Advanced Bash scripting guide Rev1.4 by Cooper M. PDF

By Cooper M.

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Y=`expr $y + 1` Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion. z=`expr substr $string $position $length` Extract substring of $length characters, starting at $position. Example 12-6. /bin/bash # Demonstrating some of the uses of 'expr' # ======================================= echo # Arithmetic Operators # ---------- --------echo "Arithmetic Operators" echo a=`expr 5 + 3` echo "5 + 3 = $a" a=`expr $a + 1` echo echo "a + 1 = $a" echo "(incrementing a variable)" a=`expr 5 % 3` # modulo echo echo "5 mod 3 = $a" echo echo # Logical Operators # ------- --------# Returns 1 if true, 0 if false, #+ opposite of normal Bash convention.

The output of nl is very similar to cat -n, however, by default nl does not list blank lines. Example 12-21. nl: A self-numbering script. /bin/bash # This script echoes itself twice to stdout with its lines numbered. # 'nl' sees this as line 3 since it does not number blank lines. # 'cat -n' sees the above line as number 5. nl `basename $0` echo; echo # Now, let's try it with 'cat -n' cat -n `basename $0` # The difference is that 'cat -n' numbers the blank lines. # Note that 'nl -ba' will also do so.

This is especially useful with the -s option, which breaks lines at word spaces (see Example 12-19 and Example A-2). fmt Simple-minded file formatter, used as a filter in a pipe to "wrap" long lines of text output. Example 12-19. Formatted file listing. /bin/bash WIDTH=40 # 40 columns wide. b=`ls /usr/local/bin` # Get a file listing... echo $b | fmt -w $WIDTH # Could also have been done by # echo $b | fold - -s -w $WIDTH exit 0 See also Example 12-4. edu/~amc/Par/. col This deceptively named filter removes reverse line feeds from an input stream.

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