Diff for "BashFAQ/100"

How do I do string manipulations in bash?

(Note: this is mostly a reiteration of material already in FAQ #73. Bash can do string operations. LOTS of string operations. This is an introduction to bash string operations for those new to Bash's special tool/feature called Parameter Expansion (PE), with a focus on typical string operations.

Note Bash's PE capability is a lot more powerful than the typical string manipulation calls you may be used to. There are some twists in the road up ahead.

Here is a list of some typical string manipulation functions/subroutines that you may already be familiar with:

• strlen returns the length of the string
• leftstr returns a string N chars long starting from the left hand side
• rightstr returns a string N chars long starting from the right hand side
• midstr returns a string N chars long starting from offset K chars from the beginning/end
• substr returns copy of the string with all instances matching a patter replaced with a new string
• basename returns the last component of a pathname (everything after the last "/")
• dirname returns everything in the pathname up to, but not including the last "/"
• getext returns a filenames extension
• dropext returns the filename without its extension.

This article will cover how to do all of these using the Bash PE and will introduce the more powerful actions available with PE's. Please note there is a BashFaq about PE's already. That FAQ covers more of the scope of PE capabilities, where this one instead focuses on string operations.

lets assume we have a string variable named fullpath whose value is "usr/home/JosephBaldwin/Its_only_Rock_and_Roll.mp3"

For a basename we use the PE expression: ${fullpath##*/} which returns "Its_only_Rock_and_Roll.mp3".

To find the dirname we use the PE expression: ${fullpath%/*} which produces "usr/home/JosephBaldwin".

To drop the filename extesnion, we use the PE expression: ${fullpath%.*} giving out "usr/home/JosephBaldwin/Its_only_Rock_and_Roll"

To get the filename's extension we use the PE expression: ${fullpath##*.} generating only "mp3".

To find the strlen the PE expression: ${#fullpath} finds it and its 49.

To get a leftstr, the PE expression: ${fullpath:0:20} grabs the first 20 chars of fullpath to make "usr/home/JosephBaldw".usr/home/JosephBaldwin/Its_only_Roll_and_Roll.mp3

To perform a rightstr in bash we use the following PE expression: ${fullpath:$(( 0 - 20 ))} which gets the last 20 chars, "ly_Rock_and_Roll.mp3".

To perform a midstr in bash we use the following PE expression: ${fullpath:10:20} making "osephBaldwin/Its_onl"

To perform a substr in bash we use the following PE expression: ${fullpath//Rock/Roll} rolling it into "usr/home/JosephBaldwin/Its_only_Roll_and_Roll.mp3".

Why aren't the PE things named more nicely

Can't I just have these a regular functions with nice names?

not totally generalizable

What can I do with PE's that I couldn't do with the string functions above?

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