query description.
  echo
  echo "Archive Listing:"
  rpm -qpl $1  #Query listing.
  echo
  rpm -i --test $1  #Query whether rpm file can be installed.
  if [ ! $? ]
  then
    echo "$1 can be installed."
  else
    echo "$1 cannot be installed."
  fi  
  echo
} > $1.test  # Redirects output of everything in block to file.

echo "Results of rpm test in file $1.test"

# See rpm man page for explanation of options.

exit 0

scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists.

command >&2 redirects output of command to stderr.

scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it will be created.

For a more detailed explanation, see Section 3.13.

echo ls -l | sh

passes the output of "ls -l" to the shell, with the same result as a simple "ls -l".

cat *.lst | sort | uniq

sorts the output of all the .lst files and deletes duplicate lines.

(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xvfp -) # Move entire file tree from one directory to another # [courtesy Alan Cox, a.cox@swansea.ac.uk] # # More elegant than, but equivalent to: # cd source-directory # tar cf - . | (cd ../target-directory; tar xzf -)

bunzip2 linux-2.2.15.tar.bz2 | tar xvf - # --uncompress tar file-- | --then pass it to "tar"-- # If "tar" has not been patched to handle "bunzip2", # this needs to be done in two discrete steps, using a pipe. # The purpose of the exercise is to unarchive "bzipped" kernel source.

Note that the "-" is not itself a Bash operator, but rather an option recognized by certain UNIX utilities.

Where a filename is expected, redirects output to stdout (mostly seen with tar cf)

#!/bin/bash

# Backs up all files in current directory
# modified within last 24 hours
# in a tarred and gzipped file.

if [ $# = 0 ]
then
  echo "Usage: `basename $0` filename"
  exit 1
fi  

tar cvf - `find . -mtime -1 -type f -print` > $1.tar
gzip $1.tar

exit 0

Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections of the script.