Advanced Bash−Scripting Guide

# Piping an empty "echo" to awk gives it dummy input, #+ and thus makes it unnecessary to supply a filename.

exit 0

9.2.2. Further Discussion

For more on string manipulation in scripts, refer to Section 9.3 and the relevant section of the expr command listing. For script examples, see:

1.Example 12−6

2.Example 9−12

3.Example 9−13

4.Example 9−14

5.Example 9−16

9.3.Parameter Substitution

Manipulating and/or expanding variables

${parameter}

Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works.

May be used for concatenating variables with strings.

your_id=${USER}−on−${HOSTNAME} echo "$your_id"

#

echo "Old \$PATH = $PATH"

PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH"

${parameter−default}

If parameter not set, use default.

echo ${username−`whoami`}

# Echoes the result of `whoami`, if variable $username is still unset.

This is almost equivalent to ${parameter:−default}. The extra : makes a difference only when parameter has been declared, but is null.

Advanced Bash−Scripting Guide

#!/bin/bash

username0=

#username0 has been declared, but is set to null. echo "username0 = ${username0−`whoami`}"

#Will not echo.

echo "username1 = ${username1−`whoami`}"

#username1 has not been declared.

#Will echo.

username2=

#username2 has been declared, but is set to null. echo "username2 = ${username2:−`whoami`}"

#Will echo because of :− rather than just − in condition test.

exit 0

${parameter=default}, ${parameter:=default}

If parameter not set, set it to default.

Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, [21] as above.

echo ${username=`whoami`}

# Variable "username" is now set to `whoami`.

${parameter+alt_value}, ${parameter:+alt_value}

If parameter set, use alt_value, else use null string.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below.

echo "###### \${parameter+alt_value} ########"

echo

echo

echo "###### \${parameter:+alt_value} ########"

echo

${parameter?err_msg}, ${parameter:?err_msg}

If parameter set, use it, else print err_msg.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.

Example 9−11. Using param substitution and :

#!/bin/bash

#Check some of the system's environmental variables.

#If, for example, $USER, the name of the person at the console, is not set, #+ the machine will not recognize you.

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

echo "Name of the machine is $HOSTNAME." echo "You are $USER."

echo "Your home directory is $HOME."

echo "Your mail INBOX is located in $MAIL." echo

echo "If you are reading this message,"

echo "critical environmental variables have been set." echo

echo

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

#The ${variablename?} construction can also check

#+ for variables set within the script.

ThisVariable=Value−of−ThisVariable

# Note, by the way, that string variables may be set #+ to characters disallowed in their names.

: ${ThisVariable?}

echo "Value of ThisVariable is $ThisVariable". echo

echo

: ${ZZXy23AB?"ZZXy23AB has not been set."}

#If ZZXy23AB has not been set,

#+ then the script terminates with an error message.

#You can specify the error message.

#: ${ZZXy23AB?"ZZXy23AB has not been set."}

Advanced Bash−Scripting Guide

echo "You will not see this message, because script terminated above."

HERE=0

exit $HERE # Will *not* exit here.

Parameter substitution and/or expansion. The following expressions are the complement to the

match in expr string operations (see Example 12−6). These particular ones are used mostly in parsing file path names.

Variable length / Substring removal

${#var}

String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array.

Exceptions:

♦${#*} and ${#@} give the number of positional parameters.

♦For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array.

Example 9−12. Length of a variable

#!/bin/bash

# length.sh

E_NO_ARGS=65

if [ $# −eq 0 ] # Must have command−line args to demo script. then

echo "Invoke this script with one or more command−line arguments." exit $E_NO_ARGS

fi

var01=abcdEFGH28ij

echo "var01 = ${var01}"

echo "Length of var01 = ${#var01}"

echo "Number of command−line arguments passed to script = ${#@}" echo "Number of command−line arguments passed to script = ${#*}"

exit 0

${var#pattern}, ${var##pattern}

Remove from $var the shortest/longest part of $pattern that matches the front end of $var.

A usage illustration from Example A−6:

Advanced Bash−Scripting Guide

#Function from "days−between.sh" example.

#Strips leading zero(s) from argument passed.

strip_leading_zero () # Better to strip possible leading zero(s)

var1=abcd12345abc6789

pattern1=a*c # * (wild card) matches everything between a − c.

Advanced Bash−Scripting Guide

#Renaming file extensions.

#rfe old_extension new_extension

#Example:

#To rename all *.gif files in working directory to *.jpg,

#rfe gif jpg

ARGS=2

E_BADARGS=65

if [ $# −ne $ARGS ] then

echo "Usage: `basename $0` old_file_suffix new_file_suffix" exit $E_BADARGS

fi

for filename in *.$1

# Traverse list of files ending with 1st argument. do

mv $filename ${filename%$1}$2

#Strip off part of filename matching 1st argument,

#then append 2nd argument.

done

exit 0

Variable expansion / Substring replacement

These constructs have been adopted from ksh.

${var:pos}

Variable var expanded, starting from offset pos.

${var:pos:len}

Expansion to a max of len characters of variable var, from offset pos. See Example A−9 for an

Advanced Bash−Scripting Guide

example of the creative use of this operator.

${var/patt/replacement}

${var//patt/replacement}

Global replacement. All matches of patt, within var replaced with replacement.

As above, if replacement is omitted, then all occurrences of patt are replaced by nothing, that is, deleted.

Example 9−15. Using pattern matching to parse arbitrary strings

#!/bin/bash

var1=abcd−1234−defg echo "var1 = $var1"

t=${var1#*−*}

echo "var1 (with everything, up to and including first − stripped out) = $t"

#t=${var1#*−} works just the same, #+ since # matches the shortest string,

#+ and * matches everything preceding, including an empty string.

#(Thanks, S. C. for pointing this out.)

t=${var1##*−*}

echo "If var1 contains a \"−\", returns empty string... var1 = $t"

t=${var1%*−*}

echo "var1 (with everything from the last − on stripped out) = $t"

echo

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

path_name=/home/bozo/ideas/thoughts.for.today

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

echo "path_name = $path_name" t=${path_name##/*/}

echo "path_name, stripped of prefixes = $t"

#If $path_name ends with a newline, then `basename $path_name` will not work, #+ but the above expression will.

#(Thanks, S.C.)

t=${path_name%/*.*}

# Same effect as t=`dirname $path_name` echo "path_name, stripped of suffixes = $t"

#These will fail in some cases, such as "../", "/foo////", # "foo/", "/".

#Removing suffixes, especially when the basename has no suffix,

#+ but the dirname does, also complicates matters.

Advanced Bash−Scripting Guide

# (Thanks, S.C.)

echo

t=${path_name:11}

echo "$path_name, with first 11 chars stripped off = $t" t=${path_name:11:5}

echo "$path_name, with first 11 chars stripped off, length 5 = $t"

echo

t=${path_name/bozo/clown}

echo "$path_name with \"bozo\" replaced by \"clown\" = $t" t=${path_name/today/}

echo "$path_name with \"today\" deleted = $t" t=${path_name//o/O}

echo "$path_name with all o's capitalized = $t" t=${path_name//o/}

echo "$path_name with all o's deleted = $t"

exit 0

${var/#patt/replacement}

If prefix of var matches replacement, then substitute replacement for patt.

${var/%patt/replacement}

If suffix of var matches replacement, then substitute replacement for patt.

Example 9−16. Matching patterns at prefix or suffix of string

#!/bin/bash

# Pattern replacement at prefix / suffix of string.

#|−−−|

#Match at suffix (end) of string.

echo

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

#Must match at beginning / end of string,

#+ otherwise no replacement results.

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−