Advanced Bash-Scripting Guide by Mendel Cooper

By Mendel Cooper

This can be the total reference at the Borne back shell (bash). quantity 1Volume 2 includes all appendices.

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Sh script for a "shiftless" #+ alternative method of stepping through the positional params. The shift command can take a numerical parameter indicating how many positions to shift. sh shift 3 # Shift 3 positions. # n=3; shift $n # Has the same effect. sh 1 2 3 4 5 4 # However, as Eleni Fragkiadaki, points out, #+ attempting a 'shift' past the number of #+ positional parameters ($#) returns an exit status of 1, #+ and the positional parameters themselves do not change. # This means possibly getting stuck in an endless loop.

K. " # Division result < 1. # Rounded off to 0. " # Illegal division by 0. # 1 # What effect does the "2>/dev/null" have? # What would happen if it were removed? # Try removing it, then rerunning the script. # ======================================= # # (( ... )) also useful in an if-then test. var1=5 var2=4 if (( var1 > var2 )) then #^ ^ Note: Not $var1, $var2. Why? 2. File test operators Returns true if... -e file exists -a file exists This is identical in effect to -e. It has been "deprecated," [28] and its use is discouraged.

This also requires indirect referencing. args=$# # Number of args passed. args} # Note: This is an *indirect reference* to $args ... ) # This is an *indirect reference* to the $# variable. $#} doesn't work. Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There must also exist symbolic links to all the alternate names of the script. See Example 15-2. If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result.

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